AD7321 [ADI]
500 kSPS, 2-Channel, Software-Selectable, True Bipolar Input, 12-Bit Plus Sign ADC; 500 kSPS时， 2通道，软件可选的真双极性输入， 12位加符号位ADC
| 型号： | AD7321 |
| 厂家： | 
ADI |
| 描述： | 500 kSPS, 2-Channel, Software-Selectable, True Bipolar Input, 12-Bit Plus Sign ADC |
| 文件： | 总36页 (文件大小：561K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
500 kSPS, 2-Channel, Software-Selectable,
True Bipolar Input, 12-Bit Plus Sign ADC
AD7321
FUNCTIONAL BLOCK DIAGRAM
FEATURES
V
REFIN/OUT
V
12-bit plus sign SAR ADC
DD
CC
True bipolar input ranges
AD7321
Software-selectable input ranges
10 V, 5 V, 2.5 V, 0 V to +10 V
500 kSPS throughput rate
2.5V
VREF
V
V
0
1
13-BIT
IN
I/P
MUX
SUCCESSIVE
APPROXIMATION
ADC
T/H
Two analog input channels with channel sequencer
IN
Single-ended, true differential, and pseudo differential
analog input capability
High analog input impedance
Low power: 17 mW
Full power signal bandwidth: 22 MHz
Internal 2.5 V reference
DOUT
SCLK
CS
CONTROL LOGIC
AND REGISTERS
CHANNEL
SEQUENCER
DIN
High speed serial interface
Power-down modes
V
DRIVE
14-lead TSSOP package
iCMOS™ process technology
AGND
V
DGND
SS
Figure 1.
GENERAL DESCRIPTION
PRODUCT HIGHLIGHTS
The AD73211 is a 2-channel, 12-bit plus sign successive
approximation ADC designed on the iCMOS (industrial
CMOS) process. iCMOS is a process combining high voltage
silicon with submicron CMOS and complementary bipolar
technologies. It enables the development of a wide range of high
performance analog ICs capable of 33 ꢁ operation in a footprint
that no previous generation of high voltage parts could achieve.
Unlike analog ICs using conventional CMOS processes, iCMOS
components can accept bipolar input signals while providing
increased performance, dramatically reduced power
1. The AD7321 can accept true bipolar analog input signals,
±1ꢀ ꢁ, ±± ꢁ, ±2.± ꢁ, and ꢀ ꢁ to +1ꢀ ꢁ unipolar signals.
2. The two analog inputs can be configured as two single-
ended inputs, one true differential input pair, or one
pseudo differential input.
3. ±ꢀꢀ kSPS serial interface. SPI®-/QSPI™-/DSP-/MICROWIRE™-
compatible interface.
4. Low power, 17 mW, at a maximum throughput rate of
±ꢀꢀ kSPS.
consumption, and reduced package size.
±. Channel sequencer.
The AD7321 can accept true bipolar analog input signals. The
AD7321 has four software-selectable input ranges, ±1ꢀ ꢁ, ±± ꢁ,
±2.± ꢁ, and ꢀ ꢁ to +1ꢀ ꢁ. Each analog input channel can be
independently programmed to one of the four input ranges.
The analog input channels on the AD7321 can be programmed
to be single-ended, true differential, or pseudo differential.
Table 1. Similar Devices
Device Throughput Number of bits
Number Rate
Number of
Channels
AD7329
AD7328
AD7327
AD7324
AD7323
AD7322
1000 kSPS
12-bit plus sign
12-bit plus sign
12-bit plus sign
12-bit plus sign
12-bit plus sign
12-bit plus sign
8
8
8
4
4
2
1000 kSPS
500 kSPS
1000 kSPS
500 kSPS
1000 kSPS
The ADC contains a 2.± ꢁ internal reference. The AD7321 also
allows for external reference operation. If a 3 ꢁ reference is
applied to the REFIN/OUT pin, the AD7321 can accept a true
bipolar ±12 ꢁ analog input. Minimum ±12 ꢁ ꢁDD and ꢁSS
supplies are required for the ±12 ꢁ input range. The ADC has a
high speed serial interface that can operate at throughput rates
up to ±ꢀꢀ kSPS.
1 Protected by U.S. Patent No. 6,731,232.
Rev. 0
Information furnished by Analog Devices is believed to be accurate and reliable. However, no
responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other
rights of third parties that may result from its use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarks and registeredtrademarks arethe property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
Fax: 781.461.3113
www.analog.com
©2006 Analog Devices, Inc. All rights reserved.
AD7321
TABLE OF CONTENTS
Features .............................................................................................. 1
Addressing Registers.................................................................. 22
Control Register ......................................................................... 23
Range Register ............................................................................ 2±
Sequencer Operation ..................................................................... 26
Reference ..................................................................................... 27
ꢁDRIꢁE ............................................................................................ 27
Modes of Operation ....................................................................... 28
Normal Mode.............................................................................. 28
Full Shutdown Mode.................................................................. 28
Auto Shutdown Mode................................................................ 29
Autostandby Mode..................................................................... 29
Power vs. Throughput Rate....................................................... 3ꢀ
Serial Interface ................................................................................ 31
Microprocessor Interfacing........................................................... 32
AD7321 to ADSP-21xx.............................................................. 32
AD7321 to ADSP-BF±3x........................................................... 32
Application Hints ........................................................................... 33
Layout and Grounding .............................................................. 33
Outline Dimensions....................................................................... 34
Ordering Guide .......................................................................... 34
Functional Block Diagram .............................................................. 1
General Description......................................................................... 1
Product Highlights ........................................................................... 1
Revision History ............................................................................... 2
Specifications..................................................................................... 3
Timing Specifications .................................................................. 7
Absolute Maximum Ratings............................................................ 8
ESD Caution.................................................................................. 8
Pin Configuration and Function Descriptions............................. 9
Typical Performance Characteristics ........................................... 1ꢀ
Terminology .................................................................................... 14
Theory of Operation ...................................................................... 16
Circuit Information.................................................................... 16
Converter Operation.................................................................. 16
Analog Input Structure.............................................................. 17
Typical Connection Diagram ................................................... 19
Analog Input ............................................................................... 19
Driver Amplifier Choice............................................................ 21
Registers........................................................................................... 22
REVISION HISTORY
1/06—Revision 0: Initial Version
Rev. 0 | Page 2 of 36
AD7321
SPECIFICATIONS
ꢁDD = 12 ꢁ to 16.± ꢁ, ꢁSS = −12 ꢁ to −16.± ꢁ, ꢁCC = 2.7 ꢁ to ±.2± ꢁ, ꢁDRIꢁE = 2.7 ꢁ to ±.2± ꢁ, ꢁREF = 2.± ꢁ to 3.ꢀ ꢁ internal/external, fSCLK = 1ꢀ
MHz, fS = ±ꢀꢀ kSPS, TA = TMAX to TMIN, unless otherwise noted.
Table 2.
B Version
Typ
Parameter1
Min
Max
Unit
Test Conditions/Comments
FIN = 50 kHz sine wave
DYNAMIC PERFORMANCE
Signal-to-Noise Ratio (SNR)2
76
75.5
72.5
dB
dB
dB
Differential mode, VCC = 4.75 V to 5.25 V
Differential mode, VCC < 4.75 V
Single-ended/pseudo differential mode; 10 V, 2.5 V
and 5 V ranges, VCC = 4.75 V to 5.25 V
72
75
74
dB
dB
Single-ended/pseudo differential mode ; 0 V to 10 V
VCC = 4.75 V to 5.25 V and all ranges at VCC < 4.75 V
Differential mode; 2.5 V and 5 V ranges
Signal-to-Noise + Distortion
(SINAD)2
Differential mode; 0 V to 10 V
Differential mode; 10 V range
Single-ended/pseudo differential mode; 2.5 V and
5 V ranges
76
dB
dB
72
72.5
dB
Single-ended/pseudo differential mode; 0 V to +10 V
and 10 V ranges
Total Harmonic Distortion (THD)2
−80
−79
dB
dB
dB
dB
dB
dB
Differential mode; 2.5 V and 5 V ranges
Differential mode; 0 V to 10 V ranges
Differential mode; 10 V range
Single-ended/pseudo differential mode; 5 V range
Single-ended/pseudo differential mode; 2.5 V range
Single-ended/pseudo differential mode; 0 V to +10 V
and 10 V ranges
−82
−80
−77
−79
Peak Harmonic or Spurious
Noise (SFDR)2
−81
−80
dB
Differential mode; 2.5 V and 5 V ranges
dB
dB
dB
Differential mode; 0 V to 10 V ranges
Differential mode; 10 V ranges
Single-ended/pseudo differential mode; 5 V range
Single-ended/pseudo differential mode; 2.5 V range
Single-ended/pseudo differential mode; 0 V to +10 V
and 10 V ranges
−82
−79
−78
−80
dB
Intermodulation Distortion
(IMD)2
fa = 50 kHz, fb = 30 kHz
Second-Order Terms
Third-Order Terms
Aperture Delay3
Aperture Jitter3
−88
−90
7
50
−79
dB
dB
ns
ps
dB
Common-Mode Rejection
Up to 100 kHz ripple frequency; see Figure 17
(CMRR)2
Channel-to-Channel Isolation2
Full Power Bandwidth
−72
22
5
dB
MHz
MHz
FIN on unselected channels up to 100 kHz; see Figure 14
At 3 dB
At 0.1 dB
Rev. 0 | Page 3 of 36
AD7321
B Version
Typ
Parameter1
DC ACCURACY4
Min
Max
Unit
Test Conditions/Comments
Resolution
No Missing Codes
13
12-bit
Bits
Bits
Differential mode
plus sign
11-bit
plus sign
Bits
Single-ended/pseudo differential mode
Differential mode; VCC = 3 V to 5.25 V, typ for VCC = 2.7 V
Single-ended/pseudo differential mode, VCC = 3 V to
5.25 V, typ for VCC = 2.7 V
Integral Nonlinearity2
1.1
1
LSB
LSB
−0.7/+1.2
−0.7/+1
LSB
Single-ended/pseudo differential mode
(LSB = FSR/8192)
Differential mode; guaranteed no missing codes to
13 bits
Single-ended mode; guaranteed no missing codes to
12 bits
Single-ended/psuedo differential mode
(LSB = FSR/8192)
Single-ended/pseudo differential mode
Differential mode
Single-ended/pseudo differential mode
Differential mode
Single-ended/pseudo differential mode
Differential mode
Single-ended/pseudo differential mode
Differential mode
Single-ended/pseudo differential mode
Differential mode
Single-ended/pseudo differential mode
Differential mode
Single-ended/pseudo differential mode
Differential mode
Single-ended/pseudo differential mode
Differential mode
Single-ended/pseudo differential mode
Differential mode
Single-ended/pseudo differential mode
Differential mode
Differential Nonlinearity2
−0.9/+1.2 LSB
0.9
LSB
LSB
Offset Error2, 5
−4/+9
−7/+10
0.6
0.5
8
LSB
LSB
LSB
LSB
LSB
LSB
LSB
LSB
LSB
LSB
LSB
LSB
LSB
LSB
LSB
LSB
LSB
LSB
LSB
LSB
Offset Error Match2, 5
Gain Error2, 5
14
Gain Error Match2, 5
0.5
0.5
4
Positive Full-Scale Error2, 6
Positive Full-Scale Error Match2, 6
Bipolar Zero Error2, 6
7
0.5
0.5
8.5
7.5
0.5
0.5
4
6
0.5
0.5
Bipolar Zero Error Match2, 6
Negative Full-Scale Error2, 6
Negative Full-Scale Error Match2, 6
Rev. 0 | Page 4 of 36
AD7321
B Version
Typ
Parameter1
Min
Max
Unit
Test Conditions/Comments
ANALOG INPUT
Input Voltage Ranges
Reference = 2.5 V; see Table 6
(Programmed via Range
Register)
10
V
VDD = 10 V min, VSS = −10 V min, VCC = 2.7 V to 5.25 V
5
2.5
0 to 10
V
V
V
VDD = 5 V min, VSS = −5 V min, VCC = 2.7 V to 5.25 V
VDD = 5 V min, VSS = −5 V min, VCC = 2.7 V to 5.25 V
VDD = 10 V min, VSS = AGND min, VCC = 2.7 V to 5.25 V
Pseudo Differential VIN(−)
Input Range
VDD = 16.5 V, VSS = −16.5 V, VCC = 5 V; see Figure 40 and
Figure 41
3.5
6
5
V
V
V
V
nA
nA
pF
pF
pF
pF
Reference = 2.5 V; range = 10 V
Reference = 2.5 V; range = 5 V
Reference = 2.5 V; range = 2.5 V
Reference = 2.5 V; range = 0 V to +10 V
VIN = VDD or VSS
Per input channel, VIN = VDD or VSS
When in track, 10 V range
When in track, 5 V and 0 V to +10 V ranges
When in track, 2.5 V range
+3/−5
DC Leakage Current
Input Capacitance3
80
3
13.5
16.5
21.5
3
When in hold, all ranges
REFERENCE INPUT/OUTPUT
Input Voltage Range
Input DC Leakage Current
Input Capacitance
2.5
3
1
V
μA
pF
V
10
2.5
Reference Output Voltage
Reference Output Voltage Error
@ 25°C
Reference Output Voltage
TMIN to TMAX
Reference Temperature
Coefficient
5
10
25
mV
mV
ppm/°C
3
7
ppm/°C
Ω
Reference Output Impedance
LOGIC INPUTS
Input High Voltage, VINH
Input Low Voltage, VINL
2.4
V
V
V
μA
pF
0.8
0.4
1
VCC = 4.75 V to 5.25 V
VCC = 2.7 to 3.6 V
VIN = 0 V or VDRIVE
Input Current, IIN
3
Input Capacitance, CIN
10
LOGIC OUTPUTS
Output High Voltage, VOH
VDRIVE
0.2 V
−
V
ISOURCE = 200 μA
ISINK = 200 μA
Output Low Voltage, VOL
Floating-State Leakage Current
Floating-State Output
Capacitance3
0.4
1
V
μA
pF
5
Output Coding
Straight natural binary
Twos complement
Coding bit set to 1 in control register
Coding bit set to 0 in control register
CONVERSION RATE
Conversion Time
Track-and-Hold Acquisition
Time2, 3
1.6
305
μs
ns
16 SCLK cycles with SCLK = 10 MHz
Full-scale step input; see the Terminology section
Throughput Rate
500
kSPS
See the Serial Interface section
Rev. 0 | Page 5 of 36
AD7321
B Version
Typ
Parameter1
Min
Max
Unit
Test Conditions/Comments
Digital inputs = 0 V or VDRIVE
See Table 6
See Table 6
See Table 6
POWER REQUIREMENTS
VDD
VSS
VCC
12
16.5
−16.5
5.25
5.25
V
V
V
V
−12
2.7
2.7
VDRIVE
Normal Mode (Static)
0.9
mA
VDD/VSS = 16.5 V, VCC/VDRIVE = 5.25 V
Normal Mode (Operational)
IDD
ISS
ICC and IDRIVE
Autostandby Mode (Dynamic)
IDD
ISS
ICC and IDRIVE
Autoshutdown Mode (Static)
IDD
ISS
ICC and IDRIVE
Full Shutdown Mode
IDD
fSAMPLE = 500 kSPS
VDD = 16.5 V
VSS = −16.5 V
VCC/VDRIVE = 5.25 V
fSAMPLE = 250 kSPS
VDD = 16.5 V
180
205
2
μA
μA
mA
100
110
0.75
μA
μA
mA
VSS = −16.5 V
VCC/VDRIVE = 5.25 V
SCLK on or off
VDD = 16.5 V
1
1
1
μA
μA
μA
VSS = −16.5 V
VCC/VDRIVE = 5.25 V
SCLK on or off
VDD = 16.5 V
VSS = −16.5 V
VCC/VDRIVE = 5.25 V
1
1
1
μA
μA
μA
ISS
ICC and IDRIVE
POWER DISSIPATION
Normal Mode (Operational)
Full Shutdown Mode
17
38.25
mW
μW
VDD = 16.5 V, VSS = −16.5 V, VCC = 5.25 V
VDD = 16.5 V, VSS = −16.5 V, VCC = 5.25 V
1 Temperature range is −40°C to +85°C.
2 See the Terminology section.
3 Sample tested during initial release to ensure compliance.
4 For dc accuracy specifications, the LSB size for differential mode is FSR/8192. For single-ended mode/pseudo differential mode, the LSB size is FSR/4096, unless
otherwise noted.
5 Unipolar 0 V to 10 V range with straight binary output coding.
6 Bipolar range with twos complement output coding.
Rev. 0 | Page 6 of 36
AD7321
TIMING SPECIFICATIONS
ꢁDD = 12 ꢁ to 16.± ꢁ, ꢁSS = −12 ꢁ to −16.± ꢁ, ꢁCC = 2.7 ꢁ to ±.2± ꢁ, ꢁDRIꢁE = 2.7 ꢁ to ±.2± ꢁ, ꢁREF = 2.± ꢁ to 3.ꢀ ꢁ internal/external,
TA = TMAX to TMIN. Timing specifications apply with a 32 pF load, unless otherwise noted.1
Table 3.
Limit at TMIN, TMAX
Description
VDRIVE ≤ VCC
Parameter VCC < 4.75 V VCC = 4.75 V to 5.25 V Unit
fSCLK
50
10
16 × tSCLK
75
50
10
16 × tSCLK
60
kHz min
MHz max
ns max
ns min
ns min
ns min
ns min
ns max
ns max
ns min
ns min
ns min
ns max
ns min
ns min
ns min
ns max
μs max
tCONVERT
tQUIET
t1
tSCLK = 1/fSCLK
CS
Minimum time between end of serial read and next falling edge of
CS
Minimum pulse width
12
5
2
t2
25
20
CS
to SCLK set-up time; bipolar input ranges ( 10 V, 5 V, 2.5 V)
Unipolar input range (0 V to 10 V)
CS
45
26
35
14
t3
t4
t5
t6
t7
t8
Delay from
until DOUT three-state disabled
57
0.4 × tSCLK
0.4 × tSCLK
13
40
10
4
2
750
500
43
0.4 × tSCLK
0.4 × tSCLK
8
22
9
4
2
750
500
Data access time after SCLK falling edge
SCLK low pulse width
SCLK high pulse width
SCLK to data valid hold time
SCLK falling edge to DOUT high impedance
SCLK falling edge to DOUT high impedance
DIN set-up time prior to SCLK falling edge
DIN hold time after SCLK falling edge
Power-up from autostandby
t9
t10
tPOWER-UP
Power-up from full shutdown/autoshutdown mode, internal
reference
25
25
μs typ
Power-up from full shutdown/autoshutdown mode, external
reference
1 Sample tested during initial release to ensure compliance. All input signals are specified with tr = tf = 5 ns (10% to 90% of VDRIVE) and timed from a voltage level of 1.6 V.
2 When using the 0 V to 10 V unipolar range, running at 500 kSPS throughput rate with t2 at 20 ns, the mark space ratio needs to be limited to 50:50.
t1
CS
tCONVERT
t2
t6
1
2
3
4
5
13
14
t5
15
16
SCLK
DOUT
IDENTIFICATION BIT
t3
t7
t8
t4
tQUIET
ZERO
ADD0
SIGN
DB11
DB10
DB2
DB1
DB0
THREE-
STATE
ZERO
THREE-STATE
t10
t9
REG
SEL
DON’T
CARE
WRITE ZERO
MSB
LSB
DIN
Figure 2. Serial Interface Timing Diagram
Rev. 0 | Page 7 of 36
AD7321
ABSOLUTE MAXIMUM RATINGS
TA = 2±°C, unless otherwise noted
Table 4.
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
Parameter
Rating
VDD to AGND, DGND
VSS to AGND, DGND
VDD to VCC
VCC to AGND, DGND
VDRIVE to AGND, DGND
AGND to DGND
Analog Input Voltage to AGND1
Digital Input Voltage to DGND
Digital Output Voltage to GND
REFIN to AGND
Input Current to Any Pin
Except Supplies2
−0.3 V to +16.5 V
+0.3 V to −16.5 V
VCC − 0.3 V to 16.5 V
−0.3 V to +7 V
−0.3 V to +7 V
−0.3 V to +0.3 V
VSS − 0.3 V to VDD + 0.3 V
−0.3 V to +7 V
−0.3 V to VDRIVE + 0.3 V
−0.3 V to VCC + 0.3 V
10 mA
Operating Temperature Range
Storage Temperature Range
Junction Temperature
TSSOP Package
−40°C to +85°C
−65°C to +150°C
150°C
θJA Thermal Impedance
θJC Thermal Impedance
Pb-Free Temperature, Soldering
Reflow
113.5°C/W
30°C/W
260(0)°C
2.5 kV
ESD
1 If the analog inputs are driven from alternative VDD and VSS supply circuitry,
Schottky diodes should be placed in series with the AD7321’s VDD and VSS
supplies.
2 Transient currents of up to 100 mA do not cause SCR latch-up.
ESD 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 this product 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.
Rev. 0 | Page 8 of 36
AD7321
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
1
2
3
4
5
6
7
14
13
12
11
10
9
CS
SCLK
DGND
DOUT
DIN
DGND
AD7321
TOP VIEW
(Not to Scale)
AGND
V
V
V
V
DRIVE
CC
REFIN/OUT
V
SS
DD
V
0
8
1
IN
IN
Figure 3. TSSOP Pin Configuration
Table 5. AD7321 Pin Function Descriptions
Pin No.
Mnemonic Description
1
CS
Chip Select. Active low logic input. This input provides the dual function of initiating conversions on
the AD7321 and frames the serial data transfer.
2
DIN
Data In. Data to be written to the on-chip registers is provided on this input and is clocked into the
register on the falling edge of SCLK (see the Registers section).
3, 13
DGND
Digital Ground. Ground reference point for all digital circuitry on the AD7321. The DGND and AGND
voltages ideally should be at the same potential and must not be more than 0.3 V apart, even on a
transient basis.
4
5
AGND
Analog Ground. Ground reference point for all analog circuitry on the AD7321. All analog input signals
and any external reference signal should be referred to this AGND voltage. The AGND and DGND
voltages ideally should be at the same potential and must not be more than 0.3 V apart, even on a
transient basis.
REFIN/OUT Reference Input/Reference Output. The on-chip reference is available on this pin for use external to the
AD7321. The nominal internal reference voltage is 2.5 V, which appears at this pin. A 680 nF capacitor
should be placed on the reference pin (see the Reference section). Alternatively, the internal reference
can be disabled and an external reference applied to this input. On power-up, the external reference
mode is the default condition.
6
VSS
Negative Power Supply Voltage. This is the negative supply voltage for the analog input section.
7, 8
VIN0 to VIN1 Analog Input 0 to Analog Input 1. The analog inputs are multiplexed into the on-chip track-and-hold.
The analog input channel for conversion is selected by programming the Channel Address Bit ADD0 in
the control register. The inputs can be configured as two single-ended inputs, one true differential
input pair, or one pseudo differential input. The configuration of the analog inputs is selected by
programming the mode bits, Bit Mode 1 and Bit Mode 0, in the control register. The input range on
each input channel is controlled by programming the range register. Input ranges of 10 V, 5 V, 2.5 V,
and 0 V to +10 V can be selected on each analog input channel when a +2.5 V reference voltage is used
(see the Registers section).
9
10
VDD
VCC
Positive Power Supply Voltage. This is the positive supply voltage for the analog input section.
Analog Supply Voltage, 2.7 V to 5.25 V. This is the supply voltage for the ADC core on the AD7321.
This supply should be decoupled to AGND.
11
12
VDRIVE
Logic Power Supply Input. The voltage supplied at this pin determines at what voltage the interface
operates. This pin should be decoupled to DGND. The voltage at this pin may be different to that at VCC
but it should not exceed VCC by more than 0.3 V.
,
DOUT
Serial Data Output. The conversion output data is supplied to this pin as a serial data stream. The bits
are clocked out on the falling edge of the SCLK input, and 16 SCLKs are required to access the data. The
data stream consists of two ZERO bits, a channel identification bit, the sign bit, and 12 bits of
conversion data. The data is provided MSB first (see the Serial Interface section).
14
SCLK
Serial Clock, Logic Input. A serial clock input provides the SCLK used for accessing the data from the
AD7321. This clock is also used as the clock source for the conversion process.
Rev. 0 | Page 9 of 36
AD7321
TYPICAL PERFORMANCE CHARACTERISTICS
1.0
0.8
0
V
= V
DRIVE
= 5V INT/EXT 2.5V REFERENCE
±10V RANGE
CC
= 25°C
4096 POINT FFT
T
A
V
V
= V
= 5V
CC
DRIVE
V
, V = ±15V
+INL = +0.55LSB
–INL = –0.68LSB
DD SS
–20
–40
, V = ±15V
DD SS
0.6
T
= 25°C
A
INT/EXT 2.5V REFERENCE
±10V RANGE
0.4
F
= 50kHz
IN
0.2
SNR = 77.30dB
SINAD = 76.85dB
THD = –86.96dB
SFDR = –88.22dB
–60
0
–80
–0.2
–0.4
–0.6
–0.8
–1.0
–100
–120
–140
0
1024
2048
3072
3584
CODE
4096
5120
6144
7168
8192
0
50
100
150
200
250
512 1536
2560
4608
5632
6656
7680
FREQUENCY (kHz)
Figure 7. Typical INL True Differential Mode
Figure 4. FFT True Differential Mode
1.0
0.8
0
–20
4096 POINT FFT
V
V
= V
= 5V
CC
DRIVE
0.6
, V = ±15V
DD SS
T
= 25°C
A
0.4
INT/EXT 2.5V REFERENCE
±10V RANGE
–40
F
= 50kHz
0.2
IN
SNR = 74.67dB
SINAD = 74.03dB
THD = –82.68dB
SFDR = –85.40dB
–60
0
–0.2
–0.4
–0.6
–0.8
–1.0
–80
–100
–120
–140
V
= V
DRIVE
= 5V
±10V RANGE
+DNL = +0.79LSB
–DNL = –0.38LSB
CC
= 25°C
T
A
V
, V = ±15V
DD SS
INT/EXT 2.5V REFERENCE
0
1024
2048
3072
3584
CODE
4096
5120
6144
7168
8192
512 1536
2560
4608
5632
6656
7680
0
50
100
150
200
250
FREQUENCY (kHz)
Figure 8. Typical DNL Single-Ended Mode
Figure 5. FFT Single-Ended Mode
1.0
0.8
1.0
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0
0
–0.2
–0.4
–0.6
–0.8
–1.0
–0.2
–0.4
–0.6
–0.8
–1.0
V
= V = 5V
DRIVE
CC
= 25°C
V
= V
DRIVE
= 5V
CC
= 25°C
T
A
T
A
V
, V = ±15V
DD SS
V
, V = ±15V
DD SS
INT/EXT 2.5V REFERENCE
±10V RANGE
+INL = +0.87LSB
–INL = –0.49LSB
INT/EXT 2.5V REFERENCE
±10V RANGE
+DNL = +0.72LSB
–DNL = –0.22LSB
0
1024
2048
3072
3584
CODE
4096
5120
6144
7168
8192
0
1024
2048
3072
3584
CODE
4096
5120
6144
7168
8192
512 1536
2560
4608
5632
6656
7680
512 1536
2560
4608
5632
6656
7680
Figure 6. Typical DNL True Differential Mode
Figure 9. Typical INL Single-Ended Mode
Rev. 0 | Page 10 of 36
AD7321
–50
–55
–60
–65
–70
–75
–80
–85
–90
–95
–100
80
75
70
65
60
55
50
V
V
= V
DRIVE
= 3V
±5V DIFF
±2.5V DIFF
CC
/V = ±12V
DD SS
T
f
= 25°C
= 500kSPS
A
±5V SE
S
0V TO +10V SE
INTERNAL REFERENCE
±2.5V SE
±10V SE
±10V DIFF
±10V DIFF
0V TO +10V DIFF
0V TO +10V SE
±10V SE
0V TO +10V DIFF
±5V SE
±5V DIFF
±2.5V DIFF
V
V
= V = 5V
DRIVE
CC
/V = ±12V
DD SS
T
= 25°C
A
±2.5V SE
f
= 500kSPS
S
INTERNAL REFERENCE
10
100
1000
10
100
ANALOG INPUT FREQUENCY (kHz)
1000
ANALOG INPUT FREQUENCY (kHz)
Figure 10. THD vs. Analog Input Frequency for Single-Ended (SE) and True
Differential Mode (Diff) at 3 V VCC
Figure 13. SINAD vs. Analog Input Frequency for Single-Ended (SE) and True
Differential Mode (Diff) at 5 V VCC
–50
–55
–50
V
V
= V = 5V
DRIVE
CC
–55
–60
–65
–70
–75
–80
–85
–90
–95
–100
/V = ±12V
DD SS
T
= 25°C
= 500kSPS
A
0V TO +10V SE
f
S
V
= 3V
–60
–65
–70
–75
–80
–85
–90
–95
CC
INTERNAL REFERENCE
V
= 5V
CC
±10V SE
±10V DIFF
0V TO +10V DIFF
±5V SE
V
/V = ±12V
DD SS
±5V DIFF
SINGLE-ENDED MODE
f
T
= 500kSPS
= 25°C
S
±2.5V SE
A
50kHz ON SELECTED CHANNEL
±2.5V DIFF
0
100
200
300
400
500
600
10
100
ANALOG INPUT FREQUENCY (kHz)
1000
FREQUENCY OF INPUT NOISE (kHz)
Figure 14. Channel-to-Channel Isolation
Figure 11. THD vs. Analog Input Frequency for Single-Ended (SE) and True
Differential Mode (Diff) at 5 V VCC
10k
9k
8k
7k
6k
5k
4k
3k
2k
1k
0
80
9469
V
V
= 5V
CC
±5V DIFF
±2.5V DIFF
/V = ±12V
DD SS
RANGE = ±10V
10k SAMPLES
75
±5V SE
±2.5V SE
T
= 25°C
A
0V TO +10V DIFF
70
±10V DIFF
±10V SE
0V TO +10V SE
65
60
V
V
= V = 3V
DRIVE
CC
/V = ±12V
DD SS
55
50
T
= 25°C
A
f
= 500kSPS
S
228
–1
303
1
INTERNAL REFERENCE
0
0
2
–2
0
10
100
ANALOG INPUT FREQUENCY (kHz)
1000
CODE
Figure 15. Histogram of Codes, True Differential Mode
Figure 12. SINAD vs. Analog Input Frequency for Single-Ended (SE) and True
Differential Mode (Diff) at 3 V VCC
Rev. 0 | Page 11 of 36
AD7321
2.0
1.5
8k
7k
6k
5k
4k
3k
2k
1k
0
7600
V
V
= 5V
CC
/V = ±12V
DD SS
RANGE = ±10V
10k SAMPLES
1.0
T
= 25°C
A
0.5
INL = 500kSPS
0
–0.5
–1.0
–1.5
–2.0
±5V RANGE
= V
1201
–1
1165
1
V
= 5V
DRIVE
CC
INTERNAL REFERENCE
SINGLE-ENDED MODE
0
23
–2
11
2
0
3
5
7
9
11
13
15
17
19
–3
0
±V /V SUPPLY VOLTAGE (V)
DD SS
CODE
Figure 16. Histogram of Codes, Single-Ended Mode
Figure 19. INL Error vs. Supply Voltage at 500 kSPS
–50
–55
–60
–65
–70
–75
–80
–85
–90
–95
–100
–50
–55
–60
–65
–70
–75
–80
–85
–90
–95
–100
100mV p-p SINE WAVE ON EACH SUPPLY
NO DECOUPLING
SINGLE-ENDED MODE
f
= 500kSPS
S
V
= 5V
CC
V
= 3V
CC
V
= 5V
CC
V
= 12V
DD
V
= 3V
CC
DIFFERENTIAL MODE
= 50kHz
V
= –12V
SS
F
IN
V
/V = ±12V
DD SS
f
T
= 500kSPS
= 25°C
S
A
0
200
400
600
800
1000
1200
0
200
400
600
800
1000
1200
SUPPLY RIPPLE FREQUENCY (kHz)
RIPPLE FREQUENCY (kHz)
Figure 20. PSRR vs. Supply Ripple Frequency Without Supply Decoupling
Figure 17. CMRR vs. Common-Mode Ripple Frequency
2.0
1.5
–50
V
V
= V
= 5V
CC
DRIVE
/V = ±12V
–55
–60
–65
–70
–75
–80
–85
–90
–95
–100
DD SS
T
= 25°C
A
±10V RANGE
INTERNAL REF
RANGE = ±10V AND ±2.5V
R
R
R
R
R
R
= 4000Ω
= 3000Ω
= 2000Ω
= 1000Ω
= 100Ω
= 12Ω
IN
IN
IN
IN
IN
IN
1.0
f
= 500kSPS
S
DIFFERENTIAL MODE
0.5
DNL = 500kSPS
0
±2.5V RANGE
R
R
R
R
R
= 9000Ω
= 5500Ω
= 2000Ω
= 100Ω
= 12Ω
IN
IN
IN
IN
IN
–0.5
–1.0
–1.5
–2.0
±5V RANGE
= V
INTERNAL REFERENCE
V
= 5V
DRIVE
CC
SINGLE-ENDED MODE
5
7
9
11
13
15
17
19
10
100
INPUT FREQUENCY (kHz)
1000
±V /V SUPPLY VOLTAGE (V)
DD SS
Figure 21. THD vs. Analog Input Frequency for Various Source Impedances,
True Differential Mode
Figure 18. DNL Error vs. Supply Voltage at 500 kSPS
Rev. 0 | Page 12 of 36
AD7321
–50
–55
–60
–65
–70
–75
–80
–85
–90
–95
–100
V
V
= V
= 5V
CC
DRIVE
/V = ±12V
DD SS
T
= 25°C
A
±10V RANGE
INTERNAL REF
RANGE = ±10V AND ±2.5V
R
R
R
R
R
= 4000Ω
= 2000Ω
= 1000Ω
= 100Ω
= 50Ω
IN
IN
IN
IN
IN
f
= 500kSPS
S
SINGLE-ENDED MODE
±2.5V RANGE
R
R
R
R
R
= 4700Ω
= 3000Ω
= 1000Ω
= 100Ω
= 50Ω
IN
IN
IN
IN
IN
10
100
INPUT FREQUENCY (kHz)
1000
Figure 22. THD vs. Analog Input Frequency for Various Source Impedances,
Single-Ended Mode
Rev. 0 | Page 13 of 36
AD7321
TERMINOLOGY
Differential Nonlinearity
Negative Full-Scale Error
This is the difference between the measured and the ideal 1 LSB
change between any two adjacent codes in the ADC.
This applies when using twos complement output coding and
any of the bipolar analog input ranges. This is the deviation of
the first code transition (1ꢀ ... ꢀꢀꢀ) to (1ꢀ ... ꢀꢀ1) from the ideal
(that is, −4 × ꢁREF + 1 LSB, −2 × ꢁREF + 1 LSB, −ꢁREF + 1 LSB)
after adjusting for the bipolar zero code error.
Integral Nonlinearity
This is the maximum deviation from a straight line passing
through the endpoints of the ADC transfer function. The
endpoints of the transfer function are zero scale (a point 1 LSB
below the first code transition) and full scale (a point 1 LSB
above the last code transition).
Negative Full-Scale Error Match
This is the difference in negative full-scale error between any
two input channels.
Offset Code Error
Track-and-Hold Acquisition Time
This applies to straight binary output coding. It is the deviation
of the first code transition (ꢀꢀ ... ꢀꢀꢀ) to (ꢀꢀ ... ꢀꢀ1) from the
ideal, that is, AGND + 1 LSB.
The track-and-hold amplifier returns into track mode after the
14th SCLK rising edge. Track-and-hold acquisition time is the
time required for the output of the track-and-hold amplifier to
reach its final value, within ±ꢂ LSB, after the end of a
conversion. For the ±2.± ꢁ range, the specified acquisition time
is the time required for the track-and-hold amplifier to settle to
within ±1 LSB.
Offset Error Match
This is the difference in offset error between any two input
channels.
Gain Error
Signal to (Noise + Distortion) Ratio
This applies to straight binary output coding. It is the deviation
of the last code transition (111 ... 11ꢀ) to (111 ... 111) from the
ideal (that is, 4 × ꢁREF − 1 LSB, 2 × ꢁREF − 1 LSB, ꢁREF −1 LSB)
after adjusting for the offset error.
This is the measured ratio of signal to (noise + distortion) at the
output of the A/D converter. The signal is the rms amplitude of
the fundamental. Noise is the sum of all non-fundamental signals
up to half the sampling frequency (fS/2), excluding dc. The ratio
is dependent on the number of quantization levels in the digi-
tization process. The more levels, the smaller the quantization
noise. Theoretically, the signal to (noise + distortion) ratio for
an ideal N-bit converter with a sine wave input is given by
Gain Error Match
This is the difference in gain error between any two input
channels.
Bipolar Zero Code Error
Signal to (Noise + Distortion) = (6.ꢀ2 N + 1.76) dB
This applies when using twos complement output coding and a
bipolar analog input. It is the deviation of the midscale
transition (all 1s to all ꢀs) from the ideal input voltage, that is,
AGND − 1 LSB.
For a 13-bit converter, this is 8ꢀ.ꢀ2 dB.
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the rms sum of
harmonics to the fundamental. For the AD7321 it is defined as
Bipolar Zero Code Error Match
This refers to the difference in bipolar zero code error between
any two input channels.
2
2
2
2
2
V2 +V3 +V4 +V± +V6
THD(dB) = 2ꢀ log
V1
Positive Full-Scale Error
where V1 is the rms amplitude of the fundamental, and V2, V3,
V4, V5, and V6 are the rms amplitudes of the second through the
sixth harmonics.
This applies when using twos complement output coding and
any of the bipolar analog input ranges. It is the deviation of the
last code transition (ꢀ11 ... 11ꢀ) to (ꢀ11 ... 111) from the ideal
(4 × ꢁREF − 1 LSB, 2 × ꢁREF − 1 LSB, ꢁREF − 1 LSB) after
adjusting for the bipolar zero code error.
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 fS/2, excluding dc) to the rms value of the
fundamental. Normally, the value of this specification is
determined by the largest harmonic in the spectrum, but for
ADCs where the harmonics are buried in the noise floor, the
largest harmonic could be a noise peak.
Positive Full-Scale Error Match
This is the difference in positive full-scale error between any
two input channels.
Rev. 0 | Page 14 of 36
AD7321
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
per the THD specification, where it is the ratio of the rms sum
of the individual distortion products to the rms amplitude of
the sum of the fundamentals expressed in decibels.
Channel-to-Channel Isolation
Channel-to-channel isolation is a measure of the level of crosstalk
between any two channels. It is measured by applying a full-scale,
1ꢀꢀ kHz sine wave signal to all unselected input channels and
determining the degree to which the signal attenuates in the
selected channel with a ±ꢀ kHz signal. Figure 14 shows the worst-
case across all eight channels for the AD7321. The analog input
range is programmed to be the same on all channels.
PSR (Power Supply Rejection)
ꢁariations in power supply affect the full-scale transition but
not the linearity of the converter. Power supply rejection is the
maximum change in the full-scale transition point due to a
change in power supply voltage from the nominal value (see the
Typical Performance Characteristics section).
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 = ꢀ, 1, 2, 3, and so on. Intermodulation distortion terms
are those for which neither m nor n are equal to ꢀ. For example,
the second-order terms include (fa + fb) and (fa − fb), whereas
the third-order terms include (2fa + fb), (2fa − fb), (fa + 2fb),
and (fa − 2fb).
CMRR (Common-Mode Rejection Ratio)
CMRR is defined as the ratio of the power in the ADC output at
full-scale frequency, f, to the power of a 1ꢀꢀ mꢁ sine wave
applied to the common-mode voltage of the ꢁIN+ and ꢁIN−
frequency, fS, as
The AD7321 is tested using the CCIF standard where two input
frequencies near the top end of the input bandwidth are used.
In this case, the second-order terms are usually distanced in
frequency from the original sine waves, whereas the third-order
CMRR (dB) = 1ꢀ log (Pf/PfS)
where Pf is the power at frequency f in the ADC output, and PfS
is the power at frequency fS in the ADC output (see Figure 17).
Rev. 0 | Page 15 of 36
AD7321
THEORY OF OPERATION
The analog inputs can be configured as two single-ended
inputs, one true differential input pair, or one pseudo
differential input. Selection can be made by programming the
mode bits, Mode ꢀ and Mode 1, in the control register.
CIRCUIT INFORMATION
The AD7321 is a fast, 2-channel, 12-bit plus sign, bipolar input,
serial A/D converter. The AD7321 can accept bipolar input
ranges that include ±1ꢀ ꢁ, ±± ꢁ, and ±2.± ꢁ; it can also accept a
ꢀ ꢁ to +1ꢀ ꢁ unipolar input range. A different analog input
range can be programmed on each analog input channel via the
on-chip registers. The AD7321 has a high speed serial interface
that can operate at throughput rates up to ±ꢀꢀ kSPS.
The serial clock input accesses data from the part and provides
the clock source for the successive approximation ADC. The
AD7321 has an on-chip 2.± ꢁ reference. However, the AD7321
can also work with an external reference. On power-up, the
external reference operation is the default option. If the internal
reference is the preferred option, the user must write to the
reference bit in the control register to select the internal
reference operation.
The AD7321 requires ꢁDD and ꢁSS dual supplies for the high
voltage analog input structures. These supplies must be equal to
or greater than the largest analog input range selected. See Table 6
for the requirements of these supplies for each analog input range.
The AD7321 requires a low voltage 2.7 ꢁ to ±.2± ꢁ ꢁCC supply
to power the ADC core.
The AD7321 also features power-down options to allow power
savings between conversions. The power-down modes are
selected by programming the on-chip control register, as
described in the Modes of Operation section.
Table 6. Reference and Supply Requirements for Each
Analog Input Range
Full-
CONVERTER OPERATION
Selected
Analog
Input Range Reference
(V)
Scale
Input
Range
The AD7321 is a successive approximation analog-to-digital
converter built around two capacitive DACs. Figure 23 and
Figure 24 show simplified schematics of the ADC in single-
ended mode during the acquisition and conversion phases,
respectively. Figure 2± and Figure 26 show simplified
schematics of the ADC in differential mode during acquisition
and conversion phases, respectively. The ADC is composed of
control logic, a SAR, and capacitive DACs. In Figure 23 (the
acquisition phase), SW2 is closed and SW1 is in Position A, the
comparator is held in a balanced condition, and the sampling
capacitor array acquires the signal on the input.
AVCC Minimum
Voltage (V) (V)
(V)
3/5
3/5
3/5
3/5
3/5
3/5
3/5
3/5
VDD/VSS (V)
10
2.5
3.0
2.5
3.0
2.5
3.0
2.5
3.0
10
10
12
12
5
5
5
6
6
2.5
2.5
3
5
5
0 to +10
0 to +10
0 to +12
+10/AGND
+12/AGND
It may be necessary to decrease the throughput rate when the
AD7321 is configured with the minimum ꢁDD and ꢁSS supplies
in order to meet the performance specifications (see the Typical
Performance Characteristics section). Figure 31 shows the
change in THD as the ꢁDD and ꢁSS supplies are reduced. For ac
performance at the maximum throughput rate, the THD degrades
slightly as ꢁDD and ꢁSS are reduced. It might therefore be necessary
to reduce the throughput rate when using minimum ꢁDD and
CAPACITIVE
DAC
COMPARATOR
C
S
B
A
V
0
IN
CONTROL
LOGIC
SW1
SW2
AGND
Figure 23. ADC Acquisition Phase (Single-Ended)
ꢁ
SS supplies so that there is less degradation of THD and the
When the ADC starts a conversion (Figure 24), SW2 opens and
SW1 moves to Position B, causing the comparator to become
unbalanced. The control logic and the charge redistribution
DAC are used to add and subtract fixed amounts of charge from
the capacitive DAC to bring the comparator back into a
balanced condition. When the comparator is rebalanced, the
conversion is complete. The control logic generates the ADC
output code.
specified performance can be maintained. The degradation is
due to an increase in the on resistance of the input multiplexer
when the ꢁDD and ꢁSS supplies are reduced. Figure 18 and
Figure 19 show the change in INL and DNL as the ꢁDD and ꢁSS
voltages are varied. For dc performance when operating at the
maximum throughput rate, as the ꢁDD and ꢁSS supply voltages
are reduced, the typical INL and DNL error remains constant.
Rev. 0 | Page 16 of 36
AD7321
CAPACITIVE
DAC
The ideal transfer characteristic for the AD7321 when twos
complement coding is selected is shown in Figure 27. The ideal
transfer characteristic for the AD7321 when straight binary
coding is selected is shown in Figure 28.
COMPARATOR
C
S
B
A
V
0
IN
CONTROL
LOGIC
SW1
SW2
AGND
011...111
011...110
Figure 24. ADC Conversion Phase (Single-Ended)
Figure 2± shows the differential configuration during the
acquisition phase. For the conversion phase, SW3 opens and
SW1 and SW2 move to Position B (see Figure 26). The output
impedances of the source driving the ꢁIN+ and ꢁIN− pins must
match; otherwise, the two inputs have different settling times,
resulting in errors.
000...001
000...000
111...111
100...010
100...001
100...000
AGND – 1LSB
–FSR/2 + 1LSB
AGND + 1LSB
+FSR/2 – 1LSB BIPOLAR RANGES
+FSR – 1LSB UNIPOLAR RANGE
CAPACITIVE
DAC
ANALOG INPUT
Figure 27. Twos Complement Transfer Characteristic
COMPARATOR
C
S
S
B
V
V
+
IN
A
A
SW1
SW2
CONTROL
LOGIC
SW3
111...111
111...110
–
IN
B
C
V
REF
CAPACITIVE
DAC
111...000
011...111
Figure 25. ADC Differential Configuration During Acquisition Phase
CAPACITIVE
DAC
000...010
000...001
000...000
COMPARATOR
C
S
S
B
–FSR/2 + 1LSB
AGND + 1LSB
+FSR/2 – 1LSB BIPOLAR RANGES
+FSR – 1LSB UNIPOLAR RANGE
V
V
+
IN
A
A
SW1
SW2
CONTROL
LOGIC
SW3
ANALOG INPUT
–
IN
Figure 28. Straight Binary Transfer Characteristic
B
C
V
REF
ANALOG INPUT STRUCTURE
CAPACITIVE
DAC
The analog inputs of the AD7321 can be configured as single-
ended, true differential, or pseudo differential via the control
register mode bits (see Table 9). The AD7321 can accept true
bipolar input signals. On power-up, the analog inputs operate as
two single-ended analog input channels. If true differential or
pseudo differential is required, a write to the control register is
necessary after power-up to change this configuration.
Figure 26. ADC Differential Configuration During Conversion Phase
Output Coding
The AD7321 default output coding is set to twos complement.
The output coding is controlled by the coding bit in the control
register. To change the output coding to straight binary coding,
the coding bit in the control register must be set. When
operating in sequence mode, the output coding for each
channel in the sequence is the value written to the coding bit
during the last write to the control register.
Figure 29 shows the equivalent analog input circuit of the
AD7321 in single-ended mode. Figure 3ꢀ shows the equivalent
analog input structure in differential mode. The two diodes
provide ESD protection for the analog inputs.
Transfer Functions
V
DD
The designed code transitions occur at successive integer
LSB values (that is, 1 LSB, 2 LSB, and so on). The LSB size is
dependent on the analog input range selected.
D
C2
R1
V
0
IN
C1
D
Table 7. LSB Sizes for Each Analog Input Range
V
Input Range
Full-Scale Range/8192 Codes
LSB Size
2.441 mV
1.22 mV
0.61 mV
1.22 mV
SS
10 V
5 V
2.5 V
0 V to +10 V
20 V
10 V
5 V
Figure 29. Equivalent Analog Input Circuit (Single-Ended)
10 V
Rev. 0 | Page 17 of 36
AD7321
V
DD
The AD7321 enters track mode on the 14th SCLK rising edge.
When running the AD7321 at a throughput rate of ±ꢀꢀ kSPS
with a 1ꢀ MHz SCLK signal, the ADC has approximately
D
C2
R1
V
+
IN
C1
D
1.± SCLK + t8 + tQUIET
V
SS
to acquire the analog input signal. The ADC goes back into
V
DD
CS
hold mode on the
falling edge.
D
C2
R1
As the ꢁDD/ꢁSS supply voltage is reduced, the on resistance of
the input multiplexer increases. Therefore, based on the
equation for tACQ, it is necessary to increase the amount of
acquisition time provided to the AD7321, and hence decrease
the overall throughput rate. Figure 31 shows that as the ꢁDD and
V
–
IN
C1
D
V
SS
Figure 30. Equivalent Analog Input Circuit (Differential)
ꢁ
SS supplies are reduced, the specified THD performance
Care should be taken to ensure that the analog input does not
exceed the ꢁDD and ꢁSS supply rails by more than 3ꢀꢀ mꢁ.
Exceeding this value causes the diodes to become forward
biased and to start conducting into either the ꢁDD supply rail or
degrades slightly. If the throughput rate is reduced when
operating with the minimum ꢁDD and ꢁSS supplies, the specified
THD performance is maintained.
–75
ꢁSS supply rail. These diodes can conduct up to 1ꢀ mA without
V
= V
= 5V
CC
DRIVE
INTERNAL REFERENCE
causing irreversible damage to the part.
T
F
= 25°C
= 10kHz
A
IN
In Figure 29 and Figure 3ꢀ, Capacitor C1 is typically 4 pF and
can primarily be attributed to pin capacitance. Resistor R1 is a
lumped component made up of the on resistance of the input
multiplexer and the track-and-hold switch. Capacitor C2 is the
sampling capacitor; its capacitance varies depending on the
analog input range selected (see the Specifications section).
±5V RANGE
SE MODE
–80
–85
–90
–95
Track-and-Hold Section
500kSPS
11
The track-and-hold on the analog input of the AD7321 allows
the ADC to accurately convert an input sine wave of full-scale
amplitude to 13-bit accuracy. The input bandwidth of the track-
and-hold is greater than the Nyquist rate of the ADC. The
AD7321 can handle frequencies up to 22 MHz.
5
7
9
13
15
17
19
±V /V SUPPLIES (V)
DD SS
Figure 31. THD vs.
VDD/VSS Supply Voltage at 500 kSPS
The track-and-hold enters its tracking mode on the 14th SCLK
Unlike other bipolar ADCs, the AD7321 does not have a
resistive analog input structure. On the AD7321, the bipolar
analog signal is sampled directly onto the sampling capacitor.
This gives the AD7321 high analog input impedance. An
approximation for the analog input impedance can be
calculated from the following formula:
CS
rising edge after the
falling edge. The time required to
acquire an input signal depends on how quickly the sampling
capacitor is charged. With ꢀ source impedance, 3ꢀ± ns is
sufficient to acquire the signal to the 13-bit level. The
acquisition time required is calculated using the following
formula:
Z = 1/(fS × CS)
t
ACQ = 1ꢀ × ((RSOURCE + R) C)
where fS is the sampling frequency, and CS is the sampling
capacitor value.
where C is the sampling capacitance and R is the resistance seen
by the track-and-hold amplifier looking back on the input. For
the AD7321, the value of R includes the on resistance of the
input multiplexer and is typically 3ꢀꢀ Ω. RSOURCE should include
any extra source impedance on the analog input.
CS depends on the analog input range chosen (see the
Specifications section). When operating at ±ꢀꢀ kSPS, the analog
input impedance is typically 14± kꢃ for the ±1ꢀ ꢁ range. As the
sampling frequency is reduced, the analog input impedance
further increases. As the analog input impedance increases, the
current required to drive the analog input therefore decreases.
Rev. 0 | Page 18 of 36
AD7321
V+
5V
TYPICAL CONNECTION DIAGRAM
Figure 32 shows a typical connection diagram for the AD7321.
In this configuration, the AGND pin is connected to the analog
ground plane of the system, and the DGND pin is connected to
the digital ground plane of the system. The analog inputs on the
AD7321 can be configured to operate in single-ended, true
differential, or pseudo differential mode. The AD7321 can operate
with either an internal or external reference. In Figure 32, the
AD7321 is configured to operate with the internal 2.± ꢁ
reference. A 68ꢀ nF decoupling capacitor is required when
operating with the internal reference.
AGND
V
+
IN
V
V
CC
DD
AD73211
V
SS
V–
ADDITIONAL PINS OMITTED FOR CLARITY.
1
Figure 33. Single-Ended Mode Typical Connection Diagram
True Differential Mode
The ꢁCC pin can be connected to either a 3 ꢁ supply voltage or a
± ꢁ supply voltage. The ꢁDD and ꢁSS are the dual supplies for the
high voltage analog input structures. The voltage on these pins
must be equal to or greater than the highest analog input range
selected on the analog input channels (see Table 6). The ꢁDRIꢁE
pin is connected to the supply voltage of the microprocessor.
The voltage applied to the ꢁDRIꢁE input controls the voltage of
the serial interface. ꢁDRIꢁE can be set to 3 ꢁ or ± ꢁ.
The AD7321 can have one true differential analog input pair.
Differential signals have some benefits over single-ended
signals, including better noise immunity based on the device’s
common-mode rejection and improvements in distortion
performance. Figure 34 defines the configuration of the true
differential analog inputs of the AD7321.
V
+
IN
+15V
V
+ 2.7V TO 5.25V
CC
+
+
0.1µF
10µF
10µF
0.1µF
AD73211
V
–
IN
1
V
V
CC
DD
+3V SUPPLY
V
DRIVE
+
10µF
0.1µF
1
ADDITIONAL PINS OMITTED FOR CLARITY.
AD7321
Figure 34. True Differential Inputs
CS
DOUT
SCLK
DIN
µC/µP
ANALOG INPUTS
±10V, ±5V, ±2.5V
0V TO +10V
The amplitude of the differential signal is the difference
between the signals applied to the ꢁIN+ and ꢁIN− pins in
each differential pair (ꢁIN+ − ꢁIN−). ꢁIN+ and ꢁIN− should
be simultaneously driven by two signals each of amplitude
±4 × ꢁREF (depending on the input range selected) that
are 18ꢀ° out of phase. Assuming the ±4 × ꢁREF mode, the
amplitude of the differential signal is −2ꢀ ꢁ to +2ꢀ ꢁ p-p
(2 × 4 × ꢁREF), regardless of the common mode.
V
V
0
IN
1
IN
DGND
SERIAL
INTERFACE
REFIN/OUT
1
680nF
AGND
V
SS
–15V
1
+
10µF
0.1µF
MINIMUM V AND V SUPPLY VOLTAGES
DD SS
DEPEND ON THE HIGHEST ANALOG INPUT
RANGE SELECTED.
The common mode is the average of the two signals
(ꢁIN+ + ꢁIN−)/2
Figure 32. Typical Connection Diagram
ANALOG INPUT
Single-Ended Inputs
and is therefore the voltage on which the two input signals are
centered.
The AD7321 has a total of two analog inputs when operating
the AD7321 in single-ended mode. Each analog input can be
independently programmed to one of the four analog input
ranges. In applications where the signal source is high
impedance, it is recommended to buffer the signal before
applying it to the ADC analog inputs. Figure 33 shows the
configuration of the AD7321 in single-ended mode.
This voltage is set up externally, and its range varies with
reference voltage. As the reference voltage increases, the
common-mode range decreases. When driving the differential
inputs with an amplifier, the actual common-mode range is
determined by the amplifier’s output swing. If the differential
inputs are not driven from an amplifier, the common-mode
range is determined by the supply voltage on the ꢁDD supply pin
and the ꢁSS supply pin.
When a conversion takes place, the common mode is rejected,
resulting in a noise-free signal of amplitude −2 × (4 × ꢁREF) to
+2 × (4 × ꢁREF) corresponding to Digital Codes −4ꢀ96 to +4ꢀ9±.
Rev. 0 | Page 19 of 36
AD7321
5
8
6
±5V RANGE
±5V RANGE
4
±2.5V
RANGE
±10V
RANGE
±5V RANGE
3
±2.5V
RANGE
4
2
±10V
RANGE
1
2
0
0
±10V
RANGE
–1
–2
–3
–4
±2.5V
RANGE
–2
–4
–6
–8
±10V
RANGE
±5V RANGE
V
V
= 3V
CC
±2.5V
RANGE
V
V
= 5V
–5
–6
CC
= 3V
REF
= 2.5V
REF
±16.5V V /V
±12V V /V
±16.5V V /V
±12V V /V
DD SS
DD SS
DD SS
DD SS
Figure 35. Common-Mode Range for VCC = 3 V and REFIN/OUT = 3 V
Figure 38. Common-Mode Range for VCC = 5 V and REFIN/OUT = 2.5 V
8
Pseudo Differential Inputs
±5V RANGE
±5V RANGE
The AD7321 can have one pseudo differential pair. The ꢁIN+
inputs are coupled to the signal source and must have an amplitude
within the selected range for that channel as programmed in the
range register. A dc input is applied to the ꢁIN− pin. The voltage
applied to this input provides an offset for the ꢁIN+ input from
ground or a pseudo ground. Pseudo differential inputs separate
the analog input signal ground from the ADC ground, allowing
cancellation of dc common-mode voltages.
6
±2.5V
RANGE
±2.5V
RANGE
4
2
±10V
RANGE
±10V
RANGE
0
–2
–4
When a conversion takes place, the pseudo ground corresponds
to Code −4ꢀ96 and the maximum amplitude corresponds to
Code +4ꢀ9±.
V
V
= 5V
CC
= 3V
REF
±16.5V V /V
±12V V /V
DD SS
DD SS
V+
5V
Figure 36. Common-Mode Range for VCC = 5 V and REFIN/OUT = 3 V
6
V
V
+
IN
V
V
CC
DD
4
±5V RANGE
±5V RANGE
AD73211
2
0
V
–
SS
IN
–2
–4
–6
–8
V–
±2.5V
±10V
RANGE RANGE
±10V
±2.5V
RANGE
1
ADDITIONAL PINS OMITTED FOR CLARITY.
RANGE
Figure 39. Pseudo Differential Inputs
V
V
= 3V
CC
Figure 4ꢀ and Figure 41 show the typical voltage range on the
ꢁIN− pin for the different analog input ranges when configured
in the pseudo differential mode.
= 2.5V
REF
±16.5V V /V
±12V V /V
DD SS
DD SS
Figure 37. Common-Mode Range for VCC = 3 V and REFIN/OUT = 2.5 V
For example, when the AD7321 is configured to operate in
pseudo differential mode and the ±± ꢁ range is selected with
±16.± ꢁ ꢁDD/ꢁSS supplies and ± ꢁ ꢁCC, the voltage on the ꢁIN−
pin can vary from −6.± ꢁ to +6.± ꢁ.
Rev. 0 | Page 20 of 36
AD7321
8
6
The driver amplifier must be able to settle for a full-scale step
to a 13-bit level, ꢀ.ꢀ122%, in less than the specified acquisition
time of the AD7321. An op amp such as the AD8ꢀ21 meets this
requirement when operating in single-ended mode. The AD8ꢀ21
needs an external compensating NPO type of capacitor. The
AD8ꢀ22 can also be used in high frequency applications where
a dual version is required. For lower frequency applications, op
amps such as the AD797, AD84±, and AD861ꢀ can be used with
the AD7321 in single-ended mode configuration.
±5V RANGE
±2.5V
RANGE
±5V RANGE
±2.5V
RANGE
±10V
RANGE
4
2
0
–2
–4
–6
–8
±10V
RANGE
Differential operation requires that ꢁIN+ and ꢁIN− be
simultaneously driven with two signals of equal amplitude that
are 18ꢀ° out of phase. The common mode must be set up
externally to the AD7321. The common-mode range is
determined by the REFIN/OUT voltage, the ꢁCC supply voltage,
and the particular amplifier used to drive the analog inputs.
Differential mode with either an ac input or a dc input provides
the best THD performance over a wide frequency range. Because
not all applications have a signal preconditioned for differential
operation, there is often a need to perform the single-ended-to-
differential conversion.
0V TO +10V
RANGE
0V TO +10V
RANGE
V
V
= 5V
REF
CC
= 2.5V
±16.5V V /V
DD SS
±12V V /V
DD SS
Figure 40. Pseudo Input Range with VCC = 5 V
4
2
±5V RANGE
±5V RANGE
±2.5V
RANGE
0
–2
–4
–6
–8
±10V
RANGE
This single-ended-to-differential conversion can be performed
using an op amp pair. Typical connection diagrams for an op
amp pair are shown in Figure 42 and Figure 43. In Figure 42,
the common-mode signal is applied to the noninverting input
of the second amplifier.
±2.5V
RANGE
±10V
RANGE
0V TO +10V
RANGE
0V TO +10V
RANGE
V
V
= 3V
= 2.5V
CC
REF
1.5kΩ
±16.5V V /V
DD SS
±12V V /V
DD SS
Figure 41. Pseudo Input Range with VCC = 3 V
3kΩ
AD845
V
IN
V+
DRIVER AMPLIFIER CHOICE
In applications where the harmonic distortion and signal-to-
noise ratio are critical specifications, the analog input of the
AD7321 should be driven from a low impedance source. Large
source impedances significantly affect the ac performance of the
ADC and can necessitate the use of an input buffer amplifier.
1.5kΩ
1.5kΩ
1.5kΩ
10kΩ
V–
V
COM
AD845
20kΩ
When no amplifier is used to drive the analog input, the source
impedance should be limited to low values. The maximum
source impedance depends on the amount of THD that can be
tolerated in the application. The THD increases as the source
impedance increases and performance degrades. Figure 21 and
Figure 22 show graphs of the THD vs. the analog input
frequency for various source impedances. Depending on the
input range and analog input configuration selected, the
AD7321 can handle source impedances of up to ±.± kΩ before
the THD starts to degrade.
Figure 42. Single-Ended-to-Differential Configuration with the AD845
442Ω
442Ω
AD8021
V
IN
V+
442Ω
442Ω
442Ω
Due to the programmable nature of the analog inputs on the
AD7321, the choice of op amp used to drive the inputs is a
function of the particular application and depends on the input
configuration and the analog input voltage ranges selected.
442Ω
V–
AD8021
100Ω
Figure 43. Single-Ended-to-Differential Configuration with the AD8021
Rev. 0 | Page 21 of 36
AD7321
REGISTERS
The AD7321 has two programmable registers, the control register and the range register. These registers are write-only registers.
ADDRESSING REGISTERS
A serial transfer on the AD7321 consists of 16 SCLK cycles. The three MSBs on the DIN line during the 16 SCLK transfer are decoded to
determine which register is addressed. The three MSBs consist of the write bit, a ZERO bit and the register select bit. The register select
bit is used to determine which of the two on-board registers is selected. The write bit determines if the data on the DIN line following the
register select bit loads into the addressed register. If the write bit is 1, the bits load into the register addressed by the register select bit. If
the write bit is ꢀ, the data on the DIN line does not load into any register.
Table 8. Decoding Register Select Bits and Write Bit
Write ZERO
Register Select
Description
0
1
0
0
0
0
Data on the DIN line during this serial transfer is ignored.
This combination selects the control register. The subsequent 12 bits are loaded into
the control register.
1
0
1
This combination selects the range register. The subsequent 6 bits are loaded into the
range register.
Rev. 0 | Page 22 of 36
AD7321
CONTROL REGISTER
The control register is used to select the analog input channel, analog input configuration, reference, coding, and power mode. The
control register is a write-only, 12-bit register. Data loaded on the DIN line corresponds to the AD7321 configuration for the next
conversion. Data should be loaded into the control register after the range register has been initialized. The bit functions of the control
register are shown in Table 9 (the power-up status of all bits is ꢀ).
MSB
LSB
0
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
Write ZERO Register Select
ZERO ZERO ADD0 Mode 1 Mode 0 PM1 PM0 Coding Ref Seq1 Seq2 ZERO
0
Table 9. Control Register Details
Bit
Mnemonic
Description
12, 11, 1
10
ZERO
ADD0
A 0 should be written to these bits at all times.
This channel address bit is used to select the analog input channel for the next conversion if the sequencer
is not being used. If the sequencer is being used, the channel address bit is used to select the final channel
in a consecutive sequence.
9, 8
Mode 1, Mode 0
These two mode bits are used to select the configuration of the two analog input pins, VIN0 to VIN1. These
pins are used in conjunction with the channel address bit. On the AD7321, the analog inputs can be
configured as two single-ended inputs, one true differential input pair, or one pseudo differential input
(see Table 10).
7, 6
5
PM1, PM0
Coding
The power management bits are used to select different power mode options on the AD7321 (see Table 11).
This bit is used to select the type of output coding the AD7321 uses for the next conversion result. If
coding = 0, the output coding is twos complement. If coding = 1, the output coding is straight binary.
When operating in sequence mode, the output coding for each channel is the value written to the coding
bit during the last write to the control register.
4
Ref
The reference bit is used to enable or disable the internal reference. If Ref = 0, the external reference is
enabled and used for the next conversion, and the internal reference is disabled. If Ref = 1, the internal
reference is used for the next conversion. When operating in sequence mode, the reference used for each
channel is the value written to the Ref bit during the last write to the control register.
3, 2
Seq1/Seq2
The Sequence 1 and Sequence 2 bits are used to control the operation of the sequencer (see Table 12).
The two analog input channels can be configured as one pseudo differential analog input, one true differential input pair, or two single-
ended analog inputs.
Table 10. Analog Input Configuration Selection
Mode 1 = 1, Mode 0 = 1
Mode 1 = 1, Mode 0 = 0
1 Fully Differential I/P
Mode 1 = 0, Mode 0 =1
1 Pseudo Differential I/P
Mode 1 = 0, Mode 0 = 0
2 Single-Ended I/Ps
Channel Address Bit
Not Allowed
ADD0
VIN+
VIN−
VIN+
VIN0
VIN0
VIN−
VIN1
VIN1
VIN+
VIN0
VIN0
VIN−
VIN1
VIN1
VIN+
VIN0
VIN1
VIN−
0
1
–
–
–
–
AGND
AGND
Rev. 0 | Page 23 of 36
AD7321
Table 11. Power Mode Selection
PM1 PM0 Description
1
1
0
0
1
0
1
0
Full Shutdown Mode. In this mode, all internal circuitry on the AD7321 is powered down. Information in the control register
is retained when the AD7321 is in full shutdown mode.
Autoshutdown Mode. The AD7321 enters autoshutdown on the 15th SCLK rising edge when the control register is updated.
All internal circuitry is powered down in autoshutdown.
Autostandby Mode. In this mode, all internal circuitry is powered down, excluding the internal reference. The AD7321 enters
autostandby mode on the 15th SCLK rising edge after the control register is updated.
Normal Mode. All internal circuitry is powered up at all times.
Table 12. Sequencer Selection
Seq1 Seq2 Description
0
0
The channel sequencer is not used. The analog channel, selected by programming the ADD0 bit in the control register,
selects the next channel for conversion.
1
0
This configuration is used in conjunction with the channel address bit in the control register. This allows continuous
conversions on a consecutive sequence of channels, from Channel 0 through a final channel selected by the channel
address bit in the control register. The range for each channel defaults to the range previously written into the range
register.
1
1
The channel sequencer is not used. The analog channel, selected by programming the ADD0 bit in the control register,
selects the next channel for conversion.
Rev. 0 | Page 24 of 36
AD7321
RANGE REGISTER
The range register is used to select one analog input range per analog input channel and to set the ranges for Channel ꢀ and Channel 1. It
is a 6-bit, write-only register with two dedicated range bits for each of the analog input channels, Channel ꢀ and Channel 1. There are four
analog input ranges, ±1ꢀ ꢁ, ±± ꢁ, ±2.± ꢁ, and ꢀ ꢁ to +1ꢀ ꢁ. A write to the range register is selected by setting the write bit to 1, the ZERO
bit to ꢀ, and the register select bit to 1. After the initial write to the range register occurs, each time an analog input is selected, the
AD7321 automatically configures the analog input to the appropriate range, as indicated by the range register. The ±1ꢀ ꢁ input range is
selected by default on each analog input channel (see Table 13).
MSB
16
LSB
1
15
14
13
12
11
0
10
0
9
8
7
0
6
0
5
0
4
0
3
0
2
0
Write
ZERO
Register Select
VIN0A
VIN0B
VIN1A
VIN1B
0
Table 13. Range Selection
VINxA
VINxB
Description
0
0
1
1
0
1
0
1
This combination selects the 10 V input range on VINx.
This combination selects the 5 V input range on VINx.
This combination selects the 2.5 V input range on VINx.
This combination selects the 0 V to +10 V input range on VINx.
Rev. 0 | Page 25 of 36
AD7321
SEQUENCER OPERATION
The AD7321 can be configured to convert a sequence of
consecutive channels (see Figure 44). This sequence begins by
converting on Channel ꢀ and ends with a final channel as selected
by Bit ADDꢀ in the control register. In this configuration, there is
no need for a write to the sequence register. To operate the AD7321
in this mode, set Seq1 to 1 and Seq2 to ꢀ, and then select the final
channel in the sequence by programming Bit ADDꢀ in the control
register.
Once the control register is configured to operate the AD7321
in this mode, the DIN line can be held low, or the write bit can
be set to ꢀ. To return to traditional multichannel operation, a
write to the control register to set Seq1 to ꢀ and Seq2 to ꢀ is
necessary.
When Seq1 and Seq2 are both set to ꢀ, or when both are set to
1, the AD7321 is configured to operate in traditional multi-
channel mode, where a write to Channel Address Bit ADDꢀ in
the control register selects the next channel for conversion.
POWER ON.
CS
DIN: WRITE TO RANGE REGISTER TO SELECT THE RANGE
FOR THE ANALOG INPUT CHANNELS.
DOUT: CONVERSION RESULT FROM CHANNEL 0, ±10V
RANGE, SINGLE-ENDED MODE.
CS
DIN: WRITE TO CONTROL REGISTER TO SELECT THE FINAL
CHANNEL IN THE CONSECUTIVE SEQUENCE, SET Seq1 = 1
AND Seq2 = 0. SELECT OUTPUT CODING FOR SEQUENCE.
DOUT: CONVERSION RESULT FROM CHANNEL 0,
RANGE SELECTED IN RANGE REGISTER 1,
SINGLE-ENDED MODE.
CS
DIN: WRITE BIT = 0 OR DIN LINE HELD LOW TO CONTINUE
TO CONVERT THROUGH THE SEQUENCE OF
CONSECUTIVE CHANNELS.
DOUT: CONVERSION RESULT FROM CHANNEL 0, RANGE AS
SELECTED IN RANGE REGISTER.
CS
DIN: WRITE BIT = 0 OR DIN LINE HELD LOW TO CONTINUE
THROUGH SEQUENCE OF CONSECUTIVE CHANNELS.
DOUT: CONVERSION RESULT FROM CHANNEL1, RANGE AS
SELECTED IN RANGE REGISTER.
DIN TIED LOW/WRITE BIT = 0.
CONTINUOUSLY CONVERT
ON CONSECUTIVE SEQUENCE
OF CHANNELS.
STOPPING
A SEQUENCE.
CS
DIN: WRITE TO CONTROL
REGISTER TO STOP THE
SEQUENCE, Seq1 = 0, Seq2 = 0.
DOUT: CONVERSION RESULT
FROM CHANNEL IN SEQUENCE.
Figure 44. Flowchart for Consecutive Sequence of Channels
Rev. 0 | Page 26 of 36
AD7321
conversion result from the first initial conversion is invalid. The
reference buffer requires ±ꢀꢀ ꢄs to power up and charge the
68ꢀ nF decoupling capacitor during the power-up time.
REFERENCE
The AD7321 can operate with either the internal 2.± ꢁ on-chip
reference or an externally applied reference. The internal
reference is selected by setting the Ref bit in the control register
to 1. On power-up, the Ref bit is ꢀ, which selects the external
reference for the AD7321 conversion. Suitable reference sources
for the AD7321 include AD78ꢀ, AD1±82, ADR431, REF193,
and ADR391.
The AD7321 is specified for a 2.± ꢁ to 3 ꢁ reference range.
When a 3 ꢁ reference is selected, the ranges are ±12 ꢁ, ±6 ꢁ,
±3 ꢁ, and ꢀ ꢁ to +12 ꢁ. For these ranges, the ꢁDD and ꢁSS supply
must be equal to or greater than the maximum analog input
range selected (see Table 6).
The internal reference circuitry consists of a 2.± ꢁ band gap
reference and a reference buffer. When operating the AD7321
in internal reference mode, the 2.± ꢁ internal reference is available
at the REFIN/OUT pin, which should be decoupled to AGND
using a 68ꢀ nF capacitor. It is recommended that the internal
reference be buffered before applying it elsewhere in the system.
The internal reference is capable of sourcing up to 9ꢀ μA.
VDRIVE
The AD7321 has a ꢁDRIꢁE feature to control the voltage at which
the serial interface operates. ꢁDRIꢁE allows the ADC to easily
interface to both 3 ꢁ and ± ꢁ processors. For example, if the
AD7321 is operated with a ꢁCC of ± ꢁ, the ꢁDRIꢁE pin can be
powered from a 3 ꢁ supply. This allows the AD7321 to accept
large bipolar input signals with low voltage digital processing.
On power-up, if the internal reference operation is required for
the ADC conversion, a write to the control register is necessary
to set the Ref bit to 1. During the control register write, the
Rev. 0 | Page 27 of 36
AD7321
MODES OF OPERATION
The AD7321 has several modes of operation that are designed
to provide flexible power management options. These options
can be chosen to optimize the power dissipation/throughput
rate ratio for different application requirements. The mode of
operation of the AD7321 is controlled by the power management
bits, Bit PM1 and Bit PMꢀ, in the control register as shown in
Table 11. The default mode is normal mode, where all internal
circuitry is fully powered up.
The AD7321 remains fully powered up at the end of the
conversion if both PM1 and PMꢀ contain ꢀ in the control
register.
To complete the conversion and access the conversion result,
16 serial clock cycles are required. At the end of the conversion,
CS
can idle either high or low until the next conversion.
Once the data transfer is complete, another conversion can be
initiated after the quiet time, tQUIET, has elapsed.
NORMAL MODE
(PM1 = PM0 = 0)
FULL SHUTDOWN MODE
(PM1 = PM0 = 1)
This mode is intended for the fastest throughput rate performance
with the AD7321 being fully powered up at all times. Figure 4±
shows the general operation of the AD7321 in normal mode.
In this mode, all internal circuitry on the AD7321 is powered
down. The part retains information in the registers during full
shutdown. The AD7321 remains in full shutdown mode until
the power management bits, Bit PM1 and Bit PMꢀ, in the
control register are changed.
CS
The conversion is initiated on the falling edge of , and the
track-and-hold section enters hold mode as described in the
Serial Interface section. Data on the DIN line during the
16 SCLK transfer is loaded into one of the on-chip registers if
the write bit is set. The register is selected by programming the
register select bits (see Table 8).
A write to the control register with PM1 = 1 and PMꢀ = 1 places
the part into full shutdown mode. The AD7321 enters full shut-
down mode on the 1±th SCLK rising edge once the control register
is updated.
CS
If a write to the control register occurs while the part is in full
shutdown mode with the power management bits, Bit PM1 and
Bit PMꢀ, set to ꢀ (normal mode), the part begins to power up
on the 1±th SCLK rising edge once the control register is
updated. Figure 46 shows how the AD7321 is configured to exit
full shutdown mode. To ensure the AD7321 is fully powered up,
1
16
SCLK
2 LEADING ZEROS, CHANNEL I.D. BIT, SIGN BIT
+ CONVERSION RESULT
DOUT
DIN
DATA INTO CONTROL/RANGE REGISTER
CS
tPOWER-UP should elapse before the next
falling edge.
Figure 45. Normal Mode
THE PART IS FULLY POWERED UP
ONCE tPOWER-UP HAS ELAPSED
PART IS IN FULL
SHUTDOWN
PART BEGINS TO POWER UP ON THE 15TH
SCLK RISING EDGE AS PM1 = PM0 = 0
tPOWER-UP
CS
1
16
1
16
SCLK
INVALID DATA
CHANNEL IDENTIFIER BITS + CONVERSION RESULT
DATA INTO CONTROL REGISTER
SDATA
DIN
DATA INTO CONTROL REGISTER
CONTROL REGISTER IS LOADED ON THE FIRST 15 CLOCKS,
PM1 = 0, PM0 = 0
TO KEEP THE PART IN NORMAL MODE, LOAD PM1 = PM0 = 0
IN CONTROL REGISTER
Figure 46. Exiting Full Shutdown Mode
Rev. 0 | Page 28 of 36
AD7321
As is the case with autoshutdown mode, the AD7321 enters
AUTO SHUTDOWN MODE
standby on the 1±th SCLK rising edge once the control register
is updated (see Figure 47). The part retains information in the
registers during standby. The AD7321 remains in standby until
(PM1 = 1, PM0 = 0)
Once the autoshutdown mode is selected, the AD7321 auto-
matically enters shutdown on the 1±th SCLK rising edge. In
autoshutdown mode, all internal circuitry is powered down.
The AD7321 retains information in the registers during
autoshutdown. The track-and-hold is in hold mode during
CS
it receives a
rising edge. The ADC begins to power up on the
CS
CS
rising edge, the track-and-hold, which
rising edge. On the
was in hold mode while the part was in standby, returns to track.
The power-up time from standby is 7ꢀꢀ ns. The user should
CS
autoshutdown. On the rising
edge, the track-and-hold,
CS
ensure that 7ꢀꢀ ns have elapsed before bringing
low to
which was in hold during shutdown, returns to track as the
AD7321 begins to power up. The power-up from autoshutdown
is ±ꢀꢀ ꢄs.
attempt a valid conversion. Once this valid conversion is
complete, the AD7321 again returns to standby on the 1±th
CS
SCLK rising edge. The
part in standby mode.
signal must remain low to keep the
When the control register is programmed to transition to
autoshutdown mode, it does so on the 1±th SCLK rising edge.
Figure 47 shows the part entering autoshutdown mode. The
Figure 47 shows the part entering autoshutdown mode. The
sequence of events is the same when entering autostandby
mode. In Figure 47, the power management bits are configured
for autoshutdown. For autostandby mode, the power
management bits, PM1 and PMꢀ, should be set to ꢀ and 1,
respectively.
CS
AD7321 automatically begins to power up on the
edge. The tPOWER-UP is required before a valid conversion, initiated
CS
rising
by bringing the
signal low, can take place. Once this valid
conversion is complete, the AD7321 powers down again on the
th
CS
1± SCLK rising edge. The
signal must remain low again to
keep the part in autoshutdown mode.
AUTOSTANDBY MODE
(PM1 = 0, PM0 =1)
In autostandby mode, portions of the AD7321 are powered
down, but the on-chip reference remains powered up. The
reference bit in the control register should be 1 to ensure that
the on-chip reference is enabled. This mode is similar to auto-
shutdown but allows the AD7321 to power up much faster,
which allows faster throughput rates.
PART BEGINS TO POWER
UP ON CS RISING EDGE
THE PART IS FULLY POWERED UP
ONCE tPOWER-UP HAS ELAPSED
PART ENTERS SHUTDOWN MODE
ON THE 15TH RISING SCLK EDGE
AS PM1 = 1, PM0 = 0
tPOWER-UP
CS
1
15 16
1
15 16
SCLK
SDATA
DIN
VALID DATA
VALID DATA
DATA INTO CONTROL REGISTER
DATA INTO CONTROL REGISTER
CONTROL REGISTER IS LOADED ON THE FIRST 15 CLOCKS,
PM1 = 1, PM0 = 0
Figure 47. Entering Autoshutdown/Autostandby Mode
Rev. 0 | Page 29 of 36
AD7321
POWER VS. THROUGHPUT RATE
20
18
16
14
12
10
8
The power consumption of the AD7321 varies with throughput
rate. The static power consumed by the AD7321 is very low, and
significant power savings can be achieved as the throughput
rate is reduced. Figure 48 and Figure 49 shows the power vs.
throughput rate for the AD7321 at a ꢁCC of 3 ꢁ and ± ꢁ,
respectively. Both plots clearly show that the average power
consumed by the AD7321 is greatly reduced as the sample
frequency is reduced. This is true whether a fixed SCLK value is
used or if it is scaled with the sampling frequency. Figure 48 and
Figure 49 show the power consumption when operating in
normal mode for a variable SCLK that scales with the sampling
frequency.
V
V
= 5V
CC
/V = ±12V
DD SS
T
= 25°C
A
INTERNAL REFERENCE
6
VARIABLE SCLK
4
2
0
0
100
200
300
400
500
12
V
V
= 3V
CC
THROUGHPUT RATE (kSPS)
/V = ±12V
DD SS
Figure 49. Power vs. Throughput Rate with 5 V VCC
T
= 25°C
A
10
8
INTERNAL REFERENCE
6
VARIABLE SCLK
4
2
0
0
100
200
300
400
500
THROUGHPUT RATE (kSPS)
Figure 48. Power vs. Throughput Rate with 3 V VCC
Rev. 0 | Page 30 of 36
AD7321
SERIAL INTERFACE
Figure ±ꢀ shows the timing diagram for the serial interface of
the AD7321. The serial clock applied to the SCLK pin provides
the conversion clock and controls the transfer of information to
and from the AD7321 during a conversion.
Data is clocked into the AD7321 on the SCLK falling edge. The
three MSBs on the DIN line are decoded to select which register
is being addressed. The control register is a 12-bit register. If the
control register is addressed by the three MSBs, the data on the
DIN line is loaded into the control on the 1±th SCLK falling
edge. If the range register is addressed, the data on the DIN line is
loaded into the addressed register on the 11th SCLK falling edge.
CS
The
signal initiates the data transfer and the conversion
CS
process. The falling edge of
puts the track-and-hold into
hold mode and takes the bus out of three-state. Then the analog
input signal is sampled. Once the conversion is initiated, it
requires 16 SCLK cycles to complete.
Conversion data is clocked out of the AD7321 on each SCLK
falling edge. Data on the DOUT line consists of two leading
ZEROs, a channel identifier bit, a sign bit, and a 12-bit conversion
result. The channel identifier bit is used to indicate which channel
corresponds to the conversion result. The first ZERO bit is clocked
The track-and-hold goes back into track mode on the 14th SCLK
rising edge. On the 16th SCLK falling edge, the DOUT line returns
to three-state. If the rising edge of occurs before 16 SCLK cycles
have elapsed, the conversion is terminated, and the DOUT line
returns to three-state. Depending on where the signal is brought
CS
CS
out on the
falling edge, and the second bit is clocked out on
the first SCLK falling edge.
CS
high, the addressed register may be updated.
t1
CS
tCONVERT
t2
t6
1
2
3
4
5
13
14
t5
15
16
SCLK
DOUT
IDENTIFICATION BIT
t7
t8
t4
t3
ZERO
tQUIET
ADD0
SIGN
DB11
DB10
DB2
DB1
DB0
THREE-
STATE
ZERO
THREE-STATE
t10
t9
REG
SEL
DON’T
CARE
WRITE ZERO
MSB
LSB
DIN
Figure 50. Serial Interface Timing Diagram (Control Register Write)
Rev. 0 | Page 31 of 36
AD7321
MICROPROCESSOR INTERFACING
The serial interface on the AD7321 allows the part to be directly
connected to a range of different microprocessors. This section
explains how to interface the AD7321 with some common
microcontroller and DSP serial interface protocols.
The frequency of the serial clock is set in the SCLKDIꢁ register.
When the instruction to transmit with TFS is given (AXꢀ = TXꢀ),
the state of the serial clock is checked. The DSP waits until the
SCLK has gone high, low, and high again before starting the
transmission. If the timer and SCLK are chosen so that the
instruction to transmit occurs on or near the rising edge of SCLK,
data can be transmitted immediately or at the next clock edge.
AD7321 TO ADSP-21xx
The ADSP-21xx family of DSPs interface directly to the AD7321
without requiring glue logic. The ꢁDRIꢁE pin of the AD7321 takes
the same supply voltage as that of the ADSP-21xx. This allows the
ADC to operate at a higher supply voltage than its serial interface.
The SPORTꢀ on the ADSP-21xx should be configured as shown
in Table 14.
For example, the ADSP-2111 has a master clock frequency of
16 MHz. If the SCLKDIꢁ register is loaded with the value 3, an
SCLK of 2 MHz is obtained, and eight master clock periods elapse
for every one SCLK period. If the timer registers are loaded with
the value 8ꢀ3, 1ꢀꢀ.± SCLKs occur between interrupts and, sub-
sequently, between transmit instructions. This situation leads to
nonequidistant sampling because the transmit instruction occurs
on an SCLK edge. If the number of SCLKs between interrupts is
an integer of N, equidistant sampling is implemented by the DSP.
Table 14. SPORT0 Control Register Setup
Setting
Description
TFSW = RFSW = 1
INVRFS = INVTFS = 1
DTYPE = 00
SLEN = 1111
ISCLK = 1
Alternative framing
Active low frame signal
Right justify data
16-bit data-word
Internal serial clock
Frame every word
AD7321 TO ADSP-BF53x
The ADSP-BF±3x family of DSPs interfaces directly to the
AD7321 without requiring glue logic, as shown in Figure ±2.
The SPORTꢀ Receive Configuration 1 register should be set
up as outlined in Table 15.
TFSR = RFSR = 1
IRFS = 0
ITFS = 1
The connection diagram is shown in Figure ±1. The ADSP-21xx
has TFSꢀ and RFSꢀ tied together. TFSꢀ is set as an output, and
RFSꢀ is set as an input. The DSP operates in alternative framing
mode, and the SPORTꢀ control register is set up as described in
Table 14. The frame synchronization signal generated on the TFS
1
AD73211
ADSP-BF53x
SCLK
RSCLK0
RFS0
DT0
CS
CS
is tied to , and, as with all signal processing applications, requires
DIN
equidistant sampling. However, as in this example, the timer
interrupt is used to control the sampling rate of the ADC, and
under certain conditions equidistant sampling cannot be achieved.
DR0
DOUT
V
DRIVE
1
AD73211
ADSP-21xx
V
DD
1
ADDITIONAL PINS OMITTED FOR CLARITY.
SCLK
SCLK0
Figure 52. Interfacing the AD7321 to the ADSP-BF53x
TFS0
RFS0
CS
Table 15. SPORT0 Receive Configuration 1 Register
DIN
DT0
DR0
Setting
Description
RCKFE = 1
LRFS = 1
RFSR = 1
Sample data with falling edge of RSCLK
Active low frame signal
Frame every word
Internal RFS used
Receive MSB first
Zero fill
Internal receive clock
Receive enable
DOUT
V
DRIVE
IRFS = 1
V
DD
RLSBIT = 0
RDTYPE = 00
IRCLK = 1
RSPEN = 1
SLEN = 1111
TFSR = RFSR = 1
1
ADDITIONAL PINS OMITTED FOR CLARITY.
Figure 51. Interfacing the AD7321 to the ADSP-21xx
The timer registers are loaded with a value that provides an
interrupt at the required sampling interval. When an interrupt
is received, a value is transmitted with TFS/DT (ADC control
word). The TFS is used to control the RFS, and hence the
reading of data.
16-bit data-word
Rev. 0 | Page 32 of 36
AD7321
APPLICATION HINTS
LAYOUT AND GROUNDING
To avoid radiating noise to other sections of the board, com-
ponents, such as clocks, with fast switching signals should be
shielded with digital ground and never run near the analog inputs.
Avoid crossover of digital and analog signals. To reduce the effects
of feedthrough within the board, traces should be run at right
angles to each other. A microstrip technique is the best method,
but its use may not be possible with a double-sided board. In
this technique, the component side of the board is dedicated
to ground planes, and signals are placed on the other side.
The printed circuit board that houses the AD7321 should be
designed so that the analog and digital sections are confined to
certain areas of the board. This design facilitates the use of
ground planes that can easily be separated.
To provide optimum shielding for ground planes, a minimum
etch technique is generally best. All AGND pins on the AD7321
should be connected to the AGND plane. Digital and analog
ground pins should be joined in only one place. If the AD7321
is in a system where multiple devices require an AGND and
DGND connection, the connection should still be made at only
one point. A star point should be established as close as possible
to the ground pins on the AD7321.
Good decoupling is also important. All analog supplies should
be decoupled with 1ꢀ ꢄF tantalum capacitors in parallel with
ꢀ.1 ꢄF capacitors to AGND. To achieve the best results from
these decoupling components, they must be placed as close as
possible to the device, ideally right up against the device. The
ꢀ.1 ꢄF capacitors should have a low effective series resistance
(ESR) and low effective series inductance (ESI), such as is
typical of common ceramic and surface mount types of
capacitors. These low ESR, low ESI capacitors provide a low
impedance path to ground at high frequencies to handle
transient currents due to internal logic switching.
Good connections should be made to the power and ground
planes. This can be done with a single via or multiple vias for
each supply and ground pin.
Avoid running digital lines under the AD7321 device because
this couples noise onto the die. However, the analog ground
plane should be allowed to run under the AD7321 to avoid
noise coupling. The power supply lines to the AD7321 device
should use as large a trace as possible to provide low impedance
paths and reduce the effects of glitches on the power supply line.
Rev. 0 | Page 33 of 36
AD7321
OUTLINE DIMENSIONS
5.10
5.00
4.90
14
8
7
4.50
4.40
4.30
6.40
BSC
1
PIN 1
0.65
BSC
1.05
1.00
0.80
0.20
0.09
1.20
MAX
0.75
0.60
0.45
8°
0°
0.15
0.05
0.30
0.19
SEATING
PLANE
COPLANARITY
0.10
COMPLIANT TO JEDEC STANDARDS MO-153-AB-1
Figure 53. 14-Lead Thin Shrink Small Outline Package [TSSOP]
(RU-14)
Dimensions show in millimeters
ORDERING GUIDE
Model
Temperature Range
–40°C to +85°C
–40°C to +85°C
Package Description
Package Option
RU-14
RU-14
AD7321BRUZ1
14-Lead TSSOP
14-Lead TSSOP
14-Lead TSSOP
Evaluation Board
Controller Board
AD7321BRUZ-REEL1
AD7321BRUZ-REEL71
EVAL-AD7321CB2
EVAL-CONTROL BRD23
–40°C to +85°C
RU-14
1 Z = Pb-free part.
2 This can be used as a standalone evaluation board or in conjunction with the EVAL-CONTROL board for evaluation/demonstration purposes.
3 This board is a complete unit allowing a PC to control and communicate with all Analog Devices evaluation boards ending in the CB designators. To order a complete
evaluation kit, the particular ADC evaluation board (for example, EVAL-AD7321CB), the EVAL-CONTROL BRD2, and a 12 V transformer must be ordered. See the relevant
evaluation board technical note for more information.
Rev. 0 | Page 34 of 36
AD7321
NOTES
Rev. 0 | Page 35 of 36
AD7321
NOTES
©2006 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D05399–0–1/06(0)
Rev. 0 | Page 36 of 36
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