a
AD7891
REV. D
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LC
2
MOS 8-Channel, 12-Bit
High Speed Data Acquisition System
FUNCTIONAL BLOCK DIAGRAM
2.5V
REFERENCE
TRACK/HOLD
VIN1A
VIN1B
VIN2A
VIN2B
VIN3A
VIN3B
VIN4A
VIN4B
VIN5A
VIN5B
VIN6A
VIN6B
VIN7A
VIN7B
VIN8A
VIN8B
CONTROL LOGIC
WR CS RD EOC CONVST MODE AGND AGND DGND
CLOCK
VDD VDD
AD7891
12-BIT
ADC
ADDRESS
DECODE
REF OUT/
REF IN REF GND
STANDBY
M
U
X
DATA/
CONTROL
LINES
FEATURES
Fast 12-Bit ADC with 1.6 ␮s Conversion Time
8 Single-Ended Analog Input Channels
Overvoltage Protection on Each Channel
Selection of Input Ranges:
ⴞ5 V, ⴞ10 V for AD7891-1
0 to +2.5 V, 0 to +5 V, ⴞ2.5 V for AD7891-2
Parallel and Serial Interface
On-Chip Track/Hold Amplifier
On-Chip Reference
Single-Supply, Low Power Operation (100 mW Max)
Power-Down Mode (75 ␮W Typ)
APPLICATIONS
Data Acquisition Systems
Motor Control
Mobile Communication Base Stations
Instrumentation
GENERAL DESCRIPTION
The AD7891 is an 8-channel, 12-bit data acquisition system
with a choice of either parallel or serial interface structure. The
part contains an input multiplexer, an on-chip track/hold ampli-
fier, a high speed 12-bit ADC, a 2.5 V reference, and a high
speed interface. The part operates from a single 5 V supply and
accepts a variety of analog input ranges across two models, the
AD7891-1 (±5V and ±10 V) and the AD7891-2 (0 V to +2.5 V,
0V to +5 V, and ±2.5 V).
The AD7891 provides the option of either a parallel or serial
interface structure determined by the MODE pin. The part
has standard control inputs and fast data access times for both
the serial and parallel interfaces, ensuring easy interfacing to
modern microprocessors, microcontrollers, and digital signal
processors.
In addition to the traditional dc accuracy specifications, such as
linearity, full-scale and offset errors, the part is also specified for
dynamic performance parameters, including harmonic distortion
and signal-to-noise ratio.
Power dissipation in normal mode is 82 mW typical; in
the standby mode, this is reduced to 75 mW typ. The part is
available in a 44-terminal MQFP and a 44-lead PLCC.
PRODUCT HIGHLIGHTS
1. The AD7891 is a complete monolithic 12-bit data acquisition
system that combines an 8-channel multiplexer, 12-bit ADC,
2.5 V reference, and track/hold amplifier on a single chip.
2. The AD7891-2 features a conversion time of 1.6 ms and an
acquisition time of 0.4 ms. This allows a sample rate of
500 kSPS when sampling one channel and 62.5 kSPS when
channel hopping. These sample rates can be achieved using
either a software or hardware convert start. The AD7891-1
has an acquisition time of 0.6 ms when using a hardware
convert start and an acquisition time of 0.7 ms when using a
software convert start. These acquisition times allow sample
rates of 454.5 kSPS and 435 kSPS, respectively, for hardware
and software convert start.
3. Each channel on the AD7891 has overvoltage protection. This
means an overvoltage on an unselected channel does not affect
the conversion on a selected channel. The AD7891-1 can
withstand overvoltages of ±17 V.
–2– REV. D
AD7891–SPECIFICATIONS
(VDD = 5 V ⴞ 5%, AGND = DGND = 0 V, REF IN = 2.5 V. All specifications TMIN to TMAX,
unless otherwise noted.)
Parameter A Version
1
B Version Y Version Unit Test Conditions/Comments
DYNAMIC PERFORMANCE
2
Sample Rate = 454.5 kSPS
3
(AD7891-1),
500 kSPS
3
(AD7891-2). Any channel.
Signal-to-(Noise + Distortion) Ratio
4
@ 25∞C707070dB min
T
MIN
to T
MAX
70 70 70 dB min
Total Harmonic Distortion
4
–78 –78 –78 dB max
Peak Harmonic or Spurious Noise
4
–80 –80 –80 dB max
Intermodulation Distortion
4
fa = 9 kHz, fb = 9.5 kHz.
Second-Order Terms –80 –80 –80 dB typ
Third-Order Terms –80 –80 –80 dB typ
Channel-to-Channel Isolation
4
–80 –80 –80 dB max
DC ACCURACY Any channel.
Resolution 12 12 12 Bits
Minimum Resolution for which
No Missing Codes Are Guaranteed 12 12 12 Bits
Relative Accuracy
4
±1±0.75 ±1LSB max
Differential Nonlinearity
4
±1±1±1LSB max
Positive Full-Scale Error
4
±3±3±3LSB max
Positive Full-Scale Error Match
4, 5
0.6 0.6 0.6 LSB typ 1.5 LSB max.
Unipolar Offset Error ±4±4±4LSB max Input ranges of 0 V to 2.5 V, 0 V to 5 V.
Unipolar Offset Error Match
5
0.1 0.1 0.1 LSB typ 1 LSB max.
Negative Full-Scale Error
4
±3±3±3LSB max Input ranges of ±2.5 V, ±5V, ±10 V.
Negative Full-Scale Error Match
4, 5
0.6 0.6 0.6 LSB typ 1.5 LSB max.
Bipolar Zero Error ±4±4±4LSB max Input ranges of ±2.5 V, ±5V, ±10 V.
Bipolar Zero Error Match
5
0.2 0.2 0.2 LSB typ 1.5 LSB max.
ANALOG INPUTS
AD7891-1 Input Voltage Range
±5±5±5V Input applied to both V
INXA
and V
INXB
.
±10 ±10 ±10 V Input applied to V
INXA
, V
INXB
= AGND.
AD7891-1 V
INXA
Input Resistance 7.5 7.5 7.5 kW min Input range of ±5V.
AD7891-1 V
INXA
Input Resistance 15 15 15 kW min Input range of ±10 V.
AD7891-2 Input Voltage Range 0 to 2.5 0 to 2.5 0 to 2.5 V Input applied to both V
INXA
and V
INXB
.
0 to 5 0 to 5 0 to 5 V Input applied to V
INXA
, V
INXB
= AGND.
±2.5 ±2.5 ±2.5 V Input applied to V
INXA
, V
INXB
= REF IN
6
.
AD7891-2 V
INXA
Input Resistance 1.5 1.5 1.5 kW min Input ranges of ±2.5 V and 0 V to 5 V.
AD7891-2 V
INXA
Input Current ±50 ±50 ±50 nA max Input range of 0 V to 2.5 V.
REFERENCE INPUT/OUTPUT
REF IN Input Voltage Range 2.375/2.625 2.375/2.625 2.375/2.625 V min/V max 2.5 V ± 5%.
Input Impedance 1.6 1.6 1.6 kW min Resistor connected to internal reference node.
Input Capacitance
5
10 10 10 pF max
REF OUT Output Voltage 2.5 2.5 2.5 V nom
REF OUT Error @ 25∞C±10 ±10 ±10 mV max
T
MIN
to T
MAX
±20 ±20 ±20 mV max
REF OUT Temperature Coefficient 25 25 25 ppm/∞C typ
REF OUT Output Impedance 5 5 5 kW nom See REF IN input impedance.
LOGIC INPUTS
Input High Voltage, V
INH
2.4 2.4 2.4 V min V
DD
= 5 V ± 5%.
Input Low Voltage, V
INL
0.8 0.8 0.8 V max V
DD
= 5 V ± 5%.
Input Current, I
INH
±10 ±10 ±10 mA max
Input Capacitance
5
C
IN
10 10 10 pF max
–3–
REV. D
AD7891
Parameter A Version
1
B Version Y Version Unit Test Conditions/Comments
LOGIC OUTPUTS
Output High Voltage, V
OH
4.0 4.0 4.0 V min I
SOURCE
= 200 mA.
Output Low Voltage, V
OL
0.4 0.4 0.4 V max I
SINK
= 1.6 mA.
DB11to DB0
Floating-State Leakage Current ±10 ±10 ±10 mA max
Floating-State Capacitance
5
15 15 15 pF max
Output Coding
Straight (Natural) Binary Data format bit of control register = 0.
Twos Complement Data format bit of control register = 1.
CONVERSION RATE
Conversion Time 1.6 1.6 1.6 ms max
Track/Hold Acquisition Time 0.6 0.6 0.6 ms max AD7891-1 hardware conversion.
0.7 0.7 0.7 ms max AD7891-1 software conversion.
0.4 0.4 0.4 ms max AD7891-2.
POWER REQUIREMENTS
V
DD
555V nom ±5% for specified performance.
I
DD
Normal Mode 20 20 21 mA max
Standby Mode 80 80 80 mA max Logic inputs = 0 V or V
DD
.
Power Dissipation V
DD
= 5 V.
Normal Mode 100 100 105 mW max Typically 82 mW.
Standby Mode 400 400 400 mW max Typically 75 mW.
NOTES
1
Temperature ranges for the A and B Versions: –40∞C to +85∞C. Temperature range for the Y Version: –55∞C to +105∞C.
2
The AD7891-1’s dynamic performance (THD and SNR) and the AD7891-2’s THD are measured with an input frequency of 10 kHz. The AD7891-2’s SNR is
evaluated with an input frequency of 100 kHz.
3
This throughput rate can only be achieved when the part is operated in the parallel interface mode. Maximum achievable throughput rate in the serial interface mode
is 357 kSPS.
4
See the Terminology section.
5
Sample tested during initial release and after any redesign or process change that may affect this parameter.
6
REF IN must be buffered before being applied to V
INXB
.
Specifications subject to change without notice.
ABSOLUTE MAXIMUM RATINGS*
(T
A
= 25∞C, unless otherwise noted)
V
DD
to AGND . . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +7 V
V
DD
to DGND . . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +7 V
Analog Input Voltage to AGND
AD7891-1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±17 V
AD7891-2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . –5 V, +10 V
Reference Input Voltage to AGND . . . . –0.3 V to V
DD
+ 0.3 V
Digital Input Voltage to DGND . . . . . . –0.3 V to V
DD
+ 0.3 V
Digital Output Voltage to DGND . . . . . –0.3 V to V
DD
+ 0.3 V
Operating Temperature Range
Commercial (A, B Versions) . . . . . . . . . . . –40∞C to +85∞C
Automotive (Y Version) . . . . . . . . . . . . . . –55∞C to +105∞C
Storage Temperature Range . . . . . . . . . . . . –65∞C to +150∞C
Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . 150∞C
MQFP Package, Power Dissipation . . . . . . . . . . . . . . 450 mW
q
JA
Thermal Impedance . . . . . . . . . . . . . . . . . . . . . . 95∞C/W
Lead Temperature, Soldering
Vapor Phase (60 sec) . . . . . . . . . . . . . . . . . . . . . . . . 215∞C
Infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . . . 220∞C
PLCC Package, Power Dissipation . . . . . . . . . . . . . . 500 mW
q
JA
Thermal Impedance . . . . . . . . . . . . . . . . . . . . . . 55∞C/W
Lead Temperature, Soldering
Vapor Phase (60 sec) . . . . . . . . . . . . . . . . . . . . . . . . 215∞C
Infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . . . 220∞C
*Stresses above those listed under Absolute Maximum Ratings may cause perma-
nent damage to the device. This is a stress rating only; functional operation of the
device at these or any other conditions above those listed in the operational
sections of this specification is not implied. Exposure to absolute maximum rating
conditions for extended periods may affect device reliability.
CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection.
Although the AD7891 features proprietary ESD protection circuitry, permanent damage may
occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD
precautions are recommended to avoid performance degradation or loss of functionality.
AD7891
–4– REV. D
TIMING CHARACTERISTICS
1, 2
Parameter A, B, Y Versions Unit Test Conditions/Comments
t
CONV
1.6 ms max Conversion Time
Parallel Interface
t
1
0ns min CS to RD/WR Setup Time
t
2
35 ns min Write Pulse Width
t
3
25 ns min Data Valid to Write Setup Time
t
4
5ns min Data Valid to Write Hold Time
t
5
0ns min CS to RD/WR Hold Time
t
6
35 ns min CONVST Pulse Width
t
7
55 ns min EOC Pulse Width
t
8
35 ns min Read Pulse Width
t
93
25 ns min Data Access Time after Falling Edge of RD
t
104
5ns min Bus Relinquish Time after Rising Edge of RD
30 ns max
Serial Interface
t
11
30 ns min RFS Low to SCLK Falling Edge Setup Time
t
123
20 ns max RFS Low to Data Valid Delay
t
13
25 ns min SCLK High Pulse Width
t
14
25 ns min SCLK Low Pulse Width
t
153
5ns min SCLK Rising Edge to Data Valid Hold Time
t
163
15 ns max SCLK Rising Edge to Data Valid Delay
t
17
20 ns min RFS to SCLK Falling Edge Hold Time
t
184
0ns min Bus Relinquish Time after Rising Edge of RFS
30 ns max
t
18A4
0ns min Bus Relinquish Time after Rising Edge of SCLK
30 ns max
t
19
20 ns min TFS Low to SCLK Falling Edge Setup Time
t
20
15 ns min Data Valid to SCLK Falling Edge Setup Time
t
21
10 ns min Data Valid to SCLK Falling Edge Hold Time
t
22
30 ns min TFS Low to SCLK Falling Edge Hold Time
NOTES
1
Sample tested during initial release and after any redesign or process change that may affect this parameter. All input signals are measured with tr = tf = 1 ns (10% to
90% of 5 V) and timed from a voltage level of 1.6 V.
2
See Figures 2, 3, and 4.
3
Measured with the load circuit of Figure 1 and defined as the time required for an output to cross 0.8 V or 2.4 V.
4
These times are derived from the measured time taken by the data outputs to change 0.5 V when loaded with the circuit of Figure 1. The measured number is then
extrapolated back to remove the effects of charging or discharging the 50 pF capacitor. This means that the times quoted in the timing characteristics are the true bus
relinquish times of the part and as such are independent of external bus loading capacitances.
Specifications subject to change without notice.
1.6mA
200␮A
1.6V
TO
OUTPUT
PIN 50pF
Figure 1. Load Circuit for Access Time and Bus Relinquish Time
AD7891
–5–
REV. D
PIN CONFIGURATIONS
PLCC
7
8
9
10
11
12
13
14
15
16
17
654321444342 41 40
39
38
37
36
35
34
33
32
31
30
29
18 19 20 21 22 23 24 25 26 27 28
PIN 1
IDENTIFIER
TOP VIEW
(Not to Scale)
NC = NO CONNECT
V
IN6A
AGND
EOC
NC
CONVST
CS
REF GND
NC
REF OUT/REF IN
V
DD
AGND
MODE
DB11/TEST
DB10/TEST
DB9/TFS
DB8/RFS
DB7/DATA IN
STANDBY
V
IN1A
DB6/SCLK
V
DD
DGND
DB5/A2/DATA OUT
DB3/A0
DB2/SWCON
DB1/SWSTBY
DB0/FORMAT
WR
RD
DB4/A1
AD7891
V
IN1B
V
IN2A
V
IN2B
V
IN3A
V
IN3B
V
IN4A
V
IN4B
V
IN5A
V
IN5B
V
IN6B
V
IN7A
V
IN7B
V
IN8A
V
IN8B
MQFP
V
IN6A
AGND
EOC
NC
CONVST
CS
REF GND
NC
REF OUT/REF IN
V
DD
AGND
MODE
DB11/TEST
DB10/TEST
DB9/TFS
DB8/RFS
DB7/DATA IN
STANDBY
V
IN1A
DB6/SCLK
V
DD
DGND
DB5/A2/DATA OUT
DB3/A0
DB2/SWCON
DB1/SWSTBY
DB0/FORMAT
WR
RD
DB4/A1
V
IN1B
V
IN2A
V
IN2B
V
IN3A
V
IN3B
V
IN4A
V
IN4B
V
IN5A
V
IN5B
V
IN6B
V
IN7A
V
IN7B
V
IN8A
V
IN8B
NC = NO CONNECT
44 43 42 41 40 39 38 37 36 35 34
1
2
3
4
5
6
7
8
9
10
11
12 13 14 15 16 17 18 19 20 21 22
33
32
31
30
29
28
27
26
25
24
23
PIN 1
IDENTIFIER
TOP VIEW
(Not to Scale)
AD7891
ORDERING GUIDE
Relative Temperature
Model Input Range Sample Rate Accuracy Range Package Option
1
AD7891ACHIPS-1 DIE
AD7891ACHIPS-2 DIE
AD7891AS-1 ±5 V or ±10 V 454 kSPS ±1 LSB –40∞C to +85∞CS-44
AD7891ASZ-1
2
±5 V or ±10 V 454 kSPS ±1 LSB –40∞C to +85∞CS-44
AD7891AP-1 ±5 V or ±10 V 454 kSPS ±1 LSB –40∞C to +85∞CP-44A
AD7891AP-1REEL ±5 V or ±10 V 454 kSPS ±1 LSB –40∞C to +85∞CP-44A
AD7891BS-1 ±5 V or ±10 V 454 kSPS ±0.75 LSB –40∞C to +85∞CS-44
AD7891BP-1 ±5 V or ±10 V 454 kSPS ±0.75 LSB –40∞C to +85∞CP-44A
AD7891BP-1REEL ±5 V or ±10 V 454 kSPS ±0.75 LSB –40∞C to +85∞CP-44A
AD7891YS-1 ±5 V or ±10 V 454 kSPS ±1 LSB –55∞C to +105∞C S-44
AD7891YS-1REEL ±5 V or ±10 V 454 kSPS ±1 LSB –55∞C to +105∞C S-44
AD7891YP-1 ±5 V or ±10 V 454 kSPS ±1 LSB –55∞C to +105∞CP-44A
AD7891YP-1REEL ±5 V or ±10 V 454 kSPS ±1 LSB –55∞C to +105∞CP-44A
AD7891AS-2 0 V to +5 V, 0 V to +2.5 V, ±2.5 V 500 kSPS ±1 LSB –40∞C to +85∞CS-44
AD7891ASZ-2
2
0 V to +5 V, 0 V to +2.5 V, ±2.5 V 500 kSPS ±1 LSB –40∞C to +85∞CS-44
AD7891AP-2 0 V to +5 V, 0 V to +2.5 V, ±2.5 V 500 kSPS ±1 LSB –40∞C to +85∞CP-44A
AD7891AP-2REEL 0 V to +5 V, 0 V to +2.5 V, ±2.5 V 500 kSPS ±1 LSB –40∞C to +85∞CP-44A
AD7891BS-2 0 V to +5 V, 0 V to +2.5 V, ±2.5 V 500 kSPS ±0.75 LSB –40∞C to +85∞CS-44
AD7891BP-2 0 V to +5 V, 0 V to +2.5 V, ±2.5 V 500 kSPS ±0.75 LSB –40∞C to +85∞CP-44A
AD7891BP-2REEL 0 V to +5 V, 0 V to +2.5 V, ±2.5 V 500 kSPS ±0.75 LSB –40∞C to +85∞CP-44A
AD7891YS-2 0 V to +5 V, 0 V to +2.5 V, ±2.5 V 500 kSPS ±1 LSB –55∞C to +105∞C S-44
AD7891YS-2REEL 0 V to +5 V, 0 V to +2.5 V, ±2.5 V 500 kSPS ±1 LSB –55∞C to +105∞C S-44
EVAL-AD7891-1CB Evaluation Board
EVAL-AD7891-2CB Evaluation Board
NOTES
1
S = Plastic Quad Flatpack (MQFP); P = Plastic Leaded Chip Carrier (PLCC).
2
Z = Pb-free part.
AD7891
–6– REV. D
PIN FUNCTION DESCRIPTIONS
PLCC MQFP
Pin No. Pin No. Mnemonic Description
1–528–43 V
INXA
, V
INXB
Analog Input Channels. The AD7891 contains eight pairs of analog input channels. Each
34–44 channel contains two input pins to allow a number of different input ranges to be used
with the AD7891. There are two possible input voltage ranges on the AD7891-1. The
±5V input range is selected by connecting the input voltage to both V
INXA
and V
INXB
,
while the ±10 V input range is selected by applying the input voltage to V
INXA
and con-
necting V
INXB
to AGND. The AD7891-2 has three possible input ranges. The 0 V to
2.5 V input range is selected by connecting the analog input voltage to both V
INXA
and V
INXB
; the
0V to 5 V input range is selected by applying the input voltage to V
INXA
and connecting
V
INXB
to AGND while the ±2.5 V input range is selected by connecting the analog input
voltage to V
INXA
and connecting V
INXB
to REF IN (provided this REF IN voltage comes
from a low impedance source). The channel to be converted is selected by the A2, A1,
and A0 bits of the control register. In the parallel interface mode, these bits are available as
three data input lines (DB3 to DB5) in a parallel write operation. While in the serial inter-
face mode, these three bits are accessed via the DATA IN line in a serial write operation.
The multiplexer has guaranteed break-before-make operation.
10, 19 4, 13 V
DD
Positive Supply Voltage, 5 V ± 5%.
11, 33 5, 27 AGND Analog Ground. Ground reference for track/hold, comparator, and DAC.
20 14 DGND Digital Ground. Ground reference for digital circuitry.
644STANDBY Standby Mode Input. TTL compatible input used to put the device into the power
save or standby mode. The STANDBY input is high for normal operation and low for
standby operation.
93 REF OUT/REF IN Voltage Reference Output/Input. The part can either be used with its own internal refer-
ence or with an external reference source. The on-chip 2.5 V reference voltage is pro-
vided at this pin. When using this internal reference as the reference source for the
part, REF OUT should be decoupled to REF GND with a 0.1 mF disc ceramic capaci-
tor. The output impedance of the reference source is typically 2 kW. When using an
external reference source as the reference voltage for the part, the reference source
should be connected to this pin. This overdrives the internal reference and provides the
reference source for the part. The reference pin is buffered on-chip but must be able to
sink or source current through this 2 kW resistor to the output of the on-chip reference.
The nominal reference voltage for correct operation of the AD7891 is 2.5 V.
71 REF GND Reference Ground. Ground reference for the part’s on-chip reference buffer. The REF
OUT pin of the part should be decoupled with a 0.1 mF capacitor to this REF GND
pin. If the AD7891 is used with an external reference, the external reference should also
be decoupled to this pin. The REF GND pin should be connected to the AGND pin
or the system’s AGND plane.
30 24 CONVST Convert Start. Edge-triggered logic input. A low-to-high transition on this input puts
the track/hold into hold and initiates conversion. When changing channels on the part,
sufficient time should be given for multiplexer settling and track/hold acquisition between
the channel change and the rising edge of CONVST.
32 26 EOC End-of-Conversion. Active low logic output indicating converter status. The end of con-
version is signified by a low-going pulse on this line. The duration of this EOC pulse is
nominally 80 ns.
12 6 MODE Interface Mode. Control input that determines the interface mode for the part. With this
pin at a logic low, the AD7891 is in its serial interface mode; with this pin at a logic high,
the device is in its parallel interface mode.
AD7891
–7–
REV. D
PARALLEL INTERFACE MODE FUNCTIONS
PLCC Pin No. MQFP Pin No. Mnemonic Description
8, 31 2, 25 NC No Connect. The two NC pins on the device can be left unconnected. If they
are to be connected to a voltage, it should be to ground potential. To ensure
correct operation of the AD7891, neither of the NC pins should be connected
to a logic high potential.
29 23 CS Chip Select Input. Active low logic input that is used in conjunction with to
enable the data outputs and with WR to allow input data to be written to the part.
28 22 RD Read Input. Active low logic input that is used in conjunction with CS low to
enable the data outputs.
27 21 WR Write Input. Active low logic input used in conjunction with CS to latch the mul-
tiplexer address and software control information. The rising edge of this input
also initiates an internal pulse. When using the software start facility, this pulse
delays the point at which the track/hold goes into hold and conversion is initiated.
This allows the multiplexer to settle and the acquisition time of the track/hold to
elapse when a channel address is changed. If the SWCON bit of the control regis-
ter is set to 1, when this pulse times out, the track/hold then goes into hold and
conversion is initiated. If the SWCON bit of the control register is set to 0, the
track/hold and conversion sequence are unaffected by WR operation.
Data I/O Lines
There are 12 data input/output lines on the AD7891. When the part is configured for parallel mode (MODE = 1), the output data
from the part is provided at these 12 pins during a read operation. For a write operation in parallel mode, these lines provide access
to the part’s control register.
Parallel Read Operation
During a parallel read operation, the 12 lines become the 12 data bits containing the conversion result from the AD7891. These
data bits are labelled Data Bit 0 (LSB) to Data Bit 11 (MSB). They are three-state, TTL compatible outputs. Output data coding
is twos complement when the data FORMAT bit of the control register is 1, and straight binary when the data FORMAT bit of
the control register is 0.
PLCC Pin No. MQFP Pin No. Mnemonic Description
13 to 18, 7 to 12, DB0 to DB11 Data Bit 0 (LSB) to Data Bit 11 (MSB). Three-state TTL compatible
21 to 26 15 to 20 outputs that are controlled by the CS and RD inputs.
Parallel Write Operation
During a parallel write operation, the following functions can be written to the control register via the 12 data input/output pins.
PLCC Pin No. MQFP Pin No. Mnemonic Description
23 17 A0 Address Input. The status of this input during a parallel write operation is
latched to the A0 bit of the control register (see Control Register section).
22 16 A1 Address Input. The status of this input during a parallel write operation is
latched to the A1 bit of the control register (see Control Register section).
21 15 A2 Address Input. The status of this input during a parallel write operation is
latched to the A2 bit of the control register (see Control Register section).
24 18 SWCON Software Conversion Start. The status of this input during a parallel write
operation is latched to the SWCONV bit of the control register (see Control
Register section).
25 19 SWSTBY Software Standby Control. The status of this input during a parallel write
operation is latched to the SWSTBY bit of the control register (see Control
Register section).
26 20 FORMAT Data Format Selection. The status of this input during a parallel write operation is
latched to the FORMAT bit of the control register (see Control Register section).
AD7891
–8– REV. D
SERIAL INTERFACE MODE FUNCTIONS
When the part is configured for serial mode (MODE = 0), five of the 12 data input/output lines provide serial interface functions.
These functions are outlined below.
PLCC Pin No. MQFP Pin No. Mnemonic Description
18 12 SCLK Serial Clock Input. This is an externally applied serial clock that is used to
load serial data to the control register and to access data from the
output register.
15 9 TFS Transmit Frame Synchronization Pulse. Active low logic input with serial
data expected after the falling edge of this signal.
16 10 RFS Receive Frame Synchronization Pulse. This is an active low logic input
with RFS provided externally as a strobe or framing pulse to access serial data
from the output register. For applications that require that data be transmitted
and received at the same time, RFS and TFS should be connected together.
21 15 DATA OUT Serial Data Output. Sixteen bits of serial data are provided with the
data FORMAT bit and the three address bits of the control register
preceding the 12 bits of conversion data. Serial data is valid on the falling
edge of SCLK for 16 edges after RFS goes low. Output conversion data
coding is twos complement when the FORMAT bit of the control register is
1 and straight binary when the FORMAT bit of the control register is 0.
17 11 DATA IN Serial Data Input. Serial data to be loaded to the control register is provided
at this input. The first six bits of serial data are loaded to the control
register on the first six falling edges of SCLK after TFS goes low. Serial
data on subsequent SCLK edges is ignored while TFS remains low.
13, 14 7, 8 TEST Test Pin. When the device is configured for serial mode of operation,
two of the pins which had been data inputs become test inputs. To ensure
correct operation of the device, both TEST inputs should be tied to a
logic low potential.
CONTROL REGISTER
The control register for the AD7891 contains six bits of information as described below. These six bits can be written to the control
register either in a parallel mode write operation or via a serial mode write operation. The default (power-on) condition of all bits in
the control register is 0. Six serial clock pulses must be provided to the part in order to write data to the control register. If TFS
returns high before six serial clock cycles, no data transfer takes place to the control register and the write cycle has to be restarted to
write data to the control register. However, if the SWCONV bit of the register was previously set to a Logic 1 and TFS is brought
high before six serial clock cycles, another conversion is initiated.
A2 Address Input. This input is the most significant address input for multiplexer channel selection.
A1 Address Input. This is the second most significant address input for multiplexer channel selection.
A0 Address Input. Least significant address input for multiplexer channel selection. When the address is written to
the control register, an internal pulse is initiated to allow for the multiplexer settling time and track/hold acquisi-
tion time before the track/hold goes into hold and conversion is initiated. When the internal pulse times out, the
track/hold goes into hold and conversion is initiated. The selected channel is given by the formula
AAA24 12 01¥+ ¥+ +
SWCONV Conversion Start. Writing a 1 to this bit initiates a conversion in a similar manner to the CONVST input. Con-
tinuous conversion starts do not take place when there is a 1 in this location. The internal pulse and the conver-
sion process are initiated when a 1 is written to this bit. With a 1 in this bit, the hardware conversion start, i.e.,
the CONVST input, is disabled. Writing a 0 to this bit enables the hardware CONVST input.
SWSTBY Standby Mode Input. Writing a 1 to this bit places the device in its standby or power-down mode. Writing a 0 to
this bit places the device in its normal operating mode.
FORMAT Data Format. Writing a 0 to this bit sets the conversion data output format to straight (natural) binary. This
data format is generally used for unipolar input ranges. Writing a 1 to this bit sets the conversion data output
format to twos complement. This output data format is generally used for bipolar input ranges.
)0BD(BSL
2A1A0AVNOCWSYBTSWSTAMROF
AD7891
–9–
REV. D
TERMINOLOGY
Signal-to-(Noise + Distortion) Ratio
This is the measured ratio of signal to (noise + distortion) at the
output of the ADC. The signal is the rms amplitude of the
fundamental. Noise is the rms sum of all nonfundamental
signals up to half the sampling frequency (f
S
/2), excluding dc.
The ratio is dependent upon the number of quantization levels
in the digitization process; the more levels, the smaller the quan-
tization noise. The theoretical signal-to-(noise +distortion) ratio
for an ideal N-bit converter with a sine wave input is given by
Signal-to-(Noise + Distortion) = (6.02N + 1.76) dB
Therefore, for a 12-bit converter, this is 74 dB.
Total Harmonic Distortion (THD)
THD is the ratio of the rms sum of harmonics to the
fundamental. For the AD7891, it is defined as
THD dB log
()
=++++
20
22222
VVVVV
V
23456
1
where V
1
is the rms amplitude of the fundamental and V
2
, V
3
,
V
4
, V
5
and V
6
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 f
S
/2 and excluding dc) to the rms value of the fundamental.
Normally, the value of this specification is determined by the larg-
est harmonic in the spectrum, but for parts 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 terms are those for
which neither m nor n are equal to zero. For example, the
second-order terms include (fa + fb) and (fa – fb), while the
third-order terms include (2fa + fb), (2fa – fb), (fa + 2fb), and
(fa – 2fb).
The AD7891 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- and third-order terms are of
different significance. The second-order terms are usually dis-
tanced 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- and-third order terms are
specified separately. The calculation of the intermodulation
distortion is as per the THD specification where it is the ratio of
the rms sum of the individual distortion products to the rms
amplitude of the fundamental expressed in dBs.
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 20 kHz (AD7891-1) or 100 kHz (AD7891-2) sine wave
signal to one input channel and determining how much that
signal is attenuated in each of the other channels. The figure
given is the worst case across all eight channels.
Relative Accuracy
Relative accuracy or endpoint nonlinearity is the maximum
deviation from a straight line passing through the endpoints of
the ADC transfer function.
Differential Nonlinearity
This is the difference between the measured and the ideal 1 LSB
change between any two adjacent codes in the ADC.
Positive Full-Scale Error (AD7891-1, ⴞ10 V and ⴞ5 V;
AD7891-2, ⴞ2.5 V)
This is the deviation of the last code transition (01. . .110 to
01. . .111) from the ideal 4 ¥ REF IN – 3/2 LSB (AD7891-1
±10 V range), 2 ¥ REF IN – 3/2 LSB (AD7891-1 ± 5V range),
or REF IN – 3/2 LSB (AD7891-2, ±2.5 V range), after the
bipolar zero error has been adjusted out.
Positive Full-Scale Error (AD7891-2, 0 V to 5 V and 0 V to 2.5 V)
This is the deviation of the last code transition (11. . .110 to
11. . .111) from the ideal 2 ¥ REF IN – 3/2 LSB (0 V to 5 V
range), or REF IN – 3/2 LSB (0 V to 2.5 V range), after the
unipolar offset error has been adjusted out.
Bipolar Zero Error (AD7891-1, ⴞ10 V and ⴞ5 V; AD7891-2, ⴞ2.5 V)
This is the deviation of the midscale transition (all 0s to all 1s)
from the ideal AGND – 1/2 LSB.
Unipolar Offset Error (AD7891-2, 0 V to 5 V and 0 V to 2.5 V)
This is the deviation of the first code transition (00. . .000 to
00. . .001) from the ideal AGND + 1/2 LSB.
Negative Full-Scale Error (AD7891-1, ⴞ10 V and ⴞ5 V;
AD7891-2, ⴞ2.5 V)
This is the deviation of the first code transition (10. . .000 to
10. . .001) from the ideal –4 ¥ REF IN + 1/2 LSB (AD7891-1
±10 V range), –2 ¥ REF IN + 1/2 LSB (AD7891-1 ± 5V range),
or –REF IN + 1/2 LSB (AD7891-2, ±2.5 V range), after bipolar
zero error has been adjusted out.
Track/Hold Acquisition Time
Track/hold acquisition time is the time required for the output of
the track/hold amplifier to reach its final value, within ±1/2 LSB,
after the end of conversion (the point at which the track/hold
returns to track mode). It also applies to situations where a
change in the selected input channel takes place or where there
is a step input change on the input voltage applied to the selected
V
IN
input of the AD7891. It means the user must wait for the
duration of the track/hold acquisition time after the end of
conversion or after a channel change/step input change to V
IN
before starting another conversion, to ensure the part operates
to specification.
AD7891
–10– REV. D
INTERFACE INFORMATION
The AD7891 provides two interface options, a 12-bit parallel
interface and a high speed serial interface. The required inter-
face mode is selected via the MODE pin. The two interface
modes are discussed in the following sections.
Parallel Interface Mode
The parallel interface mode is selected by tying the MODE
input to a logic high. Figure 2 shows a timing diagram illustrating
the operational sequence of the AD7891 in parallel mode for a
hardware conversion start. The multiplexer address is written to
the AD7891 on the rising edge of the WR input. The on-chip
track/hold goes into hold mode on the rising edge of CONVST;
conversion is also initiated at this point. When the conversion is
complete, the end of conversion line (EOC) pulses low to indi-
cate that new data is available in the AD7891’s output register.
This EOC line can be used to drive an edge-triggered interrupt
of a microprocessor. CS and RD going low accesses the 12-bit
conversion result. In systems where the part is interfaced to a
gate array or ASIC, this EOC pulse can be applied to the CS
and RD inputs to latch data out of the AD7891 and into the
gate array or ASIC. This means the gate array or ASIC does not
need any conversion status recognition logic, and it also elimi-
nates the logic required in the gate array or ASIC to generate
the read signal for the AD7891.
t6 t7
tCONV
t1 t5
t8
t1 t5
t2
VALID DATA
OUTPUT
VALID DATA
INPUT
t9 t10
t3
t4
CONVST (I)
EOC (O)
CS (O)
WR (I)
RD (I)
DB0 TO DB11
(I/O)
NOTE
I = INPUT
O = OUTPUT
Figure 2. Parallel Mode Timing Diagram
CONVERTER DETAILS
The AD7891 is an 8-channel, high speed, 12-bit data acquisi-
tion system. It provides the user with signal scaling, multiplexer,
track/hold, reference, ADC, and high speed parallel and serial
interface logic functions on a single chip. The signal condition-
ing on the AD7891-1 allows the part to accept analog input
ranges of ±5V or ±10 V when operating from a single supply.
The input circuitry on the AD7891-2 allows the part to handle
input signal ranges of 0 V to +2.5 V, 0 V to +5 V, and ±2.5 V
again while operating from a single 5 V supply. The part requires
a 2.5 V reference that can be provided from the part’s own internal
reference or from an external reference source.
Conversion is initiated on the AD7891 either by pulsing the
CONVST input or by writing a Logic 1 to the SWCONV bit of
the control register. When using the hardware CONVST input,
the on-chip track/hold goes from track to hold mode and the
conversion sequence is started on the rising edge of the CONVST
signal. When a software conversion start is initiated, an internal
pulse is generated, delaying the track/hold acquisition point and
the conversion start sequence until the pulse is timed out. This
internal pulse is initiated (goes from low to high) whenever a
write to the AD7891 control register takes place with a 1 in the
SWCONV bit. It then starts to discharge and the track/hold
cannot go into hold and conversion cannot be initiated until the
pulse signal goes low. The internal pulse duration is equal to the
track/hold acquisition time. This allows the user to obtain a
valid result after changing channels and initiating a conversion
in the same write operation.
The conversion clock for the part is internally generated and
conversion time for the AD7891 is 1.6 ms from the rising edge of
the hardware CONVST signal. The track/hold acquisition time
for the AD7891-1 is 600 ns, while the track/hold acquisition
time for the AD7891-2 is 400 ns. To obtain optimum perfor-
mance from the part, the data read operation should not occur
during the conversion or during the 100 ns prior to the next
conversion. This allows the AD7891-1 to operate at throughput
rates up to 454.5 kSPS and the AD7891-2 to operate at through-
put rates up to 500 kSPS in the parallel mode and achieve data
sheet specifications. In the serial mode, the maximum achievable
throughput rate for both the AD7891-1 and the AD7891-2 is
357 kSPS (assuming a 20 MHz serial clock).
All unused analog inputs should be tied to a voltage within the
nominal analog input range to avoid noise pickup. For mini-
mum power consumption, the unused analog inputs should be
tied to AGND.
AD7891
–11–
REV. D
Serial Interface Mode
The serial interface mode is selected by tying the MODE input
to a logic low. In this case, five of the data/control inputs of the
parallel mode assume serial interface functions.
The serial interface on the AD7891 is a 5-wire interface with
read and write capabilities, with data being read from the output
register via the DATA OUT line and data being written to the
control register via the DATA IN line. The part operates in a
slave or external clocking mode and requires an externally applied
serial clock to the SCLK input to access data from the data
register or write data to the control register. There are separate
framing signals for the read (RFS) and write (TFS) operations.
The serial interface on the AD7891 is designed to allow the part
to be interfaced to systems that provide a serial clock that is
synchronized to the serial data, such as the 80C51, 87C51,
68HC11, and 68HC05, and most digital signal processors.
When using the AD7891 in serial mode, the data lines DB11 to
DB10 should be tied to logic low, and the CS, WR, and RD
inputs should be tied to logic high. Pins DB4 to DB0 can be
tied to either logic high or logic low but must not be left floating
because this condition could cause the AD7891 to draw
large amounts of current.
Read Operation
Figure 3 shows the timing diagram for reading from the AD7891
in serial mode. RFS goes low to access data from the AD7891.
The serial clock input does not have to be continuous. The serial
data can be accessed in a number of bytes. However, RFS must
remain low for the duration of the data transfer operation. Six-
teen bits of data are transmitted in serial mode with the data
FORMAT bit first, followed by the three address bits in the
control register, followed by the 12-bit conversion result starting
with the MSB. Serial data is clocked out of the device on the
rising edge of SCLK and is valid on the falling edge of SCLK.
At the end of the read operation, the DATA OUT line is three-
stated by a rising edge on either the SCLK or RFS inputs, which-
ever occurs first.
Write Operation
Figure 4 shows a write operation to the control register of the
AD7891. The TFS input goes low to indicate to the part that a
serial write is about to occur. The AD7891 control register
requires only six bits of data. These are loaded on the first six
clock cycles of the serial clock with data on all subsequent clock
cycles being ignored. Serial data to be written to the AD7891
must be valid on the falling edge of SCLK.
Simplifying the Serial Interface
To minimize the number of interconnect lines to the AD7891
in serial mode, the user can connect the RFS and TFS lines
of the AD7891 together and read and write from the part simul-
taneously. In this case, a new control register data line selecting
the input channel and providing a conversion start command
should be provided on the DATA IN line, while the part pro-
vides the result from the conversion just completed on the
DATA OUT line.
DATA OUT (O)
SCLK (I)
RFS (I)
t18A
NOTE
I = INPUT
O = OUTPUT
FORMAT A2 A1 A0 DB11 DB10 DB0 THREE-STATE
t18
t16
t15
t14
t12
t11 t13 t17
Figure 3. Serial Mode Read Operation
DATA IN (I)
SCLK (I)
TFS (I)
NOTE
I = INPUT
FORMATA0A1A0
t
19
t
22
t
21
t
20
CONV STBY DON'T
CARE DON'T
CARE
Figure 4. Serial Mode Write Operation
AD7891
–12– REV. D
CIRCUIT DESCRIPTION
Reference
The AD7891 contains a single reference pin labeled REF OUT/
REF IN that either provides access to the part’s own 2.5 V
internal reference or to which an external 2.5 V reference can be
connected to provide the reference source for the part. The part
is specified with a 2.5 V reference voltage. Errors in the reference
source result in gain errors in the transfer function of the AD7891
and add to the specified full-scale errors on the part. They also
result in an offset error injected into the attenuator stage.
The AD7891 contains an on-chip 2.5 V reference. To use this
reference as a reference source for the AD7891, simply connect
a 0.1 mF disc ceramic capacitor from the REF OUT/REF IN pin
to REFGND. REFGND should be connected to AGND or the
analog ground plane. The voltage that appears at the REF OUT/
REF IN pin is internally buffered before being applied to the
ADC. If this reference is required for use external to the AD7891,
it should be buffered since the part has a FET switch in series
with the reference, resulting in a source impedance for this
output of 2 kW nominal. The tolerance of the internal reference
is ±10 mV at 25∞C with a typical temperature coefficient of
25 ppm/∞C and a maximum error over temperature of ±20 mV.
If the application requires a reference with a tighter tolerance
or if the AD7891 needs to be used with a system reference, an
external reference can be connected to the REF OUT/REF IN
pin. The external reference overdrives the internal reference
and thus provides the reference source for the ADC. The refer-
ence input is buffered before being applied to the ADC and
the maximum input current is ±100 mA. Suitable reference for
the AD7891 include the AD580, the AD680, the AD780, and
the REF43 precision 2.5 V references.
Analog Input Section
The AD7891 is offered as two part types: the AD7891-1 where
each input can be configured to have a ±10 V or a ±5 V input
range, and the AD7891-2 where each input can be configured
to have a 0 V to +2.5 V, 0 V to +5 V, and ±2.5 V input range.
AD7891-1
Figure 5 shows the analog input section of the AD7891-1. Each
input can be configured for ±5 V or ±10 V operation. For 5 V
operation, the V
INXA
and V
INXB
inputs are tied together and the
input voltage is applied to both. For ±10 V operation, the V
INXB
input is tied to AGND and the input voltage is applied to the
V
INXA
input. The V
INXA
and V
INXB
inputs are symmetrical and
fully interchangeable. Therefore, for ease of PCB layout on the
±10 V range, the input voltage may be applied to the V
INXB
input while the V
INXA
input is tied to AGND.
30k⍀
VINXA
VINXB
AGND
TO
MULTIPLEXER
AD7891-1
2k⍀
REF OUT/REF IN
TO ADC
REFERENCE CIRCUITRY
7.5k⍀
30k⍀
15k⍀
2.5V
REFERENCE
Figure 5. AD7891-1 Analog Input Structure
The input resistance for the ±5 V range is typically 20 kW. For
the ±10 V input range, the input resistance is typically 34.3 kW.
The resistor input stage is followed by the multiplexer, which is
followed by the high input impedance stage of the track/hold
amplifier.
The designed code transitions take place midway between suc-
cessive integer LSB values (i.e., 1/2 LSB, 3/2 LSBs, 5/2 LSBs).
LSB size is given by the formula 1 LSB = F
S
/4096. Therefore, for
the ±5 V range, 1 LSB = 10 V/4096 = 2.44 mV. For the ±10 V
range, 1 LSB = 20 V/4096 = 4.88 mV. Output coding is deter-
mined by the FORMAT bit of the control register. The ideal
input/output code transitions are shown in Table I.
AD7891-2
Figure 6 shows the analog input section of the AD7891-2. Each
input can be configured for input ranges of 0 V to +5 V, 0 V to +2.5 V,
or ±2.5 V. For the 0 V to 5 V input range, the V
INXB
input is
tied to AGND and the input voltage is applied to the V
INXA
input.
For the 0 V to 2.5 V input range, the V
INXA
and V
INXB
inputs
are tied together and the input voltage is applied to both. For
the ±2.5 V input range, the V
INXB
input is tied to 2.5 V and
the input voltage is applied to the V
INXA
input. The 2.5 V source
must have a low output impedance. If the internal reference on
the AD7891 is used, it must be buffered before being applied to
V
INXB
. The V
INXA
and V
INXB
inputs are symmetrical and fully
interchangeable. Therefore, for ease of PCB layout on the 0 V to +5 V
or ±2.5 V range, the input voltage may be applied to the V
INXB
input, while the V
INXA
input is tied to AGND or 2.5 V.
1.8k⍀
VINXA
VINXB
AGND
TO
MULTIPLEXER
AD7891-2
2k⍀
REF OUT/REF IN
TO ADC
REFERENCE
CIRCUITRY
1.8k⍀2.5V
REFERENCE
Figure 6. AD7891-2 Analog Input Structure
The input resistance for both the 0 V to +5 V and ±2.5 V ranges
is typically 3.6 kW. When an input is configured for 0 V to 2.5 V
operation, the input is fed into the high impedance stage of the
track/hold amplifier via the multiplexer and the two 1.8 kW
resistors in parallel.
The designed code transitions occur midway between successive
integer LSB values (i.e., 1/2 LSB, 3/2 LSBs, 5/2 LSBs). LSB size
is given by the formula 1 LSB = F
S
/4096. Therefore, for the 0 V
to 5 V range, 1 LSB = 5 V/4096 = 1.22 mV, for the 0 V to 2.5 V
range, 1 LSB = 2.5 V/4096 = 0.61 mV, and for the ±2.5 V range,
1 LSB = 5 V/4096 = 1.22 mV. Output coding is determined by
the FORMAT bit in the control register. The ideal input/output
code transitions for the ±2.5 V range are shown in Table I. The
ideal input/output code transitions for the 0 V to 5 V range and
the 0 V to 2.5 V range are shown in Table II.
AD7891
–13–
REV. D
Table I. Ideal Code Transition Table for the AD7891-1, ⴞ10 V and ⴞ5 V Ranges and the AD7891-2, ⴞ2.5 V Range
Digital Output Code Transition
1
Analog Input Input Voltage Twos Complement Straight Binary
+FSR
2
/2 – 3/2 LSB
3
(9.99268 V, 4.99634 V
or 2.49817 V)
4
011...110 to 011...111 111...110 to 111...111
+FSR/2 – 5/2 LSB (9.98779 V, 4.99390 V or 2.49695 V) 011...101 to 011...110 111...101 to 111...110
+FSR/2 – 7/2 LSB (9.99145 V, 4.99146 V or 2.49573 V) 011...100 to 011...101 111...100 to 111...101
AGND + 3/2 LSB (7.3242 mV, 3.6621 mV or 1.8310 mV) 000...001 to 000...010 100...001 to 100...010
AGND + 1/2 LSB (2.4414 mV, 1.2207 mV or 0.6103 mV) 000...000 to 000...001 100...000 to 100...001
AGND – 1/2 LSB (–2.4414 mV, –1.2207 mV or –0.6103 mV) 111...111 to 000...000 011...111 to 100...000
AGND – 3/2 LSB (–7.3242 mV, –3.6621 mV or –1.8310 mV) 111...110 to 111...111 011...110 to 011...111
–FSR/2 + 5/2 LSB (–9.98779 V, –4.99390 V or –2.49695 V) 100...010 to 100...011 000...010 to 000...011
–FSR/2 + 3/2 LSB (–9.99268 V, –4.99634 V or –2.49817 V) 100...001 to 100...010 000...001 to 000...010
–FSR/2 + 1/2 LSB (–9.99756 V, –4.99878 V or –2.49939 V) 100...000 to 100...001 000...000 to 000...001
NOTES
1
Output code format is determined by the FORMAT bit in the control register.
2
FSR is full-scale range and is +20 V for the ±10 V range, +10 V for the ±5 V range, and +5 V for the ±2.5 V range, with REF IN = +2.5 V.
3
1 LSB = FSR/4096 = +4.88 mV (±10 V range), +2.44 mV (±5 V range), and +1.22 mV (±2.5 V range), with REF IN = +2.5 V.
4
±10 V range, ±5 V range, or ±2.5 V range.
Table II. Ideal Code Transition Table for the AD7891-2, 0 V to 5 V and 0 V to 2.5 V Ranges
Digital Output Code Transition
1
Analog Input Input Voltage Twos Complement Straight Binary
+FSR
2
– 3/2 LSB
3
(4.99817 V or 2.49908 V)
4
011...110 to 011...111 111...110 to 111...111
+FSR – 5/2 LSB (4.99695 V or 2.49847 V) 011...101 to 011...110 111...101 to 111...110
+FSR – 7/2 LSB (4.99573 V or 2.49786 V) 011...100 to 011...101 111...100 to 111...101
AGND + 5/2 LSB (3.0518 mV or 1.52588 mV) 100...010 to 000...011 000...010 to 000...011
AGND + 3/2 LSB (1.83105 mV or 0.9155 mV) 100...001 to 000...010 000...001 to 000...010
AGND + 1/2 LSB (0.6103 mV or 0.3052 mV) 100...000 to 000...001 000...000 to 000...001
NOTES
1
Output code format is determined by the FORMAT bit in the control register.
2
FSR is the full-scale range and is 5 V for the 0 to 5 V range and 2.5 V for the 0 to 2.5 V range, with REF IN = 2.5 V.
3
1 LSB = F
S
/4096 = 1.22 mV (0 to 5 V range) or 610 mV (0 to 2.5 V range), with REF IN = 2.5 V.
4
0 V to 5 V range or 0 V to 2.5 V range.
Table III. Transfer Function M and N Values
Range Output Data Format M N
AD7891-1
±10 V Straight Binary 8 –4
±10 V Twos Complement 8 0
±5 V Straight Binary 4 –2
±5 V Twos Complement 4 0
AD7891-2
0 V to +5 V Straight Binary 2 0
0 V to +5 V Twos Complement 2 1
0 V to +2.5 V Straight Binary 1 0
0 V to +2.5 V Twos Complement 1 0.5
±2.5 V Straight Binary 2 –1
±2.5 V Twos Complement 2 0
Transfer Function of the AD7891-1 and AD7891-2
The transfer function of the AD7891-1 and AD7891-2 can be
expressed as
Input Voltage M REF IN D N REF IN=¥ ¥
()
+¥
()
/4096
D is the output data from the AD7891 and is in the range 0 to
4095 for straight binary encoding and from –2048 to +2047 for
twos complement encoding. Values for M depend upon the
input voltage range. Values for N depend upon the input voltage
range and the output data format. These values are given in
Table III. REF IN is the reference voltage applied to the AD7891.
AD7891
–14– REV. D
Track/Hold Amplifier
The track/hold amplifier on the AD7891 allows the ADC to
accurately convert an input sine wave of full-scale amplitude
to 12-bit accuracy. The input bandwidth of the track/hold is
greater than the Nyquist rate of the ADC even when the ADC is
operated at its maximum throughput rate of 454 kHz (AD7891-1)
or 500 kHz (AD7891-2). In other words, the track/hold amplifier
can handle input frequencies in excess of 227 kHz (AD7891-1)
or 250 kHz (AD7891-2).
The track/hold amplifier acquires an input signal in 600 ns
(AD7891-1) or 400 ns (AD7891-2). The operation of the track/
hold is essentially transparent to the user. The track/hold amplifier
goes from its tracking mode to its hold mode on the rising edge
of CONVST. The aperture time for the track/hold (i.e., the
delay between the external CONVST signal and the track/hold
actually going into hold) is typically 15 ns. At the end of conversion,
the part returns to its tracking mode. The track/hold starts acquiring
the next signal at this point.
STANDBY Operation
The AD7891 can be put into power save or standby mode by
using the STANDBY pin or the SWSTBY bit of the control
register. Normal operation of the AD7891 takes place when the
STANDBY input is at a Logic 1 and the SWSTBY bit is at a
Logic 0. When the STANDBY pin is brought low or a 1 is writ-
ten to the SWSTBY bit, the part goes into its standby mode of
operation, reducing its power consumption to typically 75 mW.
The AD7891 is returned to normal operation when the
STANDBY input is at a Logic 1 and the SWSTBY bit is a
Logic 0. The wake-up time of the AD7891 is normally determined
by the amount of time required to charge the 0.1 mF capacitor
between the REF OUT/REF IN pin and REF GND. If the
internal reference is being used as the reference source, this
capacitor is charged via a nominal 2 kW resistor. Assuming 10
time constants to charge the capacitor to 12-bit accuracy, this
implies a wake-up time of 2 ms.
If an external reference is used, this must be taken into account
when working out how long it will take to charge the capacitor.
If the external reference has remained at 2.5 V during the time
the AD7891 was in standby mode, the capacitor will already be
charged when the part is taken out of standby mode. Therefore,
the wake-up time is now the time required for the internal
circuitry of the AD7891 to settle to 12-bit accuracy. This typi-
cally takes 5 ms. If the external reference was also put into
standby then the wake-up time of the reference, combined with
the amount of time taken to recharge the reference capacitor
from the external reference, determines how much time must
elapse before conversions can begin again.
MICROPROCESSOR INTERFACING
AD7891 to 8X51 Serial Interface
A serial interface between the AD7891 and the 8X51
microcontroller is shown in Figure 7. TXD of the 8X51 drives
SCLK of the AD7891, while RXD transmits data to and
receives data from the part. The serial clock speed of the 8X51 is
slow compared to the maximum serial clock speed of the
AD7891, so maximum throughput of the AD7891 is not
achieved with this interface.
8X51*
DATA OUT
AD7891
*ADDITIONAL PINS OMITTED FOR CLARITY
SCLK
DATA IN
RFS
TFS
P3.4
P3.3
TXD
RXD
*
Figure 7. AD7891 to 8X51 Interface
The 8X51 provides the LSB of its SBUF register as the first bit
in the serial data stream. The AD7891 expects the MSB of the
6-bit write first. Therefore, the data in the SBUF register must
be arranged correctly so that this is taken into account. When
data is to be transmitted to the part, P3.3 is taken low. The
8X51 transmits its data in 8-bit bytes with only eight falling
clock edges occurring in the transmit cycle. One 8-bit transfer
is needed to write data to the control register of the AD7891.
After the data has been transferred, the P3.3 line is taken high
to complete the transmission.
When reading data from the AD7891, P3.4 of the 8X51 is taken
low. Two 8-bit serial reads are performed by the 8X51, and
P3.4 is taken high to complete the transfer. Again, the 8X51
expects the LSB first, while the AD7891 transmits MSB first, so
this must be taken into account in the 8X51 software.
No provision has been made in the given interface to determine
when a conversion has ended. If the conversions are initiated by
software, the 8X51 can wait a predetermined amount of time
before reading back valid data. Alternately, the falling edge of
the EOC signal can be used to initiate an interrupt service
routine that reads the conversion result from part to part.
AD7891 to 68HC11 Serial Interface
Figure 8 shows a serial interface between the AD7891 and the
68HC11 microcontroller. SCK of the 68HC11 drives SCLK of
the AD7891, the MOSI output drives DATA IN of the
AD7891, and the MISO input receives data from DATA OUT
of the AD7891. Ports PC6 and PC7 of the 68HC11 drive the
TFS and RFS lines of the AD7891, respectively.
For correct operation of this interface, the 68HC11 should be
configured such that its CPOL bit is a 1 and its CPHA bit is a 0.
When data is to be transferred to the AD7891, PC7 is taken
low. When data is to be received from the AD7891, PC6 is
taken low. The 68HC11 transmits and receives its serial data in
8-bit bytes, MSB first. The AD7891 also transmits and receives
data MSB first. Eight falling clock edges occur in a read or write
cycle from the 68HC11. A single 8-bit write with PC7 low is
required to write to the control register. When data has been
written, PC7 is taken high. When reading from the AD7891,
PC6 is left low after the first eight bits have been read. A second
byte of data is then transmitted serially from the AD7891. When
this transfer is complete, the PC6 line is taken high.
AD7891
–15–
REV. D
As in the 8X51 circuit in Figure 7, the way the 68HC11 is
informed that a conversion is completed is not shown in the
diagram. The EOC line can be used to inform the 68HC11
that a conversion is complete by using it as an interrupt signal.
The interrupt service routine reads in the result of the conver-
sion. If a software conversion start is used, the 68HC11 can
wait for 2.0 ms (AD7891-2) or 2.2 ms (AD7891-1) before read-
ing from the AD7891.
68HC11*
DATA OUT
AD7891*
*ADDITIONAL PINS OMITTED FOR CLARITY
SCLK
DATA IN
RFS
TFS
PC7
PC6
SCK
MOSI
MOSO
Figure 8. AD7891 to 68HC11 Interface
AD7891 to ADSP-21xx Serial Interface
An interface between the AD7891 and the ADSP-21xx is shown
in Figure 9. In the interface shown, either SPORT0 or SPORT1
can be used to transfer data to the AD7891. When reading
from the part, the SPORT must be set up with a serial word
length of 16 bits. When writing to the AD7891, a serial word
length of 6 bits or more can be used. Other setups for the
serial interface on the ADSP-21xx internal SCLK use alternate
framing mode and active low framing signal. Normally, the
EOC line from the AD7891 would be connected to the IRQ2
line of the ADSP-21xx to interrupt the DSP at the end of a
conversion (not shown in diagram).
ADSP-21xx*
DATA OUT
AD7891*
*ADDITIONAL PINS OMITTED FOR CLARITY
SCLK
DATA IN
RFS
TFS
RFS
TFS
SCLK
DT
DR
Figure 9. AD7891 to ADSP-21xx Serial Interface
AD7891 to DSP5600x Serial Interface
Figure 10 shows a serial interface between the AD7891 and the
DSP5600x series of DSPs. When reading from the AD7891, the
DSP5600x should be set up for 16-bit data transfers, MSB first,
normal mode synchronous operation, internally generated word
frame sync, and gated clock. When writing to the AD7891, 8-bit
or 16-bit data transfers can be used. The frame sync signal from
the DSP5600x must be inverted before being applied to the
RFS and TFS inputs of the AD7891, as shown in Figure 10.
To monitor the conversion time of the AD7891, a scheme such
as those outlined in previous interfaces with EOC can be used.
This can be implemented by connecting the EOC line directly
to the IRQA input of the DSP5600x.
DSP5600x*
DATA OUT
AD7891*
*ADDITIONAL PINS OMITTED FOR CLARITY
SCLK
DATA IN
RFS
TFS
FST (SC2)
SCK
STD
SRD
Figure 10. AD7891 to DSP5600x Serial Interface
AD7891 to TMS320xxx Serial Interface
The AD7891 can be interfaced to the serial port of TMS320xxx
DSPs, as shown in Figure 11. External timing generation circuitry
is necessary to generate the serial clock and syncs necessary for
the interface.
TMS320xxx*
DATA OUT
AD7891*
*ADDITIONAL PINS OMITTED FOR CLARITY
SCLK
DATA IN
RFS
TFS
FSX
CLKX
DX
DR
CLKR
FSR
TIMING
GENERATION
CIRCUITRY
Figure 11. AD7891 to TMS320xxx Serial Interface
AD7891
–16– REV. D
PARALLEL INTERFACING
The parallel port on the AD7891 allows the device to be interfaced
to microprocessors or DSP processors as a memory mapped
or I/O mapped device. The CS and RD inputs are common to
all memory peripheral interfacing. Typical interfaces to different
processors are shown in Figures 12 to 15. In all the interfaces
shown, an external timer controls the CONVST input of the
AD7891 and the EOC output interrupts the host DSP.
AD7891 to ADSP-21xx
Figure 12 shows the AD7891 interfaced to the ADSP-21xx
series of DSPs as a memory mapped device. A single wait state
may be necessary to interface the AD7891 to the ADSP-21xx
depending on the clock speed of the DSP. This wait state can
be programmed via the data memory wait state control register
of the ADSP-21xx (please see the ADSP-2100 family Users
Manual for details). The following instruction reads data
from the AD7891.
MR = DM (ADC)
where ADC is the address of the AD7891.
DATA BUS
ADDRESS BUS
DB11 TO DB0
AD7891*
*ADDITIONAL PINS OMITTED FOR CLARITY
CS
WR
IRQ2
D23 TO D8
EOC
RD
WR
RD
ADDR
DECODE
EN
DMS
ADSP-21xx*
A13 TO A0
Figure 12. AD7891 to ADSP-21xx Parallel Interface
AD7891 to TMS32020, TMS320C25, and TMS320C5x
Parallel interfaces between the AD7891 and the TMS32020,
TMS320C25, and TMS320C5x family of DSPs are shown in
Figure 13. The memory mapped address chosen for the
AD7891 should be chosen to fall in the I/O memory space of
the DSPs.
TMS320C25
ONLY
DATA BUS
ADDRESS BUS
DB11 TO DB0
AD7891*
*ADDITIONAL PINS OMITTED FOR CLARITY
CS
WR
INTx
D23 TO D0
EOC
RD
MSC
ADDR
DECODE
EN
IS
A15 TO A0
TMS32020/
TMS320C25/
TMS320C5x*
READY
R/W
STRB
Figure 13. AD7891 to TMS32020/TMS320C25/TMS320C5x
Parallel Interface
The parallel interface on the AD7891 is fast enough to interface
to the TMS32020 with no extra wait states. If high speed glue
logic, such as 74AS devices, are used to drive the WR and RD
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. In such a case,
this wait state can be generated using the single OR gate to
combine the CS and MSC signals to drive the READY line of
the TMS320C25, as shown in Figure 13. Extra wait states are
necessary when using the TMS320C5x at their fastest clock
speeds. Wait states can be programmed via the IOWSR and
CWSR registers (see the TMS320C5x User Guide for details).
Data is read from the ADC using the following instruction:
IN D, ADC
where D is the memory location where the data is to be stored,
and ADC is the I/O address of the AD7891.
AD7891 to TMS320C3x
Figure 14 shows a parallel interface between the AD7891 and
the TMS320C3x family of DSPs. The AD7891 is interfaced to
the expansion bus of the TMS320C3x. A single wait state is
required in this interface. This can be programmed using the
WTCNT bits of the expansion bus control register (see the
TMS320C3x Users Guide for details). Data from the AD7891
can be read using the following instruction:
LDI ARn Rx¥,
where ARn is an auxiliary register containing the lower 16 bits
of the address of the AD7891 in the TMS320C3x memory
space, and Rx is the register into which the ADC data is loaded.
EXPANSION DATA BUS
ADDRESS BUS
DB11 TO DB0
AD7891*
*ADDITIONAL PINS OMITTED FOR CLARITY
CS
WR
INTx
XD23 TO XD0
EOC
RD
ADDR
DECODE
XA15 TO XA0
XR/W
IOSTRB
TMS320C3x*
Figure 14. AD7891 to TMS320C3x Parallel Interface
AD7891
–17–
REV. D
AD7891 to DSP5600x
Figure 15 shows a parallel interface between the AD7891 and
the DSP5600x series of DSPs. The AD7891 should be mapped
into the top 64 locations of Y data memory. If extra wait states
are needed in this interface, they can be programmed using the
Port A Bus control register (see the DSP5600x Users Manual
for details). Data can be read from the AD7891 using the fol-
lowing instruction:
MOVEO Y: ADC, X0
where ADC is the address in the DSP5600x address space to
which the AD7891 has been mapped.
DATA BUS
ADDRESS BUS
DB11 TO DB0
AD7891*
*ADDITIONAL PINS OMITTED FOR CLARITY
CS
WR
IRQ
D23 TO D0
EOC
RD
WR
RD
ADDR
DECODE
DS
A15 TO A0
X/Y
DSP56000/
DSP56002*
Figure 15. AD7891 to DSP5600x Parallel Interface
Power Supply Bypassing and Grounding
In any circuit where accuracy is important, careful consideration
of the power supply and ground return layout helps to ensure
the specified performance. The PCB on which the AD7891 is
mounted should be designed such 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 separated
easily. A minimum etch technique is generally best for ground
planes because it gives the best shielding. Digital and analog
ground planes should be joined at only one place. If the AD7891
is the only device requiring an AGND to DGND connection,
then the ground planes should be connected at the AGND and
DGND pins of the AD7891. If the AD7891 is in a system where
multiple devices require an AGND to DGND connection, the
connection should still be made at one point only, a star ground
point established as close as possible to the AD7891.
Digital lines running under the device should be avoided because
these couple noise onto the die. The analog ground plane should
be allowed to run under the AD7891 to avoid noise coupling.
The power supply lines of the AD7891 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 sig-
nals like clocks should be shielded with digital ground to avoid
radiating noise to other parts of the board and should never be
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
technique but is not always possible with a double-sided board.
In this technique, the component side of the board is dedicated
to ground plane while signal traces are placed on the solder side.
The AD7891 should have ample supply bypassing located as close
to the package as possible, ideally right up against the device.
One of the V
DD
pins (Pin 10 of the PLCC package and Pin 4
on the MQFP package) mainly drives the analog circuitry on
the chip. This pin should be decoupled to the analog ground
plane with a 10 mF tantalum bead capacitor in parallel with a
0.1 mF capacitor. The other V
DD
pin (Pin 19 on the PLCC
package and Pin 13 on the MQFP package) mainly drives
digital circuitry on the chip. This pin should be decoupled to the
digital ground plane with a 0.1 mF capacitor. The 0.1 mF
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. Figure 16 shows the
recommended decoupling scheme.
VDD (PIN 10, PLCC
PIN 4, MQFP)
DGND
AD7891
AGND
AGND
VDD (PIN 19, PLCC
PIN 13, MQFP)
10␮F0.1␮F
0.1␮F
Figure 16. Recommended Decoupling Scheme for
the AD7891
AD7891
–18– REV. D
AD7891 PERFORMANCE
Linearity
The linearity of the AD7891 is primarily determined by the
on-chip 12-bit DAC. This is a segmented DAC that is laser
trimmed for 12-bit integral linearity and differential linearity.
Typical INL for the AD7891 is ±0.25 LSB while typical DNL
is ±0.5 LSB.
Noise
In an ADC, noise exhibits itself as code uncertainty in dc appli-
cations and as the noise floor (in an FFT for example) in ac
applications. In a sampling ADC, such as the AD7891, all
information about the analog input appears in the baseband from
dc to half the sampling frequency. The input bandwidth of the
track/hold amplifier exceeds the Nyquist bandwidth and,
therefore, an antialiasing filter should be used to remove
unwanted signals above f
S
/2 in the input signal in applications
where such signals exist.
Figure 17 shows a histogram plot for 16384 conversions of a dc
input signal using the AD7891-1. The analog input was set at
the center of a code transition in the following way. An initial dc
input level was selected and a number of conversions were
made. The resulting histogram was noted and the applied level
was adjusted so that only two codes were generated with an
equal number of occurrences. This indicated that the transition
point between the two codes had been found. The voltage level
at which this occurred was recorded. The other edge of one of
these two codes was then found in a similar manner. The dc
level for the center of code could then be calculated as the
average of the two transition levels. The AD7891-1 inputs
were configured for the ±5 V input range and the data was read
from the part in parallel mode after conversion. Similar results
have been found with the AD7891-1 on the ±10 V range and on
all input ranges of the AD7891-2. The same performance is
achieved in serial mode, again with the data read from the
AD7891-1 after conversion. All the codes, except for 3, appear
in one output bin, indicating excellent noise performance from
the ADC.
OUTPUT CODE
18000
16000
02148 2149
NUMBER OF OCCURRENCES
2150
8000
6000
4000
2000
12000
10000
14000
16381 CODES
1 CODE 2 CODES
Figure 17. Typical Histogram Plot (AD7891-1)
Dynamic Performance
The AD7891 contains an on-chip track/hold amplifier, allowing
the part to sample input signals of up to 250 kHz on any of its
input channels. Many of the AD7891’s applications require it to
sequence through low frequency input signals across its eight
channels. There may be some applications, however, for which
the dynamic performance of the converter on signals of up to
250 kHz input frequency is of interest. It is recommended for
these wider bandwidth signals that the hardware conversion
start method of sampling is used.
These applications require information on the spectral content
of the input signal. Signal-to-(noise + distortion), total
harmonic distortion, peak harmonic or spurious tone, and
intermodulation distortion are all specified. Figure 18 shows a
typical FFT plot of a 10 kHz, ±10 V input after being digitized
by the AD7891-1 operating at 500 kHz, with the input connected
for ±10 V operation. The signal-to-(noise + distortion) ratio is
72.2 dB and the total harmonic distortion is –87 dB. Figure 19
shows a typical FFT plot of a 100 kHz, 0 V to 5 V input after
being digitized by the AD7891-2 operating at 500 kHz, with the
input connected for 0 V to 5 V operation. The signal-to-(noise +
distortion) ratio is 71.17 dB and the total harmonic distortion
is –82.3 dB. It should be noted that reading from the part
during conversion does have a significant impact on dynamic
performance. Therefore, for sampling applications, it is
recommended not to read during conversion.
0
–30
–150
dB
–60
–90
–120
FS/2
2048 POINT FFT
SNR = 72.2dB
Figure 18. Typical AD7891-1 FFT Plot
0
–30
–150
dB
–60
–90
–120
FS/2
2048 POINT FFT
SNR = 71.17dB
Figure 19. Typical AD7891-2 FFT Plot
AD7891
–19–
REV. D
Effective Number of Bits
The formula for signal-to-(noise + distortion) ratio (see Terminology
section) is related to the resolution or number of bits of the
converter. Rewriting the formula gives a measure of performance
expressed in effective number of bits (ENOB).
ENOB SNR=-
()
176 602./.
where SNR is the signal-to-(noise + distortion) ratio.
The effective number of bits for a device can be calculated from
its measured SNR. Figure 20 shows a typical plot of effective
number of bits versus frequency for the AD7891-1 and the
AD7891-2 from dc to 200 kHz. The sampling frequency is
500 kHz. The AD7891-1 inputs were configured for ±10 V
operation. The AD7891-2 inputs were configured for 0 to 5 V
operation. The AD7891-1 plot only goes to 100 kHz as a
±10 V sine wave of sufficient quality was unavailable at higher
frequencies.
Figure 20 shows that the AD7891-1 converts an input sine wave of
100 kHz to an effective number of bits of 11 which equates to a
signal-to-(noise + distortion) level of 68.02 dBs. The AD7891-2
converts an input sine wave of 200 kHz to an effective number
of bits of 11.07, which equates to a signal-to-(noise + distortion)
level of 68.4 dBs.
FREQUENCY – kHz
12.0
11.9
11.1
40 80 120
11.5
11.4
11.3
11.2
11.7
11.6
11.8
20060100 140 160 180 200
11.0
EFFECTIVE NUMBER OF BITS
AD7891-2 ENOB
AD7891-1 ENOB
Figure 20. Effective Number of Bits vs. Frequency
44-Lead Plastic Leaded Chip Carrier [PLCC]
(P-44A)
Dimensions shown in inches and (millimeters)
BOTTOM VIEW
(PINS UP)
6
PIN 1
IDENTIFIER
7
40
39
17
18
29
28
TOP VIEW
(PINS DOWN)
0.656 (16.66)
0.650 (16.51) SQ
0.048 (1.22)
0.042 (1.07)
0.050
(1.27)
BSC
0.695 (17.65)
0.685 (17.40) SQ
0.048 (1.22)
0.042 (1.07)
0.021 (0.53)
0.013 (0.33)
0.630 (16.00)
0.590 (14.99)
0.032 (0.81)
0.026 (0.66)
0.180 (4.57)
0.165 (4.19)
0.056 (1.42)
0.042 (1.07) 0.020 (0.51)
MIN
0.120 (3.05)
0.090 (2.29)
0.040 (1.01)
0.025 (0.64) R
COMPLIANT TO JEDEC STANDARDS MO-047AC
CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETER DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN
OUTLINE DIMENSIONS
AD7891
–20– REV. D
Revision History
Location Page
4/04—Data Sheet changed from REV. C to REV. D.
Changes to SPECIFICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Changes to ORDERING GUIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Changes to PIN FUNCTION DESCRIPTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Changes to PARALLEL INTERFACE MODE FUNCTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Changes to SERIAL INTERFACE MODE FUNCTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Changes to CONTROL REGISTER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Changes to AD7891 to 8X51 Serial Interface section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Changes to Figure 10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Changes to Figure 11 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Changes to Power Supply Bypassing and Grounding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Changes to Figure 16 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Updated OUTLINE DIMENSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
01/02—Data Sheet changed from REV. B to REV. C.
Changed page 7 to page 6 and moved page 6 to page 9 to keep Pin Configurations together with Pin Function descriptions.
Edits to CONTROL REGISTER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Text added to CONVERTER DETAILS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Edits to Figure 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
02/01—Data Sheet changed from REV. A to REV. B.
PQFP designation changed to MQFP throughout.
Edit to FEATURES, Single Supply Low Power Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Edits to mW (90 to 82) in last paragraph of left hand column . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Edits to POWER REQUIREMENTS section of Specifications table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
OUTLINE DIMENSIONS
44-Lead Metric Quad Flat Package [MQFP]
(S-44-2)
Dimensions shown in millimeters
0.80
BSC
0.45
0.30
2.45
MAX
1.03
0.88
0.73
SEATING
PLANE
TOP VIEW
(PINS DOWN)
1
33
34
11
12
23
22
44
COPLANARITY
0.10
PIN 1
0.25 MIN
VIEW A
ROTATED 90ⴗ CCW
7ⴗ
0ⴗ
2.10
2.00
1.95
VIEW A
13.90
BSC SQ
10.00
BSC SQ
COMPLIANT TO JEDEC STANDARDS MO-112-AA-1
C01358–0–4/04(D)