ADS8320
DESCRIPTION
The ADS8320 is a 16-bit sampling analog-to-digital
converter (A/D) with guaranteed specifications over a
2.7V to 5.25V supply range. It requires very little
power even when operating at the full 100kHz data
rate. At lower data rates, the high speed of the device
enables it to spend most of its time in the power-down
mode—the average power dissipation is less than
100µW at 10kHz data rate.
The ADS8320 also features operation from 2.0V to
5.25V, a synchronous serial (SPI/SSI compatible) in-
terface, and a differential input. The reference voltage
can be set to any level within the range of 500mV to
VCC.
Ultra-low power and small size make the ADS8320
ideal for portable and battery-operated systems. It is
also a perfect fit for remote data acquisition mod-
ules, simultaneous multi-channel systems, and iso-
lated data acquisition. The ADS8320 is available in
an MSOP-8 package.
16-Bit, High-Speed, 2.7V to 5V
micro
Power Sampling
ANALOG-TO-DIGITAL CONVERTER
®
©1999 Burr-Brown Corporation PDS-1504C Printed in U.S.A. May, 2000
FEATURES
●100kHz SAMPLING RATE
●MICRO POWER:
1.8mW at 100kHz and 2.7V
0.3mW at 10kHz and 2.7V
●POWER DOWN: 3µA max
●MSOP-8 PACKAGE
●PIN-COMPATIBLE TO ADS7816 AND
ADS7822
●SERIAL (SPI/SSI) INTERFACE
SAR Control
Serial
Interface
D
OUT
Comparator
S/H Amp CS/SHDN
DCLOCK
+In
V
REF
–In CDAC
APPLICATIONS
●BATTERY OPERATED SYSTEMS
●REMOTE DATA ACQUISITION
●ISOLATED DATA ACQUISITION
●SIMULTANEOUS SAMPLING,
MULTI-CHANNEL SYSTEMS
●INDUSTRIAL CONTROLS
●ROBOTICS
●VIBRATION ANALYSIS
International Airport Industrial Park • Mailing Address: PO Box 11400, Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd., Tucson, AZ 85706 • Tel: (520) 746-1111
Twx: 910-952-1111 • Internet: http://www.burr-brown.com/ • Cable: BBRCORP • Telex: 066-6491 • FAX: (520) 889-1510 • Immediate Product Info: (800) 548-6132
For most current data sheet and other product
information, visit www.burr-brown.com
2
®
ADS8320
SPECIFICATIONS: +VCC = +5V
At –40°C to +85°C, VREF = +5V,–IN = GND, fSAMPLE = 100kHz, and fCLK = 24 • fSAMPLE, unless otherwise specified.
The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN
assumes no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject
to change without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not
authorize or warrant any BURR-BROWN product for use in life support devices and/or systems.
ADS8320E ADS8320EB
PARAMETER CONDITIONS MIN TYP MAX MIN TYP MAX UNITS
RESOLUTION 16 ✻Bits
ANALOG INPUT
Full-Scale Input Span +In – (–In) 0 VREF ✻✻V
Absolute Input Range +In –0.1
VCC + 0.1
✻✻V
–In –0.1 +1.0 ✻✻V
Capacitance 45 ✻pF
Leakage Current 1✻nA
SYSTEM PERFORMANCE
No Missing Codes 14 15 Bits
Integral Linearity Error ±0.008 ±0.018 ±0.006 ±0.012
% of FSR
Offset Error ±1±2±0.5 ±1mV
Offset Temperature Drift ±3✻µV/°C
Gain Error ±0.05 ±0.024 %
Gain Temperature Drift ±0.3 ✻ppm/°C
Noise 20 ✻µVrms
Power Supply Rejection Ratio +4.7V < VCC < 5.25V 3 ✻LSB(1)
SAMPLING DYNAMICS
Conversion Time 16 ✻
Clk Cycles
Acquisition Time 4.5 ✻
Clk Cycles
Throughput Rate 100 ✻kHz
Clock Frequency Range 0.024 2.9 ✻✻MHz
DYNAMIC CHARACTERISTICS
Total Harmonic Distortion VIN = 5Vp-p at 10kHz –84 –86 dB
SINAD VIN = 5Vp-p at 10kHz 82 84 dB
Spurious Free Dynamic Range VIN = 5Vp-p at 10kHz 84 86 dB
SNR 90 92 dB
REFERENCE INPUT
Voltage Range 0.5 VCC ✻✻V
Resistance
CS = GND, fSAMPLE = 0Hz
5✻GΩ
CS = VCC 5✻GΩ
Current Drain 40 80 ✻✻µA
fSAMPLE = 10kHz 0.8 ✻µA
CS = VCC 0.1
3
✻µA
DIGITAL INPUT/OUTPUT
Logic Family CMOS ✻
Logic Levels:
VIH IIH = +5µA 3.0
VCC + 0.3
✻✻V
VIL IIL = +5µA –0.3 0.8 ✻✻V
VOH IOH = –250µA 4.0 ✻V
VOL IOL = 250µA 0.4 ✻V
Data Format Straight Binary ✻
POWER SUPPLY REQUIREMENTS
VCC Specified Performance 4.75 5.25 ✻✻V
VCC Range(2) 2.0 5.25 ✻✻V
Quiescent Current 900 1700 ✻✻µA
fSAMPLE = 10kHz(3, 4) 200 ✻µA
Power Dissipation 4.5 8.5 ✻✻mW
Power Down CS = VCC 0.3 3 ✻✻µA
TEMPERATURE RANGE
Specified Performance –40 +85 ✻✻°C
✻ Specifications same as ADS8320E.
NOTES: (1) LSB means Least Significant Bit. (2) See Typical Performance Curves for more information. (3) f CLK = 2.4MHz, CS = VCC for 216 clock cycles out
of every 240. (4) See the Power Dissipation section for more information regarding lower sample rates.
3
®
ADS8320
SPECIFICATIONS: +VCC = +2.7V
At –40°C to +85°C, VREF = 2.5V, –IN = GND, fSAMPLE = 100kHz, and fCLK = 24 • fSAMPLE, unless otherwise specified.
ADS8320E ADS8320EB
PARAMETER CONDITIONS MIN TYP MAX MIN TYP MAX UNITS
RESOLUTION 16 ✻Bits
ANALOG INPUT
Full-Scale Input Span +In – (–In) 0 VREF ✻✻V
Absolute Input Range +In –0.1
VCC + 0.1
✻✻V
–In –0.1 +0.5 ✻✻V
Capacitance 45 ✻pF
Leakage Current 1✻nA
SYSTEM PERFORMANCE
No Missing Codes 14 15 Bits
Integral Linearity Error ±0.008 ±0.018 ±0.006 ±0.012
% of FSR
Offset Error ±1±2±0.5 ±1mV
Offset Temperature Drift ±3✻µV/°C
Gain Error ±0.05 ±0.024 % of FSR
Gain Temperature Drift ±0.3 ✻ppm/°C
Noise 20 ✻µVrms
Power Supply Rejection Ratio +2.7V < VCC < +3.3V 3 ✻LSB(1)
SAMPLING DYNAMICS
Conversion Time 16 ✻
Clk Cycles
Acquisition Time 4.5 ✻
Clk Cycles
Throughput Rate 100 ✻kHz
Clock Frequency Range 0.024 2.4 ✻✻MHz
DYNAMIC CHARACTERISTICS
Total Harmonic Distortion VIN = 2.7Vp-p at 1kHz –86 –88 dB
SINAD VIN = 2.7Vp-p at 1kHz 84 86 dB
Spurious Free Dynamic Range VIN = 2.7Vp-p at 1kHz 86 88 dB
SNR 88 90 dB
REFERENCE INPUT
Voltage Range 0.5 VCC ✻✻V
Resistance
CS = GND, fSAMPLE = 0Hz
5✻GΩ
CS = VCC 5✻GΩ
Current Drain 20 50 ✻✻µA
CS = VCC 0.1
3
✻✻µA
DIGITAL INPUT/OUTPUT
Logic Family CMOS ✻
Logic Levels:
VIH IIH = +5µA 2.0
VCC + 0.3
✻✻V
VIL IIL = +5µA –0.3 0.8 ✻✻V
VOH IOH = –250µA 2.1 ✻V
VOL IOL = 250µA 0.4 ✻V
Data Format Straight Binary ✻
POWER SUPPLY REQUIREMENTS
VCC Specified Performance 2.7 3.3 ✻✻V
VCC Range(3) 2.0 5.25 ✻✻V
See Note 2 2.0 2.7 ✻✻V
Quiescent Current 650 1300 ✻✻µA
fSAMPLE = 10kHz(4,5) 100 ✻µA
Power Dissipation 1.8 3.8 ✻✻mW
Power Down CS = VCC 0.3 3 ✻✻µA
TEMPERATURE RANGE
Specified Performance –40 +85 ✻✻°C
✻ Specifications same as ADS8320E.
Notes: (1) LSB means Least Significant Bit. With VREF equal to +5V, one LSB is 0.039mV. (2) The maximum clock rate of the ADS8320 is less than 2.4MHz
in this power supply range. (3) See the Typical Performance Curves for more information. (4) f CLK = 2.4MHz, CS = VCC for 216 clock cycles out of every 240.
(5) See the Power Dissipation section for more information regarding lower sample rates.
4
®
ADS8320
ELECTROSTATIC
DISCHARGE SENSITIVITY
Electrostatic discharge can cause damage ranging from per-
formance degradation to complete device failure. Burr-
Brown Corporation recommends that all integrated circuits
be handled and stored using appropriate ESD protection
methods.
ESD damage can range from subtle performance degrada-
tion to complete device failure. Precision integrated circuits
may be more susceptible to damage because very small
parametric changes could cause the device not to meet
published specifications.
ABSOLUTE MAXIMUM RATINGS(1)
VCC .......................................................................................................+6V
Analog Input ..............................................................–0.3V to (VCC + 0.3V)
Logic Input ...............................................................................–0.3V to 6V
Case Temperature ......................................................................... +100°C
Junction Temperature .................................................................... +150°C
Storage Temperature..................................................................... +125°C
External Reference Voltage .............................................................. +5.5V
NOTE: (1) Stresses above these ratings may permanently damage the device.
PIN NAME DESCRIPTION
1V
REF Reference Input.
2 +In Non Inverting Input.
3 –In Inverting Input. Connect to ground or to remote
ground sense point.
4 GND Ground.
5 CS/SHDN Chip Select when LOW, Shutdown Mode when
HIGH.
6D
OUT The serial output data word is comprised of 16
bits of data. In operation the data is valid on the
falling edge of DCLOCK. The
second clock pulse after the falling edge of CS
enables the serial output. After one null bit the
data is valid for the next 16 edges.
7 DCLOCK Data Clock synchronizes the serial data transfer
and determines conversion speed.
8+V
CC Power Supply.
PIN ASSIGNMENTS
PIN CONFIGURATION
Top View MSOP
1
2
3
4
8
7
6
5
+V
CC
DCLOCK
D
OUT
CS/SHDN
V
REF
+In
–In
GND
ADS8320
PACKAGE/ORDERING INFORMATION
MAXIMUM NO
INTEGRAL MISSING
LINEARITY CODE PACKAGE SPECIFICATION
ERROR ERROR DRAWING TEMPERATURE PACKAGE ORDERING TRANSPORT
PRODUCT (%) (LSB) PACKAGE NUMBER(1) RANGE MARKING(2) NUMBER(3) MEDIA
ADS8320E 0.018 14 MSOP-8 337 –40°C to +85°C A20 ADS8320E/250 Tape and Reel
ADS8320E """"""ADS8320E/2K5 Tape and Reel
ADS8320EB 0.012 15 MSOP-8 337 –40°C to +85°C A20 ADS8320EB/250 Tape and Reel
ADS8320EB """"""ADS8320EB/2K5 Tape and Reel
NOTE: (1) For detail drawing and dimension table, please see end of data sheet or Package Drawing File on Web. (2) Performance Grade information is marked
on the reel. (3) Models with a slash(/) are available only in Tape and reel in quantities indicated (e.g. /250 indicates 250 units per reel, /2K5 indicates 2500 devices
per reel). Ordering 2500 pieces of ”ADS8320E/2K5“ will get a single 2500-piece Tape and Reel. For detailed Tape and Reel mechanical information, refer to the
www.burr-brown.com web site under Applications and Tape and Reel Orientation and Dimensions.
5
®
ADS8320
TYPICAL PERFORMANCE CURVES
At TA = +25°C, VCC = +5V, VREF = +5V, fSAMPLE = 100kHz, fCLK = 24 • fSAMPLE, unless otherwise specified.
INTEGRAL LINEARITY ERROR vs CODE (+25°C)
2 0
1.0
0.0
–1.0
–2.0
–3.0
–4.0
–5.0
–6.0
Integral Linearity Error (LSB)
0000
H
8000
H
C000
H
4000
H
FFFF
H
Hex Code
DIFFERENTIAL LINEARITY ERROR vs CODE (+25°C)
3.0
2.0
1.0
0.0
–1.0
–2.0
–3.0
Differential Linearity Error (LSB)
0000
H
8000
H
C000
H
4000
H
FFFF
H
Hex Code
SUPPLY CURRENT vs TEMPERATURE
1200
1000
800
600
400
200
0
Supply Current (µA)
–50 –25 0 25 50 75 100
Temperature (°C)
2.7V
5V
POWER DOWN SUPPLY CURRENT
vs TEMPERATURE
600
500
400
300
200
100
0
Supply Current (nA)
–50 –25 0 25 50 75 100
Temperature (°C)
5V
QUIESCENT CURRENT vs V
CC
1200
1000
800
600
400
200
Quiescent Current (µA)
12345
V
CC
(V)
MAXIMUM SAMPLE RATE vs V
CC
1000
100
10
1
Sample Rate (kHz)
12345
V
CC
(V)
6
®
ADS8320
TYPICAL PERFORMANCE CURVES (Cont.)
At TA = +25°C, VCC = +2.7V, VREF = +2.5V, fSAMPLE = 100kHz, fCLK = 24 • fSAMPLE, unless otherwise specified.
CHANGE IN OFFSET vs REFERENCE VOLTAGE
6
5
4
3
2
1
0
–1
–2
–3
Change in Offset (LSB)
12345
Reference Voltage (V)
V
CC
= 5V
CHANGE IN OFFSET vs TEMPERATURE
3
2
1
0
–1
–2
–3
Delta from 25°C (LSB)
–50 –25 0 25 50 75 100
Temperature (°C)
5V
2.7V
CHANGE IN GAIN vs REFERENCE VOLTAGE
5
4
3
2
1
0
–1
–2
Change in Gain (LSB)
12345
Reference Voltage (V)
V
CC
= 5V
FREQUENCY SPECTRUM
(8192 Point FFT, F
IN
= 10.120kHz, –0.3dB)
0
–20
–40
–60
–80
–100
–120
–140
Amplitude (dB)
0 102030 4050
Frequency (kHz)
PEAK-TO-PEAK NOISE vs REFERENCE VOLTAGE
10
9
8
7
6
5
4
3
2
1
0
Peak-to-Peak Noise (LSB)
0.1 1 10
Reference Voltage (V)
V
CC
= 5V
CHANGE IN GAIN vs TEMPERATURE
6
4
2
0
–2
–4
–6
Delta from 25°C (LSB)
–50 –25 0 25 50 75 100
Temperature (°C)
5V
2.7V
7
®
ADS8320
TYPICAL PERFORMANCE CURVES (Cont.)
At TA = +25°C, VCC = +5V, VREF = +5V, fSAMPLE = 100kHz, fCLK = 24 • fSAMPLE, unless otherwise specified.
TOTAL HARMONIC DISTORTION vs FREQUENCY
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
Total Harmonic Distortion (dB)
1 10 100
Frequency (kHz)
SIGNAL-TO-(NOISE + DISTORTION) vs FREQUENCY
100
90
80
70
60
50
40
30
20
10
0
Signal-to-(Noise + Distortion) (dB)
1 10 50 100
Frequency (kHz)
SIGNAL-TO-(NOISE + DISTORTION) vs INPUT LEVEL
90
80
70
60
50
40
30
20
Signal-to-(Noise + Distortion) (dB)
–40 –35 –30 –25 –20 –15 –10 –5 0
Input Level (dB)
REFERENCE CURRENT vs SAMPLE RATE
70
60
50
40
30
20
10
0
Reference Current (µA)
0 20 40 60 80 100
Sample Rate (kHz)
5V
2.7V
REFERENCE CURRENT vs TEMPERATURE
70
60
50
40
30
20
10
Reference Current (µA)
–50 –25 0 25 50 75 100
Temperature (°C)
5V
2.7V
SPURIOUS FREE DYNAMIC RANGE AND
SIGNAL-TO-NOISE RATIO vs FREQUENCY
100
90
80
70
60
50
40
30
20
10
0
Spurious Free Dynamic Range
and Signal-to-Noise Ratio (dB)
1 10 10050
Frequency (kHz)
Signal-to-Noise Ratio
Spurious Free Dynamic Range
8
®
ADS8320
THEORY OF OPERATION
The ADS8320 is a classic successive approximation register
(SAR) analog-to-digital (A/D) converter. The architecture is
based on capacitive redistribution which inherently includes
a sample/hold function. The converter is fabricated on a 0.6µ
CMOS process. The architecture and process allow the
ADS8320 to acquire and convert an analog signal at up to
100,000 conversions per second while consuming less than
4.5mW from +VCC.
The ADS8320 requires an external reference, an external
clock, and a single power source (VCC). The external refer-
ence can be any voltage between 500mV and VCC. The value
of the reference voltage directly sets the range of the analog
input. The reference input current depends on the conversion
rate of the ADS8320.
The external clock can vary between 24kHz (1kHz through-
put) and 2.4MHz (100kHz throughput). The duty cycle of
the clock is essentially unimportant as long as the minimum
high and low times are at least 200ns (VCC = 2.7V or
greater). The minimum clock frequency is set by the leakage
on the capacitors internal to the ADS8320.
The analog input is provided to two input pins: +In and –In.
When a conversion is initiated, the differential input on these
pins is sampled on the internal capacitor array. While a
conversion is in progress, both inputs are disconnected from
any internal function.
The digital result of the conversion is clocked out by the
DCLOCK input and is provided serially, most significant bit
first, on the DOUT pin. The digital data that is provided on the
DOUT pin is for the conversion currently in progress—there
is no pipeline delay. It is possible to continue to clock the
ADS8320 after the conversion is complete and to obtain the
serial data least significant bit first. See the digital timing
section for more information.
ANALOG INPUT
The +In and –In input pins allow for a differential input
signal. Unlike some converters of this type, the –In input is
not re-sampled later in the conversion cycle. When the
converter goes into the hold mode, the voltage difference
between +In and –In is captured on the internal capacitor
array.
The range of the –In input is limited to –0.1V to +1V (–0.1V
to +0.5V when using a 2.7V supply). Because of this, the
differential input can be used to reject only small signals that
are common to both inputs. Thus, the –In input is best used
to sense a remote signal ground that may move slightly with
respect to the local ground potential.
The input current on the analog inputs depends on a number
of factors: sample rate, input voltage, source impedance, and
power-down mode. Essentially, the current into the ADS8320
charges the internal capacitor array during the sample pe-
riod. After this capacitance has been fully charged, there is
no further input current. The source of the analog input
voltage must be able to charge the input capacitance (45pF)
to a 16-bit settling level within 4.5 clock cycles. When the
converter goes into the hold mode or while it is in the power-
down mode, the input impedance is greater than 1GΩ.
Care must be taken regarding the absolute analog input
voltage. To maintain the linearity of the converter, the –In
input should not drop below GND – 100mV or exceed
GND + 1V. The +In input should always remain within the
range of GND – 100mV to VCC + 100mV. Outside of these
ranges, the converter’s linearity may not meet specifications.
To minimize noise, low bandwidth input signals with low-
pass filters should be used.
REFERENCE INPUT
The external reference sets the analog input range. The
ADS8320 will operate with a reference in the range of
500mV to VCC. There are several important implications of
this.
As the reference voltage is reduced, the analog voltage
weight of each digital output code is reduced. This is often
referred to as the Least Significant Bit (LSB) size and is
equal to the reference voltage divided by 65,536. This means
that any offset or gain error inherent in the A/D converter
will appear to increase, in terms of LSB size, as the reference
voltage is reduced.
The noise inherent in the converter will also appear to
increase with lower LSB size. With a +5V reference, the
internal noise of the converter typically contributes only 1.5
LSB peak-to-peak of potential error to the output code.
When the external reference is 500mV, the potential error
contribution from the internal noise will be 10 times larger—
15 LSBs. The errors due to the internal noise are gaussian in
nature and can be reduced by averaging consecutive conver-
sion results.
For more information regarding noise, consult the typical
performance curve “Peak-to-Peak Noise vs Reference Volt-
age.” Note that the Effective Number of Bits (ENOB) figure
is calculated based on the converter’s signal-to-(noise +
distortion) ratio with a 1kHz, 0dB input signal. SINAD is
related to ENOB as follows:
SINAD = 6.02 • ENOB + 1.76
With lower reference voltages, extra care should be taken to
provide a clean layout including adequate bypassing, a clean
power supply, a low-noise reference, and a low-noise input
signal. Because the LSB size is lower, the converter will also
be more sensitive to external sources of error such as nearby
digital signals and electromagnetic interference.
9
®
ADS8320
NOISE
The noise floor of the ADS8320 itself is extremely low, as
can be seen from Figures 1 and 2, and is much lower than
competing A/D converters. It was tested by applying a low
noise DC input and a 5.0V reference to the ADS8320 and
initiating 5000 conversions. The digital output of the A/D
2
2510
3
2490
4
Code
56
0000
1
FIGURE 1. Histogram of 5000 Conversions of a DC Input
at the Code Transition.
FIGURE 2. Histogram of 5000 Conversions of a DC Input
at the Code Center.
2
72
3
4864
4
Code
56
64 000
1
converter will vary in output code due to the internal noise
of the ADS8320. This is true for all 16-bit SAR-type A/D
converters. Using a histogram to plot the output codes, the
distribution should appear bell-shaped with the peak of the
bell curve representing the nominal code for the input value.
The ±1σ, ±2σ, and ±3σ distributions will represent the
68.3%, 95.5%, and 99.7%, respectively, of all codes. The
transition noise can be calculated by dividing the number of
codes measured by 6 and this will yield the ±3σ distribution
or 99.7% of all codes. Statistically, up to 3 codes could fall
outside the distribution when executing 1000 conversions.
The ADS8320, with < 3 output codes for the ±3σ distribu-
tion, will yield a < ±0.5LSB transition noise. Remember, to
achieve this low noise performance, the peak-to-peak noise
of the input signal and reference must be < 50µV.
AVERAGING
The noise of the A/D converter can be compensated by
averaging the digital codes. By averaging conversion re-
sults, transition noise will be reduced by a factor of 1/√n,
where n is the number of averages. For example, averaging
4 conversion results will reduce the transition noise by 1/2
to ±0.25 LSBs. Averaging should only be used for input
signals with frequencies near DC.
For AC signals, a digital filter can be used to low pass filter
and decimate the output codes. This works in a similar
manner to averaging; for every decimation by 2, the signal-
to-noise ratio will improve 3dB.
DIGITAL INTERFACE
SIGNAL LEVELS
The digital inputs of the ADS8320 can accommodate logic
levels up to 5.5V regardless of the value of VCC. Thus, the
ADS8320 can be powered at 3V and still accept inputs from
logic powered at 5V.
The CMOS digital output (DOUT) will swing 0V to VCC. If
VCC is 3V and this output is connected to a 5V CMOS logic
input, then that IC may require more supply current than
normal and may have a slightly longer propagation delay.
10
®
ADS8320
SYMBOL DESCRIPTION MIN TYP MAX UNITS
tSMPL Analog Input Sample Time 4.5 5.0
Clk Cycles
tCONV Conversion Time 16
Clk Cycles
tCYC Throughput Rate 100 kHz
tCSD CS Falling to 0 ns
DCLOCK LOW
tSUCS CS Falling to 20 ns
DCLOCK Rising
thDO DCLOCK Falling to 5 15 ns
Current DOUT Not Valid
tdDO DCLOCK Falling to Next 30 50 ns
DOUT Valid
tdis CS Rising to DOUT Tri-State 70 100 ns
ten DCLOCK Falling to DOUT 20 50 ns
Enabled
tfDOUT Fall Time 5 25 ns
trDOUT Rise Time 7 25 ns
FIGURE 3. ADS8320 Basic Timing Diagrams.
TABLE I. Timing Specifications (VCC = 2.7V and above,
–40°C to +85°C.
DESCRIPTION ANALOG VALUE
Full Scale Range VREF
Least Significant VREF/65,536
Bit (LSB) BINARY CODE HEX CODE
Full Scale VREF –1 LSB 1111 1111 1111 1111 FFFF
Midscale VREF/2 1000 0000 0000 0000 8000
Midscale – 1LSB VREF/2 – 1 LSB 0111 1111 1111 1111 7FFF
Zero 0V 0000 0000 0000 0000 0000
DIGITAL OUTPUT
STRAIGHT BINARY
TABLE II. Ideal Input Voltages and Output Codes.
SERIAL INTERFACE
The ADS8320 communicates with microprocessors and other
digital systems via a synchronous 3-wire serial interface as
shown in Figure 3 and Table I. The DCLOCK signal syn-
chronizes the data transfer with each bit being transmitted on
the falling edge of DCLOCK. Most receiving systems will
capture the bitstream on the rising edge of DCLOCK. How-
ever, if the minimum hold time for DOUT is acceptable, the
system can use the falling edge of DCLOCK to capture each
bit.
A falling CS signal initiates the conversion and data transfer.
The first 4.5 to 5.0 clock periods of the conversion cycle are
used to sample the input signal. After the fifth falling
DCLOCK edge, DOUT is enabled and will output a LOW
value for one clock period. For the next 16 DCLOCK
periods, DOUT will output the conversion result, most signifi-
cant bit first. After the least significant bit (B0) has been
output, subsequent clocks will repeat the output data but in
a least significant bit first format.
After the most significant bit (B15) has been repeated, DOUT
will tri-state. Subsequent clocks will have no effect on the
converter. A new conversion is initiated only when CS has
been taken HIGH and returned LOW.
DATA FORMAT
The output data from the ADS8320 is in Straight Binary
format as shown in Table II. This table represents the ideal
output code for the given input voltage and does not include
the effects of offset, gain error, or noise.
POWER DISSIPATION
The architecture of the converter, the semiconductor fabrica-
tion process, and a careful design allow the ADS8320 to
convert at up to a 100kHz rate while requiring very little
power. Still, for the absolute lowest power dissipation, there
are several things to keep in mind.
The power dissipation of the ADS8320 scales directly with
conversion rate. Therefore, the first step to achieving the
lowest power dissipation is to find the lowest conversion
rate that will satisfy the requirements of the system.
In addition, the ADS8320 is in power down mode under two
conditions: when the conversion is complete and whenever
CS is HIGH (see Figure 3). Ideally, each conversion should
occur as quickly as possible, preferably at a 2.4MHz clock
rate. This way, the converter spends the longest possible
time in the power-down mode. This is very important as the
converter not only uses power on each DCLOCK transition
(as is typical for digital CMOS components) but also uses
some current for the analog circuitry, such as the compara-
tor. The analog section dissipates power continuously, until
the power down mode is entered.
CS/SHDN
D
OUT
DCLOCK
Complete Cycle
Power Down
ConversionSample
Use positive clock edge for data transfer
t
SUCS
t
CONV
t
SMPL
NOTE: Minimum 22 clock cycles required for 16-bit conversion. Shown are 24 clock cycles.
If CS remains LOW at the end of conversion, a new datastream with LSB-first is shifted out again.
B15
(MSB)B14 B13 B12 B11 B10 B9 B8 B0
(LSB)
B7 B1B6 B2B5 B3B4
Hi-Z 0Hi-Z
t
CSD
11
®
ADS8320
FIGURE 4. Timing Diagrams and Test Circuits for the Parameters in Table I.
D
OUT
1.4V
Test Point
3kΩ
100pF
C
LOAD
Load Circuit for t
dDO
, t
r
, and t
f
Voltage Waveforms for D
OUT
Rise and Fall Times, t
r
, t
f
Voltage Waveforms for D
OUT
Delay Times, t
dDO
Voltage Waveforms for t
dis
NOTES: (1) Waveform 1 is for an output with internal conditions such that the output
is HIGH unless disabled by the output control. (2) Waveform 2 is for an output with
internal conditions such that the output is LOW unless disabled by the output control.
Voltage Waveforms for t
en
Load Circuit for t
dis
and t
en
t
r
D
OUT
V
OH
V
OL
t
f
D
OUT
Test Point
t
dis
Waveform 2, t
en
V
CC
t
dis
Waveform 1
100pF
C
LOAD
3kΩ
t
dis
CS/SHDN
D
OUT
Waveform 1
(1)
D
OUT
Waveform 2
(2)
90%
10%
V
IH
41
B11
5
t
en
CS/SHDN
DCLOCK
V
OL
D
OUT
t
dDO
D
OUT
DCLOCK
V
OH
V
OL
V
IL
t
hDO
12
®
ADS8320
Figure 5 shows the current consumption of the ADS8320
versus sample rate. For this graph, the converter is clocked
at 2.4MHz regardless of the sample rate—CS is HIGH for
the remaining sample period. Figure 6 also shows current
consumption versus sample rate. However, in this case, the
DCLOCK period is 1/24th of the sample period—CS is
HIGH for one DCLOCK cycle out of every 16.
There is an important distinction between the power-down
mode that is entered after a conversion is complete and the
full power-down mode which is enabled when CS is HIGH.
CS LOW will shut down only the analog section. The digital
section is completely shutdown only when CS is HIGH.
Thus, if CS is left LOW at the end of a conversion and the
converter is continually clocked, the power consumption
will not be as low as when CS is HIGH. See Figure 7 for
more information.
Power dissipation can also be reduced by lowering the
power supply voltage and the reference voltage. The
ADS8320 will operate over a VCC range of 2.0V to 5.25V.
However, at voltages below 2.7V, the converter will not run
at a 100kHz sample rate. See the typical performance curves
for more information regarding power supply voltage and
maximum sample rate.
SHORT CYCLING
Another way of saving power is to utilize the CS signal to
short cycle the conversion. Because the ADS8320 places the
latest data bit on the DOUT line as it is generated, the
converter can easily be short cycled. This term means that
the conversion can be terminated at any time. For example,
if only 14 bits of the conversion result are needed, then the
conversion can be terminated (by pulling CS HIGH) after
the 14th bit has been clocked out.
This technique can be used to lower the power dissipation
(or to increase the conversion rate) in those applications
where an analog signal is being monitored until some con-
dition becomes true. For example, if the signal is outside a
predetermined range, the full 16-bit conversion result may
not be needed. If so, the conversion can be terminated after
the first n bits, where n might be as low as 3 or 4. This results
in lower power dissipation in both the converter and the rest
of the system, as they spend more time in the power-down
mode.
LAYOUT
For optimum performance, care should be taken with the
physical layout of the ADS8320 circuitry. This will be
particularly true if the reference voltage is low and/or the
conversion rate is high. At a 100kHz conversion rate, the
ADS8320 makes a bit decision every 416ns. That is, for each
subsequent bit decision, the digital output must be updated
with the results of the last bit decision, the capacitor array
appropriately switched and charged, and the input to the
comparator settled to a 16-bit level all within one clock
cycle.
FIGURE 5. Maintaining fCLK at the Highest Possible Rate
Allows Supply Current to Drop Linearly with
Sample Rate.
FIGURE 6. Scaling fCLK Reduces Supply Current Only
Slightly with Sample Rate.
FIGURE 7. Shutdown Current with CS HIGH is 50nA
Typically, Regardless of the Clock. Shutdown
Current with CS LOW Varies with Sample
Rate.
1000
800
600
400
200
0.0
0.00
Supply Current (µA)
0.1 1 10 100
Sample Rate (kHz)
T
A
= 25°C
V
CC
= 5.0V
V
REF
= 5.0V
f
CLK
= 24 • f
SAMPLE
CS LOW (GND)
CS HIGH (V
CC
)
0.250
1000
100
10
1
Supply Current (µA)
0.1 1 10 100
Sample Rate (kHz)
T
A
= 25°C
V
CC
= 5.0V
V
REF
= 5.0V
f
CLK
= 24 • f
SAMPLE
1000
100
10
1
Supply Current (µA)
0.1 1 10 100
Sample Rate (kHz)
V
CC
= 5.0V
V
REF
= 5.0V V
CC
= 2.7V
V
REF
= 2.5V
T
A
= 25°C
f
CLK
= 2.4MHz
13
®
ADS8320
The basic SAR architecture is sensitive to spikes on the
power supply, reference, and ground connections that occur
just prior to latching the comparator output. Thus, during
any single conversion for an n-bit SAR converter, there are
n “windows” in which large external transient voltages can
easily affect the conversion result. Such spikes might origi-
nate from switching power supplies, digital logic, and high
power devices, to name a few. This particular source of error
can be very difficult to track down if the glitch is almost
synchronous to the converter’s DCLOCK signal—as the
phase difference between the two changes with time and
temperature, causing sporadic misoperation.
With this in mind, power to the ADS8320 should be clean
and well bypassed. A 0.1µF ceramic bypass capacitor should
be placed as close to the ADS8320 package as possible. In
addition, a 1 to 10µF capacitor and a 5Ω or 10Ω series
resistor may be used to lowpass filter a noisy supply.
The reference should be similarly bypassed with a 0.1µF
capacitor. Again, a series resistor and large capacitor can be
used to lowpass filter the reference voltage. If the reference
voltage originates from an op amp, be careful that the op
amp can drive the bypass capacitor without oscillation (the
series resistor can help in this case). Keep in mind that while
the ADS8320 draws very little current from the reference on
average, there are still instantaneous current demands placed
on the external input and reference circuitry.
Burr-Brown’s OPA627 op amp provides optimum perfor-
mance for buffering both the signal and reference inputs. For
low cost, low voltage, single-supply applications, the
OPA2350 or OPA2340 dual op amps are recommended.
Also, keep in mind that the ADS8320 offers no inherent
rejection of noise or voltage variation in regards to the
reference input. This is of particular concern when the
reference input is tied to the power supply. Any noise and
ripple from the supply will appear directly in the digital
results. While high frequency noise can be filtered out as
described in the previous paragraph, voltage variation due to
the line frequency (50Hz or 60Hz), can be difficult to
remove.
The GND pin on the ADS8320 should be placed on a clean
ground point. In many cases, this will be the “analog”
ground. Avoid connecting the GND pin too close to the
grounding point for a microprocessor, microcontroller, or
digital signal processor. If needed, run a ground trace di-
rectly from the converter to the power supply connection
point. The ideal layout will include an analog ground plane
for the converter and associated analog circuitry.
APPLICATION CIRCUITS
Figure 8 shows a basic data acquisition system. The ADS8320
input range is 0V to VCC, as the reference input is connected
directly to the power supply. The 5Ω resistor and 1µF to
10µF capacitor filter the microcontroller “noise” on the
supply, as well as any high-frequency noise from the supply
itself. The exact values should be picked such that the filter
provides adequate rejection of the noise.
FIGURE 8. Basic Data Acquisition System.
ADS8320
V
CC
CS
D
OUT
DCLOCK
V
REF
+In
–In
GND
+
+
5Ω
1µF to
10µF
1µF to
10µF
0.1µF
Microcontroller
+2.7V to +5.25V
