8-Channel, 625 kSPS, 12-Bit
Parallel ADCs with a Sequencer
Data Sheet AD7938-6
Rev. C
Information furnished by Analog Devices is believed to be accurate and reliable. However, no
responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other
rights of third parties that may result from its use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarks and registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700 www.analog.com
Fax: 781.461.3113 ©2004–2011 Analog Devices, Inc. All rights reserved.
FEATURES
Throughput rate: 625 kSPS
Specified for VDD of 2.7 V to 5.25 V
Power consumption
3.6 mW maximum at 625 kSPS with 3 V supplies
7.5 mW maximum at 625 kSPS with 5 V supplies
8 analog input channels with a sequencer
Software-configurable analog inputs
8-channel single-ended inputs
4-channel fully differential inputs
4-channel pseudo differential inputs
7-channel pseudo differential inputs
Accurate on-chip 2.5 V reference
±0.2% maximum @ 25°C, 25 ppm/°C maximum
69 dB SINAD at 50 kHz input frequency
No pipeline delays
High speed parallel interface with word/byte modes
Full shutdown mode: 2 μA maximum
32-lead LFCSP and TQFP packages
FUNCTIONAL BLOCK DIAGRAM
V
04751-001
V
IN
7
T/H
PARALLEL INTERFACE/CONTROL REGISTER
SEQUENCER
12-BIT
SAR ADC
AND
CONTROL
I/P
MUX
2.5V
V
REF
V
DRIVE
DD
AD7938-6
V
IN
0
AGND
V
REFIN
/
V
REFOUT
CLKIN
BUSY
CONVST
DB0 DB11 CS DGNDRD WR W/B
Figure 1.
GENERAL DESCRIPTION
The AD7938-6 is a 12-bit, high speed, low power, successive
approximation (SAR) analog-to-digital converter (ADC). The
part operates from a single 2.7 V to 5.25 V power supply and
features throughput rates up to 625 kSPS. The part contains a
low noise, wide bandwidth, differential track-and-hold
amplifier that can handle input frequencies up to 50 MHz.
The AD7938-6 features eight analog input channels with a
channel sequencer that allows a preprogrammed selection of
channels to be converted sequentially. The part can operate
with either single-ended, fully differential, or pseudo differential
analog inputs.
The conversion process and data acquisition are controlled
using standard control inputs that allow easy interfacing with
microprocessors and DSPs. The input signal is sampled on the
falling edge of CONVST and the conversion is initiated at
this point.
The AD7938-6 has an accurate on-chip 2.5 V reference that
can be used as the reference source for the analog-to-digital
conversion. Alternatively, this pin can be overdriven to provide
an external reference.
The AD7938-6 uses advanced design techniques to achieve very
low power dissipation at high throughput rates. The part also
features flexible power management options. An on-chip
control register allows the user to set up different operating
conditions, including analog input range and configuration,
output coding, power management, and channel sequencing.
PRODUCT HIGHLIGHTS
1. High throughput with low power consumption.
2. Eight analog inputs with a channel sequencer.
3. Accurate on-chip 2.5 V reference.
4. Single-ended, pseudo differential, or fully differential
analog inputs that are software selectable.
5. Single-supply operation with VDRIVE function. The VDRIVE
function allows the parallel interface to connect directly to
3 V or 5 V processor systems independent of VDD.
6. No pipeline delay.
7. Accurate control of the sampling instant via a CONVST
input and once-off conversion control.
Table 1.
Similar Device No. of Bits No. of Channels Speed
AD7938/AD7939 12/10 8 1.5 MSPS
AD7933/AD7934 10/12 4 1.5 MSPS
AD7934-6 12 4 625 kSPS
AD7938-6 Data Sheet
Rev. C | Page 2 of 32
TABLE OF CONTENTS
Features .............................................................................................. 1
Functional Block Diagram .............................................................. 1
General Description ......................................................................... 1
Product Highlights ....................................................................... 1
Revision History ............................................................................... 2
Specifications ..................................................................................... 3
Timing Specifications .................................................................. 5
Absolute Maximum Ratings ............................................................ 6
ESD Caution .................................................................................. 6
Pin Configuration and Function Descriptions ............................. 7
Typical Performance Characteristics ............................................. 9
Terminology .................................................................................... 11
On-Chip Registers .......................................................................... 13
Control Register .......................................................................... 13
Sequencer Operation ................................................................. 14
Shadow Register .......................................................................... 14
Circuit Information ........................................................................ 16
Converter Operation .................................................................. 16
ADC Transfer Function ............................................................. 16
Typical Connection Diagram ................................................... 17
Analog Input Structure .............................................................. 17
Analog Inputs ............................................................................. 18
Analog Input Selection .............................................................. 20
Reference ..................................................................................... 21
Parallel Interface ......................................................................... 23
Power Modes of Operation ....................................................... 26
Power vs. Throughput Rate ....................................................... 27
Microprocessor Interfacing ....................................................... 27
Application Hints ........................................................................... 29
Grounding and Layout .............................................................. 29
PCB Design Guidelines for Chip Scale Package .................... 29
Evaluating the AD7938-6 Performance .................................. 29
Outline Dimensions ....................................................................... 30
Ordering Guide .......................................................................... 31
REVISION HISTORY
10/11—Rev. B to Rev. C
Change to Features Section ............................................................. 1
10/11—Rev. A to Rev. B
Changes to Table 2 ............................................................................ 3
Changes to Figure 2 and Table 5 ..................................................... 7
Added Exposed Pad Notation to Outline Dimensions ............. 30
Changes to Ordering Guide .......................................................... 31
2/07—Rev. 0 to Rev. A
Changes to Specifications ................................................................ 3
Changes to Figure 13 ...................................................................... 10
Changes to Sequencer Operation Section ................................... 14
Changes to Analog Inputs Section ............................................... 18
Updated Outline Dimensions ....................................................... 30
10/04—Revision 0: Initial Version
Data Sheet AD7938-6
Rev. C | Page 3 of 32
SPECIFICATIONS
VDD = VDRIVE = 2.7 V to 5.25 V, internal/external VREF = 2.5 V, unless otherwise noted; fCLKIN = 10 MHz, fSAMPLE = 625 kSPS; TA = TMIN to
TMAX1, unless otherwise noted.
Table 2.
Parameter Value1Unit Test Conditions/Comments
DYNAMIC PERFORMANCE fIN = 50 kHz sine wave
Signal-to-Noise + Distortion (SINAD)269 dB min Differential mode
67 dB min Single-ended mode
Signal-to-Noise Ratio (SNR)2
71 dB min Differential mode
69 dB min Single-ended mode
Total Harmonic Distortion (THD)2
−73 dB max −85 dB typ, differential mode
−69.5 dB max −80 dB typ, single-ended mode
Peak Harmonic or Spurious Noise (SFDR)2
−72 dB max −82 dB typ
Intermodulation Distortion (IMD)2
fa = 30 kHz, fb = 50 kHz
Second-Order Terms −86 dB typ
Third-Order Terms −90 dB typ
Channel-to-Channel Isolation −85 dB typ fIN = 50 kHz, fNOISE = 300 kHz
Aperture Delay2
5 ns typ
Aperture Jitter2
72 ps typ
Full Power Bandwidth2
50 MHz typ @ 3 dB
10 MHz typ @ 0.1 dB
DC ACCURACY
Resolution 12 Bits
Integral Nonlinearity2
±1 LSB max Differential mode
±1.5 LSB max Single-ended mode
Differential Nonlinearity2
Differential Mode ±0.95 LSB max Guaranteed no missed codes to 12 bits
Single-Ended Mode −0.95/+1.5 LSB max Guaranteed no missed codes to 12 bits
Single-Ended and Pseudo Differential Input Straight binary output coding
Offset Error2
±12 LSB max
Offset Error Match2
±3 LSB max
Gain Error2
±3 LSB max
Gain Error Match2
±2 LSB max
Fully Differential Input Twos complement output coding
Positive Gain Error2
±3 LSB max
Positive Gain Error Match2
±1.5 LSB typ
Zero-Code Error2
±9.5 LSB max
Zero-Code Error Match2
±1 LSB typ
Negative Gain Error2
±3 LSB max
Negative Gain Error Match2
±1.5 LSB typ
ANALOG INPUT
Single-Ended Input Range 0 to VREF V RANGE bit = 0
0 to 2 × VREF V RANGE bit = 1
Pseudo Differential Input Range:
VIN+ 0 to VREF V RANGE bit = 0
0 to 2 × VREF V RANGE bit = 1
VIN− −0.3 to +0.7 V typ VDD = 3 V
−0.3 to +1.8 V typ VDD = 5 V
Fully Differential Input Range
VIN+ and VIN− V
CM ± VREF/2 V VCM = common-mode voltage3 = VREF/2
VIN+ and VIN− V
CM ± VREF V VCM = VREF, VIN+ or VIN− must remain within GND/VDD
AD7938-6 Data Sheet
Rev. C | Page 4 of 32
Parameter Value1Unit Test Conditions/Comments
DC Leakage Current4±1 μA max
Input Capacitance 45 pF typ When in track
10 pF typ When in hold
REFERENCE INPUT/OUTPUT
VREF Input Voltage52.5 V ±1% for specified performance
DC Leakage Current ±1 μA max
VREFOUT Output Voltage 2.5 V ±0.2% max @ 25°C
VREFOUT Temperature Coefficient 25 ppm/°C max
5 ppm/°C typ
VREF Noise 10 μV typ 0.1 Hz to 10 Hz bandwidth
130 μV typ 0.1 Hz to 1 MHz bandwidth
VREF Output Impedance 10 Ω typ
VREF Input Capacitance 15 pF typ When in track
25 pF typ When in hold
LOGIC INPUTS
Input High Voltage, VINH 2.4 V min
Input Low Voltage, VINL 0.8 V max
Input Current, IIN ±5 μA max Typically 10 nA, VIN = 0 V or VDRIVE
Input Capacitance, CIN4
10 pF max
LOGIC OUTPUTS
Output High Voltage, VOH 2.4 V min ISOURCE = 200 μA
Output Low Voltage, VOL 0.4 V max ISINK = 200 μA
Floating-State Leakage Current ±3 μA max
Floating-State Output Capacitance4
10 pF max
Output Coding Straight (Natural) Binary CODING bit = 0
Twos Complement CODING bit = 1
CONVERSION RATE
Conversion Time t2 + 13 tCLKIN ns
Track-and-Hold Acquisition Time 125 ns max Full-scale step input
80 ns typ Sine wave input
Throughput Rate 625 kSPS max
POWER REQUIREMENTS
VDD 2.7/5.25 V min/max
VDRIVE 2.7/5.25 V min/max
IDD6 Digital inputs = 0 V or VDRIVE
Normal Mode (Static) 0.8 mA typ VDD = 2.7 V to 5.25 V, SCLK on or off
Normal Mode (Operational) 1.5 mA max VDD = 4.75 V to 5.25 V
1.2 mA max VDD = 2.7 V to 3.6 V
Autostandby Mode 0.3 mA typ fSAMPLE = 100 kSPS, VDD = 5 V
160 μA typ Static
Full/Autoshutdown Mode (Static) 2 μA max SCLK on or off
Power Dissipation
Normal Mode (Operational) 7.5 mW max VDD = 5 V
3.6 mW max VDD = 3 V
Autostandby Mode (Static) 800 μW typ VDD = 5 V
480 μW typ VDD = 3 V
Full/Autoshutdown Mode (Static) 10 μW max VDD = 5 V
6 μW max VDD = 3 V
1 Temperature range is −40°C to +85°C.
2 See the Terminology section.
3 For full common-mode range, see Figure 25 and Figure 26.
4 Sample tested during initial release to ensure compliance.
5 This device is operational with an external reference in the range of 0.1 V to VDD. See the Reference section for more information.
6 Measured with a midscale dc analog input.
Data Sheet AD7938-6
Rev. C | Page 5 of 32
TIMING SPECIFICATIONS
VDD = VDRIVE = 2.7 V to 5.25 V, internal/external VREF = 2.5 V, unless otherwise noted; fCLKIN = 10MHz, fSAMPLE = 625 kSPS; TA = TMIN to
TMAX, unless otherwise noted.
Table 3.
Parameter1Limit at TMIN, TMAX Unit Description
fCLKIN2700 kHz min CLKIN frequency
10 MHz
max
tQUIET 30 ns min
Minimum time between end of read and start of next conversion, that is, time from when
the data bus goes into three-state until the next falling edge of CONVST.
t1 10 ns min CONVST pulse width.
t2 15 ns min CONVST falling edge to CLKIN falling edge setup time.
t3 50 ns max CLKIN falling edge to BUSY rising edge.
t4 0 ns min CS to WR setup time.
t5 0 ns min
CS to WR hold time.
t6 10 ns min
WR pulse width.
t7 10 ns min Data setup time before WR.
t8 10 ns min Data hold after WR.
t9 10 ns min New data valid before falling edge of BUSY.
t10 0 ns min CS to RD setup time.
t11 0 ns min CS to RD hold time.
t12 30 ns min RD pulse width.
t13330 ns max Data access time after RD.
t1443 ns min Bus relinquish time after RD.
50 ns max Bus relinquish time after RD.
t15 0 ns min HBEN to RD setup time.
t16 0 ns min HBEN to RD hold time.
t17 10 ns min Minimum time between reads/writes.
t18 0 ns min HBEN to WR setup time.
t19 10 ns min HBEN to WR hold time.
t20 40 ns max CLKIN falling edge to BUSY falling edge.
t21 15.7 ns min CLKIN low pulse width.
t22 7.8 ns min CLKIN high pulse width.
1 Sample tested during initial release to ensure compliance. All input signals are specified with tRISE = tFALL = 5 ns (10% to 90% of VDD) and timed from a voltage level of
1.6 V. All timing specifications given above are with a 25 pF load capacitance (see Figure 35, Figure 36, Figure 37, and Figure 38).
2 Minimum CLKIN for specified performance, with slower CLKIN frequencies performance specifications apply typically.
3 The time required for the output to cross 0.4 V or 2.4 V.
4 t14 is derived from the measured time taken by the data outputs to change 0.5 V. The measured number is then extrapolated back to remove the effects of charging or
discharging the 25 pF capacitor. This means that the time, t14, quoted in the timing characteristics is the true bus relinquish time of the part and is independent of the
bus loading.
AD7938-6 Data Sheet
Rev. C | Page 6 of 32
ABSOLUTE MAXIMUM RATINGS
TA = 25°C, unless otherwise noted.
Table 4.
Parameter Rating
VDD to AGND/DGND −0.3 V to +7 V
VDRIVE to AGND/DGND −0.3 V to VDD + 0.3 V
Analog Input Voltage to AGND −0.3 V to VDD + 0.3 V
Digital Input Voltage to DGND −0.3 V to +7 V
VDRIVE to VDD −0.3 V to VDD + 0.3 V
Digital Output Voltage to DGND −0.3 V to VDRIVE + 0.3 V
VREFIN to AGND −0.3 V to VDD + 0.3 V
AGND to DGND −0.3 V to +0.3 V
Input Current to Any Pin Except Supplies1±10 mA
Operating Temperature Range,
Commercial (B Version) −40°C to +85°C
Storage Temperature Range −65°C to +150°C
Junction Temperature 150°C
θJA Thermal Impedance 108.2°C/W (LFCSP)
121°C/W (TQFP)
θJC Thermal Impedance 32.71°C/W (LFCSP)
45°C/W (TQFP)
Lead Temperature, Soldering
Reflow Temperature (10 sec to 30 sec) 255°C
ESD 1.5 kV
1 Transient currents of up to 100 mA do not cause SCR latch-up.
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
ESD CAUTION
Data Sheet AD7938-6
Rev. C | Page 7 of 32
04751-006
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
DB0
1
V
DRIVE
9
DGND
10
DB8/HBEN
11
DB9
12
DB10
13
DB11
14
BUSY
15
CLKIN
16
W/B
32
V
DD
31
V
IN
7
30
V
IN
6
29
V
IN
5
28
V
IN
4
27
V
IN
3
26
V
IN
2
25
DB1
2
DB2
3
DB3
4
DB4
5
DB5
6
DB6
7
DB7
8
V
IN
1
24
V
IN
0
23
V
REFIN
/V
REFOUT
22
AGND
21
20
19
WR
18
CONVST
17
AD7938-6
TOP VIEW
(Not to Scale)
RD
CS
PIN 1
IDENTIFIER
NOTES
1. THE EXPOSED PAD IS LOCATED ON THE UNDERSIDE OF
THE PACKAGE. CONNECT THE EPAD TO THE GROUND PLANE
OF THE PCB USING MULTIPLE VIAS.
Figure 2. Pin Configuration
Table 5. Pin Function Description
Pin No. Mnemonic Description
1 to 8 DB0 to DB7 Data Bit 0 to Data Bit 7. Three-state parallel digital I/O pins that provide the conversion result and allow the
control and shadow registers to be programmed. These pins are controlled by CS, RD, and WR. The logic
high/low voltage levels for these pins are determined by the VDRIVE input.
9 VDRIVE Logic Power Supply Input. The voltage supplied at this pin determines at what voltage the parallel interface of
the AD7938-6 operates. This pin should be decoupled to DGND. The voltage at this pin may be different to that
at VDD but should never exceed VDD by more than 0.3 V.
10 DGND Digital Ground. This is the ground reference point for all digital circuitry on the AD7938-6. This pin should
connect to the DGND plane of a system. The DGND and AGND voltages should ideally be at the same potential
and must not be more than 0.3 V apart, even on a transient basis.
11 DB8/HBEN Data Bit 8/High Byte Enable. When W/B is high, this pin acts as Data Bit 8, a three-state I/O pin that is controlled
by CS, RD, and WR. When W/B is low, this pin acts as the high byte enable pin. When HBEN is low, the low byte of
data being written to or read from the AD7938-6 is on DB0 to DB7. When HBEN is high, the top four bits of the
data being written to or read from the AD7938-6 are on DB0 to DB3. When reading from the device, DB4 to DB6
of the high byte contains the ID of the channel to which the conversion result corresponds (see the channel
address bits in ). When writing to the device, DB4 to DB7 of the high byte must be all 0s. Table 9
12 to 14 DB9 to DB11 Data Bit 9 to Data Bit 11. Three-state parallel digital I/O pins that provide the conversion result and allow the
control and shadow registers to be programmed in word mode. These pins are controlled by CS, RD, and WR.
The logic high/low voltage levels for these pins are determined by the VDRIVE input.
15 BUSY Busy Output. Logic output indicating the status of the conversion. The BUSY output goes high following the
falling edge of CONVST and stays high for the duration of the conversion. Once the conversion is complete and
the result is available in the output register, the BUSY output goes low. The track-and-hold returns to track mode
just prior to the falling edge of BUSY on the 13th rising edge of CLKIN (see ). Figure 35
16 CLKIN Master Clock Input. The clock source for the conversion process is applied to this pin. Conversion time for the
AD7938-6 takes 13 clock cycles + t2. The frequency of the master clock input therefore determines the
conversion time and achievable throughput rate. The CLKIN signal may be a continuous or burst clock.
17 CONVST Conversion Start Input. A falling edge on CONVST is used to initiate a conversion. The track-and-hold goes from
track mode to hold mode on the falling edge of CONVST and the conversion process is initiated at this point.
Following power-down, when operating in autoshutdown or autostandby modes, a rising edge on CONVST is
used to power up the device.
18 WR Write Input. Active low logic input used in conjunction with CS to write data to the internal registers.
AD7938-6 Data Sheet
Rev. C | Page 8 of 32
Pin No. Mnemonic Description
19 RD Read Input. Active low logic input used in conjunction with CS to access the conversion result. The conversion
result is placed on the data bus following the falling edge of RD read while CS is low.
20 CS Chip Select. Active low logic input used in conjunction with RD and WR to read conversion data or to write data
to the internal registers.
21 AGND Analog Ground. This is the ground reference point for all analog circuitry on the AD7938-6. All analog
input signals and any external reference signal should be referred to this AGND voltage. The AGND and
DGND voltages should ideally be at the same potential and must not be more than 0.3 V apart, even on a
transient basis.
22 VREFIN/VREFOUT Reference Input/Output. This pin is connected to the internal reference and is the reference source for the
ADC. The nominal internal reference voltage is 2.5 V and this appears at this pin. It is recommended that this pin
be decoupled to AGND with a 470 nF capacitor. This pin can be overdriven by an external reference. The input
voltage range for the external reference is 0.1 V to VDD; however, care must be taken to ensure that the analog
input range does not exceed VDD + 0.3 V. See the Reference section.
23 to 30 VIN0 to VIN7 Analog Input 0 to Analog Input 7. Eight analog input channels that are multiplexed into the on-chip track-and-
hold. The analog inputs can be programmed to be eight single-ended inputs, four fully differential pairs, four
pseudo differential pairs, or seven pseudo differential inputs by setting the MODE bits in the control register
appropriately (see Table 9). The analog input channel to be converted can either be selected by writing to the
address bits (ADD2 to ADD0) in the control register prior to the conversion or the on-chip sequencer can be
used. The SEQ and SHDW bits in conjunction with the address bits in the control register allow the shadow
register to be programmed. The input range for all input channels can either be 0 V to VREF or 0 V to 2 × VREF, and
the coding can be binary or twos complement, depending on the states of the RANGE and CODING bits in the
control register. Any unused input channels should be connected to AGND to avoid noise pickup.
31 VDD Power Supply Input. The VDD range for the AD7938-6 is 2.7 V to 5.25 V. The supply should be decoupled to AGND
with a 0.1 μF capacitor and a 10 μF tantalum capacitor.
32 W/B Word/Byte Input. When this input is logic high, data is transferred to and from the AD7938-6 in 12-bit words on
Pin DB0 to Pin DB11. When this pin is logic low, byte transfer mode is enabled. Data and the channel ID are
transferred on Pin DB0 to Pin DB7, and Pin DB8/HBEN assumes its HBEN functionality. Unused data lines when
operating in byte transfer mode should be tied off to DGND.
EPAD Exposed Pad. The exposed pad is located on the underside of the package. Connect the EPAD to the ground
plane of the PCB using multiple vias.
Data Sheet AD7938-6
Rev. C | Page 9 of 32
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C, unless otherwise noted.
SUPPLY RIPPLE FREQUENCY (kHz)
PSRR (dB)
–
60
–70
–80
–90
–110
–100
–120
10 210 610410 810 1010
04751-007
100mV p-p SINE WAVE ON V
DD
AND/OR V
DRIVE
NO DECOUPLING
DIFFERENTIAL/SINGLE-ENDED MODE
INT REF
EXT REF
FREQUENCY (kHz)
0
–10
–20
–50
–40
–30
–90
–100
–80
–70
–60
–110
0
100
200
300
400
500
600
700
04751-009
4096 POINT FFT
VDD = 5V
FSAMPLE = 625kSPS
FIN = 49.62kHz
SINAD = 70.94dB
THD = –90.09dB
DIFFERENTIAL MODE
(dB)
Figure 3. PSRR vs. Supply Ripple Frequency Without Supply Decoupling
NOISE FREQUENCY (kHz)
NOISE ISOL
A
TION (dB)
–
70
–75
–90
–85
–80
–195
0 100 400200 300 600500 800700
04751-021
INTERNAL/EXTERNAL REFERENCE
V
DD
= 5V
Figure 4. AD7938-6 Channel-to-Channel Isolation
FREQUENCY (kHz)
SINAD (dB)
80
70
60
30
40
50
20
0 100 400200 300 600500 1000700 800 900
04751-008
F
SAMPLE
= 625kSPS
RANGE = 0 TO V
REF
DIFFERENTIAL MODE
V
DD
= 5V
V
DD
= 3V
Figure 5. AD7938-6 SINAD vs. Analog Input
Frequency for Various Supply Voltages
Figure 6. AD7938-6 FFT @ VDD = 5 V
CODE
DNL ERROR (LSB)
1.0
0.8
0.6
0.4
0.2
–0.2
0
–0.8
–0.6
–0.4
–1.0
0 500 20001000 1500 30002500 40003500
04751-010
VDD = 5V
DIFFERENTIAL MODE
Figure 7. AD7938-6 Typical DNL @ VDD = 5 V
CODE
INL ERROR (LSB)
1.0
0.8
0.6
0.4
0.2
–0.2
0
–0.8
–0.6
–0.4
–1.0
0 500 20001000 1500 30002500 40003500
04751-011
VDD = 5V
DIFFERENTIAL MODE
Figure 8. AD7938-6 Typical INL @ VDD = 5 V
AD7938-6 Data Sheet
Rev. C | Page 10 of 32
VREF (V)
DNL (LSB)
6
5
4
3
1
2
0
–1
0.25 0.50 1.250.75 1.00 2.001.751.50 2.752.502.25
04751-012
CODE
???
10000
9000
7000
8000
3000
4000
5000
6000
2000
1000
0
2046 2047 2048 2049 2050
04751-015
9997
CODES
INTERNAL
REF
DIFFERENTIAL MODE
SINGLE-ENDED MODE
POSITIVE DNL
3 CODES
NEGATIVE DNL
Figure 9. AD7938-6 DNL vs. VREF for VDD = 3 V
V (V)
EFFECTIVE NUMBER OF BITS
12
11
10
8
9
7
6
00.5 1.51.0 2.52.0 4.03.53.0
04751-013
REF
VDD = 5V
DIFFERENTIAL MODE
VDD = 5V
SINGLE-ENDED MODE
VDD = 3V
SINGLE-ENDED MODE
VDD = 3V
DIFFERENTIAL MODE
Figure 10. AD7938-6 ENOB vs. VREF
VREF (V)
–4.5
–5.0
00.5 1.51.0 2.52.0 3.53.0
04751-014
OFFSET (LSB)
0
–0.5
–1.5
–1.0
–3.5
–3.0
–2.5
–2.0
–4.0
SINGLE-ENDED MODE
VDD = 5V
VDD = 3V
Figure 11. AD7938-6 Offset vs. VREF
Figure 12. AD7938-6 Histogram of Codes for
10,000 Samples @ VDD = 5 V with Internal Reference
RIPPLE FREQUENCY (kHz)
CMRR (dB)
60
70
80
100
90
110
120
0 200 400 800600 12001000
04751-017
DIFFERENTIAL MODE
Figure 13. CMRR vs. Input Frequency with VDD = 5 V and 3 V
Data Sheet AD7938-6
Rev. C | Page 11 of 32
TERMINOLOGY
Integral Nonlinearity (INL)
This is the maximum deviation from a straight line passing
through the endpoints of the ADC transfer function. The
endpoints of the transfer function are zero scale, 1 LSB below
the first code transition, and full scale, 1 LSB above the last code
transition.
Differential Nonlinearity
This is the difference between the measured and the ideal 1 LSB
change between any two adjacent codes in the ADC.
Offset Error
This is the deviation of the first code transition (00…000) to
(00…001) from the ideal (that is, AGND + 1 LSB).
Offset Error Match
This is the difference in offset error between any two channels.
Gain Error
This is the deviation of the last code transition (111…110) to
(111…111) from the ideal (that is, VREF – 1 LSB) after the offset
error has been adjusted out.
Gain Error Match
This is the difference in gain error between any two channels.
Zero-Code Error
This applies when using the twos complement output coding
option, in particular to the 2 × VREF input range with −VREF to
+VREF biased about the VREFIN point. It is the deviation of the
midscale transition (all 0s to all 1s) from the ideal VIN voltage,
(that is, VREF).
Zero-Code Error Match
This is the difference in zero-code error between any two channels.
Positive Gain Error
This applies when using the twos complement output coding
option, in particular to the 2 × VREF input range with −VREF to
+VREF biased about the VREFIN point. It is the deviation of the last
code transition (011…110) to (011…111) from the ideal (that
is, +VREF − 1 LSB) after the zero-code error has been adjusted out.
Positive Gain Error Match
This is the difference in positive gain error between any
two channels.
Negative Gain Error
This applies when using the twos complement output coding
option, in particular to the 2 × VREF input range with −VREF to
+VREF biased about the VREF point. It is the deviation of the first
code transition (100…000) to (100…001) from the ideal
(that is, −VREF + 1 LSB) after the zero-code error has been
adjusted out.
Negative Gain Error Match
This is the difference in negative gain error between any
two channels.
Channel-to-Channel Isolation
Channel-to-channel isolation is a measure of the level of
crosstalk between channels. It is measured by applying a full-
scale sine wave signal to all seven nonselected input channels
and applying a 50 kHz signal to the selected channel. The
channel-to-channel isolation is defined as the ratio of the power
of the 50 kHz signal on the selected channel to the power of the
noise signal on the unselected channels that appears in the FFT
of this channel. The noise frequency on the unselected channels
varies from 40 kHz to 740 kHz. The noise amplitude is at 2 × VREF,
while the signal amplitude is at 1 × VREF. See Figure 4.
Power Supply Rejection Ratio (PSRR)
PSRR is defined as the ratio of the power in the ADC output at
full-scale frequency, f, to the power of a 100 mV p-p sine wave
applied to the ADC VDD supply of frequency, fS. The frequency
of the noise varies from 1 kHz to 1 MHz.
PSRR (dB) = 10 log(Pf/PfS)
where:
Pf is the power at frequency f in the ADC output.
PfS is the power at frequency fS in the ADC output.
Common-Mode Rejection Ratio (CMRR)
CMRR is defined as the ratio of the power in the ADC output at
full-scale frequency, f, to the power of a 100 mV p-p sine wave
applied to the common-mode voltage of VIN+ and VIN− of
frequency, fS.
CMRR (dB) = 10 log(Pf/PfS)
where:
Pf is the power at frequency f in the ADC output.
PfS is the power at frequency fS in the ADC output.
AD7938-6 Data Sheet
Rev. C | Page 12 of 32
()
Track-and-Hold Acquisition Time
The track-and-hold amplifier returns to track mode at the end
of conversion. The track-and-hold acquisition time is the time
required for the output of the track-and-hold amplifier to reach
its final value, within ±1/2 LSB, after the end of conversion.
Signal to Noise and Distortion Ratio (SINAD)
This is the measured ratio of signal-to-noise and distortion at
the output of the ADC. The signal is the rms amplitude of the
fundamental. Noise is the sum of all nonfundamental signals up
to half the sampling frequency (fSAMPLE/2), excluding dc. The
ratio is dependent on the number of quantization levels in the
digitization process; the more levels, the smaller the
quantization noise.
The theoretical SINAD ratio for an ideal N-bit converter with a
sine wave input is given by
SINAD = (6.02 N + 1.76) dB
Thus, for a 12-bit converter, SINAD is 74 dB.
Total Harmonic Distortion (THD)
THD is the ratio of the rms sum of harmonics to the
fundamental. For the AD7938-6, it is defined as:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛++++
−=
1
65432
V
VVVVV
THD
22222
log20dB
where:
V1 is the rms amplitude of the fundamental.
V2, V3, V4, V5, and V6 are the rms amplitudes of the second
through the sixth harmonics.
Peak Harmonic or Spurious Noise
Peak harmonic or spurious noise is defined as the ratio of the
rms value of the next largest component in the ADC output
spectrum (up to fSAMPLE/2 and excluding dc) to the rms value of
the fundamental. Normally, the value of this specification is
determined by the largest harmonic in the spectrum, but for
ADCs where the harmonics are buried in the noise floor, it is a
noise peak.
Intermodulation Distortion
With inputs consisting of sine waves at two frequencies, fa and
fb, any active device with nonlinearities creates distortion
products at sum and difference frequencies of mfa ± nfb where
m, n = 0, 1, 2, 3, and so on. Intermodulation distortion terms
are those for which neither m nor n are equal to 0. For example,
the second-order terms include (fa + fb) and (fa − fb), and the
third-order terms include (2fa + fb), (2fa − fb), (fa + 2fb) and
(fa − 2fb).
The AD7938-6 is tested using the CCIF standard where two
input frequencies near the top end of the input bandwidth are
used. In this case, the second-order terms are usually distanced
in frequency from the original sine waves while the third-order
terms are usually at a frequency close to the input frequencies.
As a result, the second-order and third-order terms are
specified separately. The intermodulation distortion is
calculated per the THD specification, as the ratio of the rms
sum of the individual distortion products to the rms amplitude
of the sum of the fundamentals, expressed in dB.
Data Sheet AD7938-6
Rev. C | Page 13 of 32
ON-CHIP REGISTERS
The AD7938-6 has two on-chip registers that are necessary for
the operation of the device. These are the control register, which
is used to set up different operating conditions, and the shadow
register, which is used to program the analog input channels to
be converted.
CONTROL REGISTER
The control register on the AD7938-6 is a 12-bit, write-only
register. Data is written to this register using the CS and WR
pins. The control register is shown in and the functions
of the bits are described in . At power-up, the default bit
settings in the control register are all 0s. When writing to the
control register between conversions, ensure that
Table 6
Table 7
CONVST
returns high before the write is performed.
Table 6. Control Register Bits
MSB LSB
DB11 DB10 DB9 DB8 DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0
PM1 PM0 CODING REF ADD2 ADD1 ADD0 MODE1 MODE0 SHDW SEQ RANGE
Table 7. Control Register Bit Function Description
Bit No. Mnemonic Description
11, 10 PM1, PM0 Power Management Bits. These two bits are used to select the power mode of operation. The user can choose
between either normal mode or various power-down modes of operation as shown in Table 8.
9 CODING This bit selects the output coding of the conversion result. If this bit is set to 0, the output coding is straight
(natural) binary. If this bit is set to 1, the output coding is twos complement.
8 REF This bit selects whether the internal or external reference is used to perform the conversion. If this bit is Logic 0, an
external reference should be applied to the VREF pin. If this bit is Logic 1, the internal reference is selected. See the
Reference section.
7 to 5 ADD2 to
ADD0
These three address bits are used to either select which analog input channel is converted in the next conversion if
the sequencer is not used, or to select the final channel in a consecutive sequence when the sequencer is used as
described in Table 10. The selected input channel is decoded as shown in Table 9.
4, 3 MODE1,
MODE0
The two mode pins select the type of analog input on the eight VIN pins. The AD7938-6 can have either eight single-
ended inputs, four fully differential inputs, four pseudo differential inputs, or seven pseudo differential inputs. See
Table 9.
2 SHDW The SHDW bit in the control register is used in conjunction with the SEQ bit to control the sequencer function and
access the SHDW register. See Table 10.
1 SEQ The SEQ bit in the control register is used in conjunction with the SHDW bit to control the sequencer function and
access the SHDW register. See Table 10.
0 RANGE This bit selects the analog input range of the AD7938-6. If it is set to 0, then the analog input range extends from
0 V to VREF. If it is set to 1, the analog input range extends from 0 V to 2 × VREF. When this range is selected, VDD must
be 4.75 V to 5.25 V if a 2.5 V reference is used; otherwise, care must be taken to ensure that the analog input remains
within the supply rails. See the Analog Inputs section for more information.
Table 8. Power Mode Selection using the Power Management Bits in the Control Register
PM1 PM0 Mode Description
0 0 Normal Mode When operating in normal mode, all circuitry is fully powered up at all times.
0 1 Autoshutdown
When operating in autoshutdown mode, the AD7938-6 enters full shutdown mode at the end of each
conversion. In this mode, all circuitry is powered down.
1 0 Autostandby
When the AD7938-6 enters this mode, all circuitry is powered down except for the reference and reference
buffer. This mode is similar to autoshutdown mode, but it allows the part to power up in 7 μs (or 600 ns if
an external reference is used). See the Power Modes of Operation section for more information.
1 1 Full Shutdown
When the AD7938-6 enters this mode, all circuitry is powered down. The information in the control register
is retained.
AD7938-6 Data Sheet
Rev. C | Page 14 of 32
Table 9. Analog Input Type Selection
Channel Address
MODE0 = 0, MODE1 = 0 MODE0 = 0, MODE1 = 1 MODE0 = 1, MODE1 = 0 MODE0 = 1, MODE1 = 1
Eight Single-Ended
Input Channels
Four Fully Differential
Input Channels
Four Pseudo Differential Input
Channels (Pseudo Mode 1)
Seven Pseudo Differential
Input Channels (Pseudo Mode 2)
ADD2 ADD1 ADD0 VIN+ V
IN− V
IN+ V
IN− V
IN+ V
IN− V
IN+ V
IN−
0 0 0 VIN0 AGND VIN0 VIN1 VIN0 VIN1 VIN0 VIN7
0 0 1 VIN1 AGND VIN1 VIN0 VIN1 VIN0 VIN1 VIN7
0 1 0 VIN2 AGND VIN2 VIN3 VIN2 VIN3 VIN2 VIN7
0 1 1 VIN3 AGND VIN3 VIN2 VIN3 VIN2 VIN3 VIN7
1 0 0 VIN4 AGND VIN4 VIN5 VIN4 VIN5 VIN4 VIN7
1 0 1 VIN5 AGND VIN5 VIN4 VIN5 VIN4 VIN5 VIN7
1 1 0 VIN6 AGND VIN6 VIN7 VIN6 VIN7 VIN6 VIN7
1 1 1 VIN7 AGND VIN7 VIN6 VIN7 VIN6 Not Allowed Not Allowed
SEQUENCER OPERATION
The configuration of the SEQ and SHDW bits in the control
register allows the user to select a particular mode of operation
of the sequencer function. Tabl e 10 outlines the four modes of
operation of the sequencer.
Writing to the Control Register to Program the
Sequencer
The AD7938-6 needs 13 full CLKIN periods to perform a
conversion. If the ADC does not receive the full 13 CLKIN
periods, the conversion aborts. If a conversion is aborted after
applying 12.5 CLKIN periods to the ADC, ensure that a rising
edge of CONVST or a falling edge of CLKIN is applied to the
part before writing to the control register to program the
sequencer. If these conditions are not met, the sequencer will
not be in the correct state to handle being reprogrammed for
another sequence of conversions and the performance of the
converter is not guaranteed.
SHADOW REGISTER
The shadow register on the AD7938-6 is an 8-bit, write-only
register. Data is loaded from DB0 to DB7 on the rising edge of
WR. The eight LSBs load into the shadow register. The
information is written into the shadow register provided that
the SEQ and SHDW bits in the control register were set to 0 and
1, respectively, in the previous write to the control register. Each
bit represents an analog input from Channel 0 through Channel 7.
A sequence of channels can be selected through which the
AD7938-6 cycles with each consecutive conversion after the
write to the shadow register.
To select a sequence of channels to be converted, if operating in
single-ended mode or Pseudo Mode 2, the associated channel
bit in the shadow register must be set for each required analog
input. When operating in fully differential mode or Pseudo
Mode 1, the associated pair of channel bits must be set for each
pair of analog inputs required in the sequence.
With each consecutive CONVST pulse after the sequencer has
been set up, the AD7938-6 progresses through the selected
channels in ascending order, beginning with the lowest channel.
This continues until a write operation occurs with the SEQ and
SHDW bits configured in any way except 1, 0 (see ).
When a sequence is set up in differential mode or Pseudo
Mode 1, the ADC does not convert on the inverse pairs (that is,
VIN1, VIN0). The bit functions of the shadow register are
outlined in . See the section for
further information on using the sequencer.
Table 10
Table 11 Analog Input Selection
Data Sheet AD7938-6
Rev. C | Page 15 of 32
Table 10. Sequence Selection
SEQ SHDW Sequence Type
0 0 This configuration is selected when the sequence function is not used. The analog input channel selected on each
individual conversion is determined by the contents of the channel address bits, ADD2 to ADD0, in each prior write
operation. This mode of operation reflects the traditional operation of a multichannel ADC, without the sequencer function
being used, where each write to the AD7938-6 selects the next channel for conversion.
0 1 This configuration selects the shadow register for programming. The following write operation loads the data on DB0 to
DB7 to the shadow register. This programs the sequence of channels to be converted continuously after each CONVST
falling edge (see the section and ). Shadow Register Table 11
1 0 If the SEQ and SHDW bits are set in this way, the sequence function is not interrupted upon completion of the write
operation. This allows other bits in the control register to be altered between conversions while in a sequence without
terminating the cycle.
1 1 This configuration is used in conjunction with the channel address bits (ADD2 to ADD0) to program continuous
conversions on a consecutive sequence of channels from Channel 0 through to a selected final channel as determined by
the channel address bits in the control register.
Table 11. Shadow Register Bit Functions
MSB LSB
DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0
VIN7 VIN6 VIN5 VIN4 VIN3 VIN2 VIN1 VIN0
AD7938-6 Data Sheet
Rev. C | Page 16 of 32
CIRCUIT INFORMATION
The AD7938-6 is a fast, 8-channel, 12-bit, single-supply,
successive approximation analog-to-digital converter. The part
can operate from a 2.7 V to 5.25 V power supply and features
throughput rates up to 625 kSPS.
The AD7938-6 provides the user with an on-chip track-and-hold,
an accurate internal reference, an analog-to-digital converter, and
a parallel interface housed in a 32-lead LFCSP or TQFP package.
The AD7938-6 has eight analog input channels that can be
configured to be eight single-ended inputs, four fully differential
pairs, four pseudo differential pairs, or seven pseudo differential
inputs with respect to one common input. There is an on-chip
user-programmable channel sequencer that allows the user to select
a sequence of channels through which the ADC can progress and
cycle with each consecutive falling edge of CONVST.
The analog input range for the AD7938-6 is 0 V to VREF or 0 V to
2 × VREF depending on the status of the RANGE bit in the control
register. The output coding of the ADC can be either straight
binary or twos complement, depending on the status of the
CODING bit in the control register.
The AD7938-6 provides flexible power management options to
allow the user to achieve the best power performance for a given
throughput rate. These options are selected by programming the
power management bits, PM1 and PM0, in the control register.
CONVERTER OPERATION
The AD7938-6 is a successive approximation ADC based
around two capacitive digital-to-analog converters. Figure 14
and Figure 15 show simplified schematics of the ADC in
acquisition and conversion phase, respectively. The ADC
comprises control logic, an SAR, and two capacitive digital-to-
analog converters. Both figures show the operation of the ADC
in differential/pseudo differential mode. Single-ended mode
operation is similar but VIN− is internally tied to AGND. In
acquisition phase, SW3 is closed, SW1 and SW2 are in Position
A, the comparator is held in a balanced condition and the
sampling capacitor arrays acquire the differential signal on the
input.
04751-023
V
IN+
V
IN–
A
B
SW1
SW3
COMPARATOR
CONTROL
LOGIC
CAPACITIVE
DAC
CAPACITIVE
DAC
C
S
C
S
V
REF
SW2
B
A
Figure 14. ADC Acquisition Phase
When the ADC starts a conversion (see Figure 15), SW3 opens
and SW1 and SW2 move to Position B, causing the comparator
to become unbalanced. Both inputs are disconnected once the
conversion begins. The control logic and the charge
redistribution DACs are used to add and subtract fixed amounts
of charge from the sampling capacitor arrays to bring the
comparator back into a balanced condition. When the comparator
is rebalanced, the conversion is complete. The control logic
generates the output code of the ADC. The output impedances
of the sources driving the VIN+ and the VIN− pins must match;
otherwise, the two inputs have different settling times, resulting
in errors.
04751-024
V
IN+
V
IN–
A
B
SW1
SW3
COMPARATOR
CONTROL
LOGIC
CAPACITIVE
DAC
C
S
C
S
V
REF
SW2
B
A
CAPACITIVE
DAC
Figure 15. ADC Conversion Phase
ADC TRANSFER FUNCTION
The output coding for the AD7938-6 is either straight binary or
twos complement, depending on the status of the CODING bit
in the control register. The designed code transitions occur at
successive LSB values (that is, 1 LSB, 2 LSBs, and so on) and the
LSB size is VREF/4096. The ideal transfer characteristics of the
AD7938-6 for both straight binary and twos complement
output coding are shown in Figure 16 and Figure 17,
respectively.
04751-025
000...000
111...111
1 LSB = V
REF
/4096
1 LSB +V
REF
– 1 LSB
ANALOG INPUT
ADC CODE
0V
NOTE: V
REF
IS EITHER V
REF
OR 2 × V
REF
000...001
000...010
111...110
111...000
011...111
Figure 16. AD7938-6 Ideal Transfer Characteristic
with Straight Binary Output Coding
Data Sheet AD7938-6
Rev. C | Page 17 of 32
04751-026
100...000
011...111
1 LSB = 2 × V
REF
/4096
ADC CODE
–V
REF
+ 1 LSB V
REF
+V
REF
– 1 LSB
100...001
100...010
011...110
000...001
000...000
111...111
Figure 17. AD7938-6 Ideal Transfer Characteristic
with Twos Complement Output Coding and 2 × VREF Range
TYPICAL CONNECTION DIAGRAM
Figure 18 shows a typical connection diagram for the
AD7938-6. The AGND and DGND pins are connected together
at the device for good noise suppression. The VREFIN/VREFOUT pin
is decoupled to AGND with a 0.47 μF capacitor to avoid noise
pickup if the internal reference is used. Alternatively, VREFIN/VREFOUT
can be connected to an external reference source. In this case,
the reference pin should be decoupled with a 0.1 μF capacitor.
In both cases, the analog input range can either be 0 V to VREF
(RANGE bit = 0) or 0 V to 2 × VREF (RANGE bit = 1). The
analog input configuration can be either eight single-ended
inputs, four differential pairs, four pseudo differential pairs, or
seven pseudo differential inputs (see Table 9). The VDD pin is
connected to either a 3 V or 5 V supply. The voltage applied to
the VDRIVE input controls the voltage of the digital interface and
here, it is connected to the same 3 V supply of the microprocessor
to allow a 3 V logic interface (see the Digital Inputs section).
04751-027
0.1µF 10µF
3V/5
V
SUPPLY
MICROCONTROLLER/
MICROPROCESSOR
AD7938-6
0.1µF
0TO V
REF
/
3V
SUPPLY
0.1µF EXTERNAL V
REF
0.47µF INTERNAL V
REF
0
TO 2 × V
REF
AGND
DGND
W/B
CLKIN
CS
V
DRIVE
V
IN
0
V
DD
V
REFIN
/V
REFOUT
V
IN
7
10µF
2.5V
V
REF
RD
CONVST
WR
BUSY
DB0
DB11/DB9
Figure 18. Typical Connection Diagram
ANALOG INPUT STRUCTURE
Figure 19 shows the equivalent circuit of the analog input
structure of the AD7938-6 in differential/pseudo differential
mode. In single-ended mode, VIN− is internally tied to AGND.
The four diodes provide ESD protection for the analog inputs.
Care must be taken to ensure that the analog input signals never
exceed the supply rails by more than 300 mV. This causes these
diodes to become forward-biased and start conducting into the
substrate. These diodes can conduct up to 10 mA without
causing irreversible damage to the part.
The C1 capacitors in Figure 19 are typically 4 pF and can
primarily be attributed to pin capacitance. The resistors are
lumped components made up of the on resistance of the
switches. The value of these resistors is typically about 100 Ω.
The C2 capacitors are the ADC sampling capacitors and
typically have a capacitance of 45 pF.
For ac applications, removing high frequency components from
the analog input signal is recommended by the use of an RC
low-pass filter on the relevant analog input pins. In applications
where harmonic distortion and signal-to-noise ratio are critical,
the analog input should be driven from a low impedance
source. Large source impedances significantly affect the ac
performance of the ADC. This may necessitate the use of an
input buffer amplifier. The choice of the op amp is a function of
the particular application.
V
R1 C2
VIN+
DD
C1
D
D
04751-028
R1 C2
VIN–
VDD
C1
D
D
Figure 19. Equivalent Analog Input Circuit,
Conversion Phase: Switches Open, Track Phase: Switches Closed
When no amplifier is used to drive the analog input, the source
impedance should be limited to low values. The maximum
source impedance depends on the amount of THD that can be
tolerated. The THD increases as the source impedance increases
and performance degrades. Figure 20 and Figure 21 show a
graph of the THD vs. source impedance with a 50 kHz input
tone for both VDD = 5 V and VDD = 3 V in single-ended mode
and differential mode, respectively.
AD7938-6 Data Sheet
Rev. C | Page 18 of 32
–
R
SOURCE
(Ω)
40
–45
–50
–55
–80
–75
–70
–65
–60
–85
751-018
–90
10 100 1k 10k
04
THD (dB)
F
IN
= 50kHz
V
DD
= 5V
V
DD
= 3V
Figure 20. THD vs. Source Impedance in Single-Ended Mode
R
SOURCE
(Ω)
THD (dB)
–
60
–80
–75
–70
–65
–100
–85
–90
–95
10 100 1k 10k
04751-019
F
IN
= 50kHz
V
DD
= 5V
V
DD
= 3V
Figure 21. THD vs. Source Impedance in Differential Mode
Figure 22 shows a graph of the THD vs. the analog input
frequency for various supplies while sampling at 625 kHz with
an SCLK of 10 MHz. In this case, the source impedance is 10 Ω.
INPUT FREQUENCY (kHz)
THD (dB)
–
50
–60
–70
–80
–110
–100
–90
–120
0 100 400200 300 600500 700
04751-020
F
SAMPLE
= 625kSPS
RANGE = 0 TO V
REF
V
DD
= 3V
SINGLE-ENDED MODE
V
DD
= 5V/3V
DIFFERENTIAL MODE
V
DD
= 5V
SINGLE-ENDED MODE
Figure 22. THD vs. Analog Input Frequency for Various Supply Voltages
ANALOG INPUTS
The AD7938-6 has software-selectable analog input
configurations. The user can choose either eight single-ended
inputs, four fully differential pairs, four pseudo differential
pairs, or seven pseudo differential inputs. The analog input
configuration is chosen by setting the MODE0/MODE1 bits in
the internal control register (see Table 9 ).
Single-Ended Mode
The AD7938-6 can have eight single-ended analog input
channels by setting the MODE0 and MODE1 bits in the control
register to 0. In applications where the signal source has a high
impedance, it is recommended to buffer the analog input before
applying it to the ADC. An op amp suitable for this function is
the AD8021. The analog input range can be programmed to be
either 0 V to VREF or 0 V to 2 × VREF.
If the analog input signal to be sampled is bipolar, the internal
reference of the ADC can be used to externally bias up this
signal to make it the correct format for the ADC.
Figure 23 shows a typical connection diagram when operating
the ADC in single-ended mode. This diagram shows a bipolar
signal of amplitude ±1.25 V being preconditioned before it is
applied to the AD7938-6. In cases where the analog input
amplitude is ±2.5 V, the 3R resistor can be replaced with a
resistor of value R. The resultant voltage on the analog input of
the AD7938-6 is a signal ranging from 0 V to 5 V. In this case,
the 2 × VREF mode can be used.
0.47µF
+1.25V
V
IN
R
R
3R
0V
–1.25V
+2.5
V
0V
V
IN0
V
IN7
V
REFOUT
AD7938-6*
*ADDITIONAL PINS OMITTED FOR CLARITY.
04751-031
Figure 23. Single-Ended Mode Connection Diagram
Differential Mode
The AD7938-6 can have four fully differential analog input
pairs by setting the MODE0 and MODE1 bits in the control
register to 0 and 1, respectively.
Differential signals have some benefits over single-ended
signals, including noise immunity based on the device’s
common-mode rejection and improvements in distortion
performance. Figure 24 defines the fully differential analog
input of the AD7938-6.
Data Sheet AD7938-6
Rev. C | Page 19 of 32
04751-032
*ADDITIONAL PINS OMITTED FOR CLARITY.
V
REF
p-p V
IN+
V
IN–
V
REF
p-p
AD7938-6*
V
REF
(V)
COMMON-MODE RANGE (V)
4.5
4.0
3.0
1.5
2.0
2.5
3.5
1.0
0.5
0
0.1 0.6 1.61.1 2.1 2.6
04751-034
T
A
= 25°C
COMMON-MODE
VOLTAGE
Figure 24. Differential Input Definition
The amplitude of the differential signal is the difference between
the signals applied to the VIN+ and VIN− pins in each differential
pair (that is, VIN+ − VIN−). VIN+ and VIN− should be simultaneously
driven by two signals each of amplitude VREF (or 2 × VREF,
depending on the range chosen) that are 180° out of phase. The
amplitude of the differential signal is therefore −VREF to +VREF
peak-to-peak (that is, 2 × VREF). This is regardless of the
common mode (CM). The common mode is the average of the
two signals (that is, (VIN+ + VIN−)/2) and is therefore the voltage
on which the two inputs are centered. This results in the span of
each input being CM ± VREF/2. This voltage has to be set up
externally and its range varies with the reference value VREF.
As the value of VREF increases, the common-mode range
decreases. When driving the inputs with an amplifier, the
actual common-mode range is determined by the amplifier’s
output voltage swing.
Figure 25 and Figure 26 show how the common-mode range
typically varies with VREF for a 5 V power supply using the 0 V
to VREF range or 2 × VREF range, respectively. The common
mode must be in this range to guarantee the functionality of the
AD7938-6.
When a conversion takes place, the common mode is rejected,
resulting in a virtually noise-free signal of amplitude −VREF to
+VREF corresponding to the digital codes of 0 to 4096. If the
2 × VREF range is used, the input signal amplitude extends from
−2 VREF to +2 VREF after conversion.
V
REF
(V)
0
00.5 1.51.0 2.0 2.5 3.0
04751-03
COMMON-MODE RANGE (V)
3.5
3.0
2.0
1.5
2.5
1.0
0.5
3
T
A
= 25°C
Figure 25. Input Common-Mode Range vs. VREF (0 V to VREF Range, VDD = 5 V)
Figure 26. Input Common-Mode Range vs. VREF (2 × VREF Range, VDD = 5 V)
Driving Differential Inputs
Differential operation requires that VIN+ and VIN− be
simultaneously driven with two equal signals that are 180° out
of phase. The common mode must be set up externally and has
a range that is determined by VREF, the power supply, and the
particular amplifier used to drive the analog inputs. Differential
modes of operation with either an ac or dc input provide the
best THD performance over a wide frequency range. Since not
all applications have a signal preconditioned for differential
operation, there is often a need to perform single-ended-to-
differential conversion.
Using an Op Amp Pair
An op amp pair can be used to directly couple a differential
signal to one of the analog input pairs of the AD7938-6. The
circuit configurations shown in Figure 27 and Figure 28 show
how a dual op amp can be used to convert a single-ended signal
into a differential signal for both a bipolar and unipolar input
signal, respectively.
The voltage applied to Point A sets up the common-mode
voltage. In both diagrams, it is connected in some way to the
reference, but any value in the common-mode range can be
input here to set up the common mode. A suitable dual op amp
that can be used in this configuration to provide differential
drive to the AD7938-6 is the AD8022.
It is advisable to take care when choosing the op amp; the
selection depends on the required power supply and system
performance objectives. The driver circuits in Figure 27 and
Figure 28 are optimized for dc coupling applications requiring
best distortion performance.
The differential op amp driver circuit in Figure 27 is configured
to convert and level shift a single-ended, ground-referenced
(bipolar) signal to a differential signal centered at the VREF level
of the ADC.
The circuit configuration shown in Figure 28 converts a
unipolar, single-ended signal into a differential signal.
AD7938-6 Data Sheet
220Ω
2 × V
REF
p-p
GND
440Ω
220
Ω
Rev. C | Page 20 of 32
10kΩ
220Ω
20kΩ
220Ω
27Ω
27Ω
V+
V–
V+
V–
A
V
IN+
V
IN–
V
REF
3.75V
2.5V
1.25V
AD7938-6
0.47µF
04751-035
3.75V
2.5V
1.25V
Figure 27. Dual Op Amp Circuit to Convert a Single-Ended
Bipolar Signal into a Differential Unipolar Signal
10kΩ
V
REF
p-p
V
REF
GND
440Ω
220
Ω
220Ω
20kΩ
220Ω
27Ω
27Ω
V+
V–
V+
V–
A
V
IN+
V
IN–
V
REF
AD7938-6
0.47µF
04751-036
3.75V
2.5V
1.25V
3.75V
2.5V
1.25V
Figure 28. Dual Op Amp Circuit to Convert a Single-Ended
Unipolar Signal into a Differential Signal
Another method of driving the AD7938-6 is to use the AD8138
(or equivalent) differential amplifier. The AD8138 can be used
as a single-ended-to-differential amplifier or as a differential-to-
differential amplifier. The device is as easy to use as an op amp
and greatly simplifies differential signal amplification and driving.
Pseudo Differential Mode
The AD7938-6 can have four pseudo differential pairs (Pseudo
Mode 1) or seven pseudo differential inputs (Pseudo Mode 2)
by setting the MODE0 and MODE1 bits in the control register
to 1, 0 and 1, 1, respectively. In the case of the four pseudo
differential pairs, VIN+ is connected to the signal source, which
must have an amplitude of VREF (or 2 × VREF depending on the
range chosen) to make use of the full dynamic range of the part.
A dc input is applied to the VIN− pin. The voltage applied to this
input provides an offset from ground or a pseudo ground for
the VIN+ input. In the case of the seven pseudo differential
inputs, the seven analog input signals inputs are referred to a dc
voltage applied to VIN7.
The benefit of pseudo differential inputs is that they separate
the analog input signal ground from the ADC ground allowing
dc common-mode voltages to be cancelled. The specified
voltage range for the VIN− pin while in pseudo differential mode
is −0.1 V to +0.4 V; however, typically this range can extend to
−0.3 V to +0.7 V when VDD = 3 V, or −0.3 V to +1.8 V when
VDD = 5 V. Figure 29 shows a connection diagram for pseudo
differential mode.
VIN+
VIN–
VREF
AD7938-6*
*ADDITIONAL PINS OMITTED FOR CLARITY.
04751-037
VREF p-p
DC INPUT
VOLTAGE 0.47µF
Figure 29. Pseudo Differential Mode Connection Diagram
ANALOG INPUT SELECTION
As shown in Tabl e 9, the user can set up their analog input
configuration by setting the values in the MODE0 and MODE1
bits in the control register. Assuming the configuration has been
chosen, there are different ways of selecting the analog input to
be converted depending on the state of the SEQ and SHDW bits
in the control register.
Traditional Multichannel Operation (SEQ = 0, SHDW = 0)
Any one of eight analog input channels or four pairs of channels
can be selected for conversion in any order by setting the SEQ
and SHDW bits in the control register to 0. The channel to be
converted is selected by writing to the address bits, ADD2 to
ADD0, in the control register to program the multiplexer prior
to the conversion. This mode of operation is that of a traditional
multichannel ADC where each data write selects the next
channel for conversion. Figure 30 shows a flow chart of this
mode of operation. The channel configurations are shown in
Table 9 .
POWER ON
WRITE TO THE CONTROL REGISTER TO
SET UP OPERATING MODE, ANALOG INPUT
AND OUTPUT CONFIGURATION
SET SEQ = SHDW = 0. SELECT THE DESIRED
CHANNEL TO CONVERT (ADD2 TO ADD0).
ISSUE CONVST PULSE TO INITIATE A CONVERSION
ON THE SELECTED CHANNEL.
INITIATE A READ CYCLE TO READ THE DATA
FROM THE SELECTED CHANNEL.
INITIATE A WRITE CYCLE TO SELECT THE NEXT
CHANNEL TO BE CONVERTED BY
CHANGING THE VALUES OF BITS ADD2 TO ADD0
IN THE CONTROL REGISTER. SEQ = SHDW = 0.
04751-038
Figure 30. Traditional Multichannel Operation Flow Chart
Data Sheet AD7938-6
Rev. C | Page 21 of 32
Using the Sequencer: Programmable Sequence
(SEQ = 0, SHDW = 1 )
The AD7938-6 can be configured to automatically cycle through a
number of selected channels using the on-chip programmable
sequencer by setting SEQ = 0 and SHDW = 1 in the control
register. The analog input channels to be converted are selected
by setting the relevant bits in the shadow register to 1 (see
Table 11).
Once the shadow register has been programmed with the required
sequence, the next conversion executed is on the lowest channel
programmed in the SHDW register. The next conversion
executed is on the next highest channel in the sequence, and so
on. When the last channel in the sequence is converted, the
internal multiplexer returns to the first channel selected in the
shadow register and commences the sequence again.
It is not necessary to write to the control register again once a
sequencer operation has been initiated. The WR input must be
kept high to ensure that the control register is not accidentally
overwritten or that a sequence operation is not interrupted.
If the control register is written to at any time during the
sequence, ensure that the SEQ and SHDW bits are set
to 1, 0 to avoid interrupting the conversion sequence. The
sequence program remains in force until such time as the
AD7938-6 is written to and the SEQ and SHDW bits are
configured with any bit combination except 1, 0.
shows a flow chart of the programmable sequence operation.
Figure 31
POWER ON
WRITE TO THE CONTROL REGISTER TO
SET UP OPERATING MODE, ANALOG INPUT
AND OUTPUT CONFIGURATION
SET SEQ = 0 SHDW = 1.
INITIATE A WRITE CYCLE.
THIS WRITE CYCLE IS TO PROGRAM THE SHADOW REGISTER.
SET RELEVANT BITS TO SELECT
THE CHANNELS TO BE INCLUDED IN THE SEQUENCE.
SEQ BIT = 1
SHDW BIT = 0
CONTINUOUSLY CONVERT
CONSECUTIVE
CHANNELS SELECTED
WITH EACH CONVST PULSE
BUT ALLOWS THE RANGE,
CONTINUOUSLY CONVERT
CONSECUTIVE
CHANNELS SELECTED
IN THE SHADOW REGISTER
WITH EACH CONVST PULSE.
CODING, ANALOG INPUT TYPE,
ETC BITS IN THE CONTROL
REGISTER TO BE CHANGED
WITHOUT INTERRUPTING
THE SEQUENCE.
04751-039
WR = HIGH
SEQ BIT = 0
SHDW BIT = 1
Figure 31. Programmable Sequence Flow Chart
Consecutive Sequence (SEQ = 1, SHDW = 1)
A sequence of consecutive channels can be converted beginning
with Channel 0 and ending with a final channel selected by
writing to the ADD2 to ADD0 bits in the control register.
This is done by setting the SEQ and SHDW bits in the control
register to 1. In this mode, the sequencer can be used without
having to write to the shadow register. In this mode, once the
control register is written to, the next conversion is on Channel
0, then Channel 1, and so on until the channel selected by the
address bits (ADD2 to ADD0) is reached. The cycle begins
again provided the WR input is tied high. If low, the SEQ and
SHDW bits must be set to 1, 0 to allow the ADC to continue its
preprogrammed sequence uninterrupted. shows the
flow chart of the consecutive sequence mode.
Figure 32
POWER ON
WRITE TO THE CONTROL REGISTER TO
SET UP OPERATING MODE, ANALOG INPUT
AND OUTPUT CONFIGURATION SELECT
FINAL CHANNEL (ADD2 TO ADD0) IN
CONSECUTIVE SEQUENCE.
SET SEQ = 1 SHDW = 1.
CONTINUOUSLY CONVERT A CONSECUTIVE
SEQUENCE OF CHANNELS FROM CHANNEL 0
UP TO AND INCLUDING THE PREVIOUSLY
SELECTED FINAL CHANNEL ON ADD2 TO ADD0
WITH EACH CONVST PULSE.
SEQ BIT = 1
SHDW BIT = 0
CONTINUOUSLY CONVERT
CONSECUTIVE CHANNELS SELECTED
WITH EACH CONVST PULSE BUT
ALLOWS THE RANGE, CODING, ANALOG
INPUT TYPE, ETC BITS IN THE
CONTROL REGISTER TO BE CHANGED
WITHOUT INTERRUPTING
THE SEQUENCE.
04751-040
Figure 32. Consecutive Sequence Mode Flow Chart
REFERENCE
The AD7938-6 can operate with either the on-chip reference or
external reference. The internal reference is selected by setting
the REF bit in the internal control register to 1. A block diagram
of the internal reference circuitry is shown in Figure 33. The
internal reference circuitry includes an on-chip 2.5 V band gap
reference and a reference buffer. When using the internal
reference, the VREFIN/VREFOUT pin should be decoupled to AGND
with a 0.47 μF capacitor. This internal reference not only
provides the reference for the analog-to-digital conversion, but
it can also be used externally in the system. It is recommended
that the reference output be buffered using an external precision
op amp before applying it anywhere in the system.
REFERENCE
AD7938-6
ADC
BUFFER
04751-041
V
REFIN
/
V
REFOUT
Figure 33. Internal Reference Circuit Block Diagram
Alternatively, an external reference can be applied to the
VREFIN/VREFOUT pin of the AD7938-6. An external reference input
is selected by setting the REF bit in the internal control register
to 0. The external reference input range is 0.1 V to VDD. It is
important to ensure that, when choosing the reference value,
the maximum analog input range (VIN MAX) is never greater than
VDD + 0.3 V to comply with the maximum ratings of the device.
AD7938-6 Data Sheet
Rev. C | Page 22 of 32
e d r en e is 2.5 V.
eference values is
f suitable voltage references from
Table 12. Examples of Suitable Voltage References
Reference
ting
)
For example, if operating in differential mode and the reference
is sourced from VDD, the 0 V to 2 × VREF range cannot be used.
This is because the analog input signal range now extends to 2 ×
VDD, which exceeds maximum rating conditions. In the pseudo
differential modes, the user must ensure that VREF + VIN− ≤ VDD
when using the 0 V to VREF range, or when using the 2 × VREF
range that 2 × VREF +VIN− ≤ VDD.
In all cases, the sp cifie efer c
The performance of the part with different r
shown in Figure 9 to Figure 11. The value of the reference sets
the analog input span and the common-mode voltage range.
Errors in the reference source result in gain errors in the
AD7938-6 transfer function and add to specified full-scale
errors on the part.
Table 12 lists examples o
Analog Devices that can be used. Figure 34 shows a typical
connection diagram for an external reference.
Output Initial Accuracy Opera
Voltage (% Maximum) Current (μA
AD780 2.5/3 0.04 1000
ADR421 2.5 0.04 500
ADR420 8 2.04 0.05 500
Digital Inputs
The digital inputs applied to the AD7938-6 are not limited by
the maximum ratings that limit the analog inputs. Instead, the
digital inputs applied can go to 7 V and are not restricted by the
VDD + 0.3 V limit as on the analog inputs.
Another advantage of the digital inputs not being restricted by
the VDD + 0.3 V limit is the fact that power supply sequencing
issues are avoided. If any of these inputs are applied before VDD,
there is no risk of latch-up as there would be on the analog
inputs if a signal greater than 0.3 V were applied prior to VDD.
VDRIVE Input
The AD7938-6 has a VDRIVE feature. VDRIVE controls the voltage
at which the parallel interface operates. VDRIVE allows the ADC
to easily interface to 3 V and 5 V processors.
For example, if the AD7938-6 is operated with an AVDD of 5 V
and the VDRIVE pin is powered from a 3 V supply, the AD7938-6
has better dynamic performance with a VDD of 5 V while still
being able to interface directly to 3 V processors. Care should
be taken to ensure VDRIVE does not exceed VDD by more than
0.3 V (see the Absolute Maximum Ratings section).
AD7938-6*
1
AD780
NC
8
04751-042
2
+V
IN
NC
7
3
GND
6
4
TEMP
5
V
REF
O/PSELECT
TRIM
V
OUT
NC
2.5V
NC
V
DD
NC = NO CONNECT
10nF 0.1µF 0.1µF
0.1µF
*ADDITIONAL PINS OMITTED FOR CLARITY.
Figure 34. Typical VREF Connection Diagram
Data Sheet AD7938-6
Rev. C | Page 23 of 32
At the end of the conversion, BUSY goes low and can be used to
activate an interrupt service routine. The CS and RD lines are
then activated in parallel to read the 12 bits of conversion data.
When power supplies are first applied to the device, a rising
edge on CONVST is necessary to put the track-and-hold into
track. The acquisition time of 125 ns minimum must be allowed
before CONVST is brought low to initiate a conversion. The
ADC then goes into hold on the falling edge of CONVST and
back into track on the 13th rising edge of CLKIN after this (see
). When operating the device in autoshutdown or
autostandby mode, where the ADC powers down at the end of
each conversion, a rising edge on the
Figure 35
CONVST signal is used to
power up the device.
PARALLEL INTERFACE
The AD7938-6 has a flexible, high speed, parallel interface. This
interface is 12-bits wide and is capable of operating in either
word (W/B tied high) or byte (W/B tied low) mode. The
CONVST signal is used to initiate conversions, and when
operating in autoshutdown or autostandby mode, it is used to
initiate power-up.
A falling edge on the CONVST signal is used to initiate
conversions and it puts the ADC track-and-hold into track.
Once the CONVST signal goes low, the BUSY signal goes high
for the duration of the conversion. In between conversions,
CONVST must be brought high for a minimum time of t1. This
must happen after the 14th falling edge of CLKIN; otherwise, the
conversion is aborted and the track-and-hold goes back into track.
t
2
t
3
t
20
t
14
t
11
t
9
t
13
t
12
t
10
t
CONVERT
t
ACQUISITION
t
QUIET
t
1
12 345 121314
B
A
DATA
DATAOLD DATADB0 TO DB11
DB0 TO DB11
RD
THREE-STATE
THREE-STATE
CS
INTERNAL
TRACK/HOLD
BUSY
CLKIN
CONVST
WITH CS AND RD TIED LOW
04751-004
Figure 35. AD7938-6 Parallel Interface—Conversion and Read Cycle Timing in Word Mode (W/B = 1)
AD7938-6 Data Sheet
Rev. C | Page 24 of 32
Reading Data from the AD7938-6
With the W/B pin tied logic high, the AD7938-6 interface operates
in word mode. In this case, a single read operation from the
device accesses the conversion data-word on Pin DB0 to Pin
DB11. The DB8/HBEN pin assumes its DB8 function. With the
W/B pin tied to logic low, the AD7938-6 interface operates in
byte mode. In this case, the DB8/HBEN pin assumes its HBEN
function. Conversion data from the AD7938-6 must be accessed
in two read operations with eight bits of data provided on DB0
to DB7 for each of the read operations. The HBEN pin determines
whether the read operation accesses the high byte or the low
byte of the12-bit word. For a low byte read, DB0 to DB7 provide
the eight LSBs of the 12-bit word. For a high byte read, DB0 to
DB3 provide the four MSBs of the 12-bit word, DB4 to DB6
provide the Channel ID and DB7 is always a 0. shows
the read cycle timing diagram for a 12-bit transfer. When operating
in word mode, the HBEN input does not exist, and only the first
read operation is required to access data from the device. When
operating in byte mode, the two read cycles shown in
are required to access the full data-word from the device.
Figure 35
Figure 36
The CS and RD signals are gated internally and the level is
triggered active low. In either word mode or byte mode, CS and
RD can be tied together as the timing specifications for t10 and
t11 are 0 ns minimum. This means the bus would be constantly
driven by the AD7938-6.
The data is placed onto the data bus a time, t13, after both CS
and RD go low. The RD rising edge can be used to latch data
out of the device. After a time, t14, the data lines become
three-stated.
Alternatively, CS and RD can be tied permanently low and the
conversion data is valid and placed onto the data bus a time, t9,
before the falling edge of BUSY.
Note that if RD is pulsed during the conversion time this causes
a degradation in linearity performance of approximately 0.25 LSB.
Reading during conversion by way of tying CS and RD low does
not cause any degradation.
t
15
t
11
t
10
t
13
t
16
t
15
t
16
t
14
t
12
t
17
LOW BYTE HIGH BYTE
DB0 TO DB7
HBEN/DB8
RD
CS
04751-005
Figure 36. AD7938-6 Parallel Interface—Read Cycle Timing for Byte Mode Operation (W/B = 0)
Data Sheet AD7938-6
Rev. C | Page 25 of 32
Writing Data to the AD7938-6
With W/B tied logic high, a single write operation transfers the
full data-word on DB0 to DB11 to the control register on the
AD7938-6. The DB8/HBEN pin assumes its DB8 function. Data
written to the AD7938-6 should be provided on the DB0 to
DB11 inputs with DB0 being the LSB of the data-word. With
W/B tied logic low, the AD7938-6 requires two write operations
to transfer a full 12-bit word. DB8/HBEN assumes its HBEN
function. Data written to the AD7938-6 should be provided on
the DB0 to DB7 inputs. HBEN determines whether the byte
written is high byte or low byte data. The low byte of the data-
word should be written first with DB0 being the LSB of the full
data-word. For the high byte write, HBEN should be high and
the data on the DB0 input should be Bit 8 of the 12-bit word. In
both word mode and byte mode, a single write operation to the
shadow register is always sufficient since it is only 8-bits wide.
Figure 37 shows the write cycle timing diagram of the AD7938-6
in word mode. When operating in word mode, the HBEN input
does not exist and only one write operation is required to write
the word of data to the device. Data should be provided on DB0
to DB11. When operating in byte mode, the two write cycles
shown in Figure 38 are required to write the full data-word to
the AD7938-6. In Figure 38, the first write transfers the lower
eight bits of the data-word from DB0 to DB7, and the second
write transfers the upper four bits of the data-word. When
writing to the AD7938-6, the top four bits in the high byte must
be 0s.
The data is latched into the device on the rising edge of WR.
The data needs to be set up a time, t7, before the WR rising edge
and held for a time, t8, after the WR rising edge. The CS and
WR signals are gated internally. CS and WR can be tied
together as the timing specifications for t4 and t5 are 0 ns
minimum (assuming CS and RD have not already been tied
together).
t8
t5
t7
t6
t4
DATA
DB0 TO DB11
WR
CS
04751-002
Figure 37. AD7938-6 Parallel Interface—Write Cycle Timing for Word Mode Operation (W/B = 1)
t5
t4
t7
t18 t18 t19
t19
t8
t6
t17
LOW BYTE HIGH BYTE
DB0 TO DB11
HBEN/DB8
WR
CS
04751-003
Figure 38. AD7938-6 Parallel Interface—Write Cycle Timing for Byte Mode Operation (W/B = 0)
AD7938-6 Data Sheet
Rev. C | Page 26 of 32
POWER MODES OF OPERATION
The AD7938-6 has four different power modes of operation.
These modes are designed to provide flexible power
management options. Different options can be chosen to
optimize the power dissipation/throughput rate ratio for
differing applications. The mode of operation is selected by the
power management bits, PM1 and PM0, in the control register,
as detailed in Table 8. When power is first applied to the
AD7938-6 an on-chip, power-on reset circuit ensures that the
default power-up condition is normal mode.
Note that, after power-on, the track-and-hold is in hold mode
and the first rising edge of CONVST places the track-and-hold
into track mode.
Normal Mode (PM1 = PM0 = 0)
This mode is intended for the fastest throughput rate
performance. The user does not have to worry about any
power-up times associated with the AD7938-6 because it
remains fully powered up at all times. At power-on reset, this
mode is the default setting in the control register.
Autoshutdown (PM1 = 0; PM0 = 1)
In this mode of operation, the AD7938-6 automatically enters
full shutdown at the end of each conversion, which is shown at
Point A in Figure 35 and Figure 39. In shutdown mode, all
internal circuitry on the device is powered down. The part
retains information in the control register during shutdown.
The track-and-hold also goes into hold at this point and
remains in hold as long as the device is in shutdown. The
AD7938-6 remains in shutdown mode until the next rising
edge of CONVST (see Point B in and ). In
order to keep the device in shutdown for as long as possible,
Figure 35 Figure 39
CONVST should idle low between conversions, as shown in
. On this rising edge, the part begins to power-up and
the track-and-hold returns to track mode. The power-up time
required is 10 ms minimum regardless of whether the user is
operating with the internal or external reference. The user
should ensure that the power-up time has elapsed before
initiating a conversion.
Figure 39
Autostandby (PM1 = 1; PM0 = 0)
In this mode of operation, the AD7938-6 automatically enters
standby mode at the end of each conversion, which is shown as
Point A in Figure 35. When this mode is entered, all circuitry
on the AD7938-6 is powered down except for the reference and
reference buffer. The track-and-hold goes into hold at this point
also and remains in hold as long as the device is in standby. The
part remains in standby until the next rising edge of CONVST
powers up the device. The power-up time required depends on
whether the internal or external reference is used. With an
external reference, the power-up time required is a minimum of
600 ns, while when using the internal reference, the power-up
time required is a minimum of 7 μs. The user should ensure this
power-up time has elapsed before initiating another conversion
as shown in . This rising edge of Figure 39 CONVST also places
the track-and-hold back into track mode.
Full Shutdown Mode (PM1 =1; PM0 = 1)
When this mode is programmed, all circuitry on the AD7938-6
is powered down upon completion of the write operation, that
is, on the rising edge of WR. The track-and-hold enters hold
mode at this point. The part retains the information in the
control register while the part is in shutdown. The AD7938-6
remains in full shutdown mode, with the track-and-hold in
hold mode, until the power management bits (PM1 and PM0)
in the control register are changed. If a write to the control
register occurs while the part is in full shutdown mode, and the
power management bits are changed to PM0 = PM1 = 0, that is,
normal mode, the part begins to power-up on the WR rising
edge and the track-and-hold returns to track. To ensure the part
is fully powered up before a conversion is initiated, the power-
up time of 10 ms minimum should be allowed before the next
CONVST falling edge; otherwise, invalid data is read.
Note that all power-up times quoted apply with a 470 nF
capacitor on the VREFIN pin.
t
POWER-UP
1 114 14
BUSY
CLKIN
CONVST
04751-049
AB
Figure 39. Autoshutdown/Autostandby Mode
Data Sheet AD7938-6
Rev. C | Page 27 of 32
THROUGHPUT (kSPS)
POWER (mW)
7
1
2
3
4
5
6
00 100 200 300 400 500 700600
04751-030
POWER vs. THROUGHPUT RATE
T
A
= 25°C
A considerable advantage of powering the ADC down after a
conversion is that the power consumption of the part is
significantly reduced at lower throughput rates. When using the
different power modes, the AD7938-6 is only powered up for
the duration of the conversion. Therefore, the average power
consumption per cycle is significantly reduced. Figure 40 shows
a plot of the power vs. the throughput rate when operating in
autostandby mode for both VDD = 5 V and 3 V.
V
DD
= 5V
V
DD
= 3V
AD7938-6*
For example, if the device runs at a throughput rate of 10 kSPS,
then the overall cycle time would be 100 μs. If the maximum
CLKIN frequency of 10 MHz is used, the conversion time accounts
for only 1.315 μs of the overall cycle time while the AD7938-6
remains in standby mode for the remainder of the cycle.
If an external reference is used, the power-up time reduces to
600 ns; therefore, the AD7938-6 remains in standby for a
greater length of time in every cycle. Additionally, the current
consumption, when converting, would be lower than the
specified maximum of 1.5 mA with VDD = 5 V, or 1.2 mA with
VDD = 3 V.
Figure 41 shows a plot of the power vs. the throughput rate
when operating in normal mode for both VDD = 5 V and
VDD = 3 V. Again, when using an external reference, the current
consumption when converting will be lower than the specified
maximum. In both plots, the figures apply when using the
internal reference.
THROUGHPUT (kSPS)
0
0.2
0 20 40 60 80 120100
04751-029
POWER (mW)
2.0
0.8
1.0
1.2
1.4
1.6
1.8
0.6
0.4
T
A
= 25°C
V
DD
= 5V
V
DD
= 3V
Figure 40. Power vs. Throughput in Autostandby
Mode Using Internal Reference
Figure 41. Power vs. Throughput in Normal Mode Using Internal Reference
MICROPROCESSOR INTERFACING
AD7938-6 to ADSP-21xx Interface
Figure 42 shows the AD7938-6 interfaced to the ADSP-21xx
series of DSPs as a memory mapped device. A single wait state
may be necessary to interface the AD7938-6 to the ADSP-21xx
depending on the clock speed of the DSP. The wait state can be
programmed via the data memory wait state control register of
the ADSP-21xx (see the ADSP-21xx family User’s Manual for
details). The following instruction reads from the AD7938-6:
MR = DM (ADC)
where ADC is the address of the AD7938-6.
ADSP-21xx*
WR
RD
DB0 TO DB11
D0 TO D23
A0 TO A15
DMS
IRQ2 BUSY
CONVST
DSP/USER SYSTEM
CS
WR
RD
*ADDITIONAL PINS OMITTED FOR CLARITY.
ADDRESS BUS
DATA BUS
ADDRESS
DECODER
04751-045
Figure 42. Interfacing to the ADSP-21xx
AD7938-6 Data Sheet
Rev. C | Page 28 of
AD7938-6 to ADSP-21065L Interface
32
Figure 43 shows a typical interface between the AD7938-6 and
the ADSP-21065L SHARC® processor. This interface is an
example of one of three DMA handshake modes. The MSx
control line is actually three memory select lines. Internal
ADDR25 to 24 are decoded into MS3 to 0, these lines are then
asserted as chip selects. The DMAR1 (DMA request 1) is used in
this setup as the interrupt to signal the end of conversion. The
rest of the interface is a standard handshaking operation.
AD7938-6*
ADSP-21065L*
WR
DB0 TO DB11
D0 TO D31
A0 TO A23
MS
X
DMAR
1
BUSY
CS
CONVST
DSP/USER SYSTEM
WR
RDRD
*
ADDITION
A
L PINS REMOVED FOR CLARITY.
ADDRESS BUS
ADDRESS BUS
DATA BUS
ADDRESS
LATCH
ADDRESS
DECODER
04751-046
Figure 43. Interfacing to the ADSP-21065L
AD7938-6 to TMS32020, TMS320C25, and TMS320C5x
Interface
Parallel interfaces between the AD7938-6 and the TMS32020,
TMS320C25, and TMS320C5x family of DSPs are shown in
Figure 44. The memory mapped address chosen for the AD7938-6
should be chosen to fall in the I/O memory space of the DSPs.
The parallel interface on the AD7938-6 is fast enough to
interface to the TMS32020 with no extra wait states. If high
speed glue logic, such as 74AS devices, is used to drive the RD
and the WR lines when interfacing to the TMS320C25, then
again, no wait states are necessary. However, if slower logic is
used, data accesses may be slowed sufficiently when reading
from, and writing to, the part to require the insertion of one
wait state. Extra wait states are necessary when using the
TMS320C5x at their fastest clock speeds (see the TMS320C5x
User’s Guide for details).
Data is read from the ADC using the following instruction
IN D, ADC
where:
D is the data memory address.
ADC is the AD7938-6 address.
AD7938-6*
TMS32020/
TMS320C25/
TMS320C50*
WR
RD
DB11 TO DB0DMD0 TO DMD15
A0 TO A15
IS
READY
INT
X
BUSY
CSEN
CONVST
DSP/USER SYSTEM
TMS320C25
ONLY
R/W
STRB
*ADDITIONAL PINS OMITTED FOR CLARITY.
ADDRESS BUS
ADDRESS
DECODER
MSC
DATA BUS
04751-047
Figure 44. Interfacing to the TMS32020/TMS320C25/TMS320C5x
AD7938-6 to 80C186 Interface
Figure 45 shows the AD7938-6 interfaced to the 80C186
microprocessor. The 80C186 DMA controller provides two
independent high speed DMA channels where data transfer
can occur between memory and I/O spaces. Each data transfer
consumes two bus cycles, one cycle to fetch data and the other
to store data. After the AD7938-6 has finished a conversion, the
BUSY line generates a DMA request to Channel 1 (DRQ1).
Because of the interrupt, the processor performs a DMA read
operation that also resets the interrupt latch. Sufficient priority
must be assigned to the DMA channel to ensure that the DMA
request is serviced before the completion of the next
conversion.
AD7938-6*
80C186*
WR
DB0 TO DB11
AD0 TO AD15
A16 TO A19
ALE
DRQ1
BUSY
CS
QR
S
CONVST
MICROPROCESSOR/
USER SYSTEM
WR
RDRD
L PINS OMITTED FOR CLARITY
ADDRESS/DATA BUS
ADDRESS BUS
*
ADDITION
A
DATA BUS
ADDRESS
LATCH
ADDRESS
DECODER
04751-048
Figure 45. Interfacing to the 80C186
Data Sheet AD7938-6
Rev. C | Page 29 of 32
APPLICATION HINTS
GROUNDING AND LAYOUT
The printed circuit board that houses the AD7938-6 should be
designed so that the analog and digital sections are separated
and confined to certain areas of the board. This facilitates the
use of ground planes that can be easily separated. Generally, a
minimum etch technique is best for ground planes since it gives
the best shielding. Digital and analog ground planes should be
joined in only one place, and the connection should be a star
ground point established as close to the ground pins on the
AD7938-6 as possible. Avoid running digital lines under the
device as this couples noise onto the die. The analog ground
plane should be allowed to run under the AD7938-6 to avoid
noise coupling. The power supply lines to the AD7938-6 should
use as large a trace as possible to provide low impedance paths
and reduce the effects of glitches on the power supply line.
Fast switching signals, such as clocks, should be shielded with
digital ground to avoid radiating noise to other sections of the
board, and clock signals should never run near the analog
inputs. Avoid crossover of digital and analog signals. Traces on
opposite sides of the board should run at right angles to each
other. This reduces the effects of feedthrough through the
board. A microstrip technique is by far the best but is not
always possible with a double-sided board. In this technique,
the component side of the board is dedicated to ground planes,
while signals are placed on the solder side.
Good decoupling is also important. All analog supplies should
be decoupled with 10 μF tantalum capacitors in parallel with
0.1 μF capacitors to GND. To achieve the best performance
from these decoupling components, they must be placed as
close as possible to the device, ideally right up against the
device. The 0.1 μF capacitors should have low effective series
resistance (ESR) and effective series inductance (ESI), such as
the common ceramic types or surface-mount types, which
provide a low impedance path to ground at high frequencies to
handle transient currents due to internal logic switching.
PCB DESIGN GUIDELINES FOR CHIP SCALE
PACKAGE
The lands on the chip scale package (CP-32-2) are rectangular.
The printed circuit board pad for these should be 0.1 mm
longer than the package land length and 0.05 mm wider than
the package land width. The land should be centered on the
pad. This ensures that the solder joint size is maximized. The
bottom of the chip scale package has a thermal pad. The
thermal pad on the printed circuit board should be at least as
large as this exposed pad. On the printed circuit board, there
should be a clearance of at least 0.25 mm between the thermal
pad and the inner edges of the pad pattern. This ensures that
shorting is avoided. Thermal vias can be used on the printed
circuit board thermal pad to improve thermal performance of
the package. If vias are used, they should be incorporated in the
thermal pad at 1.2 mm pitch grid. The via diameter should be
between 0.3 mm and 0.33 mm, and the via barrel should be
plated with 1 oz. copper to plug the via. The user should
connect the printed circuit board thermal pad to AGND.
EVALUATING THE AD7938-6 PERFORMANCE
The recommended layout for the AD7938-6 is outlined in the
evaluation board documentation. The evaluation board
package includes a fully assembled and tested evaluation board,
documentation, and software for controlling the board from the
PC via the evaluation board controller. The evaluation board
controller can be used in conjunction with the AD7938-6
evaluation board, as well as many other Analog Devices evaluation
boards ending in the CB designator, to demonstrate/evaluate
the ac and dc performance of the AD7938-6.
The software allows the user to perform ac (fast Fourier
transform) and dc (histogram of codes) tests on the AD7938-6.
The software and documentation are on the CD that ships with
the evaluation board.
AD7938-6 Data Sheet
Rev. C | Page 30 of 32
3.25
3.10 SQ
2.95
FOR PROPER CONNECTION OF
THE EXPOSED PAD, REFER TO
THE PIN CONFIGURATION AND
FUNCTION DESCRIPTIONS
SECTION OF THIS DATA SHEET.
OUTLINE DIMENSIONS
COMPLIANT TO JEDEC STANDARDS MO-220-VHHD-2
1
32
8
9
25
24
17
16
COPLANARITY
0.08
3.50 REF
0.50
BSC
PIN 1
INDICATOR
PIN 1
INDICATOR
0.30
0.25
0.18
0.20 REF
12° MAX 0.80 MAX
0.65 TYP
1.00
0.85
0.80 0.05 MAX
0.02 NOM
S
EATING
PLANE
0.50
0.40
0.30
5.00
BSC SQ
4.75
BSC SQ
0.60 MAX
0.60 MAX
0.25 MIN
05-25-2011-A
TOP VIEW
EXPOSED
PAD
BOTTOM VIEW
Figure 46. 32-Lead Lead Frame Chip Scale Package [LFCSP_VQ]
5 × 5 mm Body, Very Thin Quad
(CP-32-2)
Dimensions shown in millimeters
0.45
0.37
0.30
0.80
7.00
BSC SQ
9.00 BSC SQ
BSC
LEAD PITCH
124
25
32
8
9
17
16
1.20
MAX
0.75
0.60
0.45
1.05
1.00
0.95
0.20
0.09
0.08 MAX
COPLANARITY
SEATING
PLANE
0° MIN
7°
3.5°
0°
0.15
0.05
VIEW A
ROTATED 90° CCW
VIEW A
PIN 1
TOP VIEW
(PINS DOWN)
020607-A
COMPLIANT TO JEDEC STANDARDS MS-026-AB A
Figure 47. 32-Lead Thin Plastic Quad Flat Package [TQFP]
(SU-32-2)
Dimensions shown in millimeters
Data Sheet AD7938-6
Rev. C | Page 31 of 32
ORDERING GUIDE
Model1 Temperature Range Linearity Error (LSB)2Package Description Package Option
AD7938BCPZ-6 –40°C to +85°C ±1 32-Lead LFCSP_VQ CP-32-2
AD7938BCPZ-6REEL7 –40°C to +85°C ±1 32-Lead LFCSP_VQ CP-32-2
AD7938BSUZ-6 –40°C to +85°C ±1 32-Lead TQFP SU-32-2
AD7938BSUZ-6REEL7 –40°C to +85°C ±1 32-Lead TQFP SU-32-2
1 Z = RoHS Compliant Part.
2 Linearity error here refers to integral linearity error.
