3 V/5 V, 1 mW, 2-/3-Channel,
16-Bit, Sigma-Delta ADCs
AD7705/AD7706
FEATURES
AD7705: 2 fully differential input channel ADCs
AD7706: 3 pseudo differential input channel ADCs
16 bits no missing codes
0.003% nonlinearity
Programmable gain front end: gains from 1 to 128
3-wire serial interface
SPI®-, QSPI™-, MICROWIRE™-, and DSP-compatible
Schmitt-trigger input on SCLK
Ability to buffer the analog input
2.7 V to 3.3 V or 4.75 V to 5.25 V operation
Power dissipation 1 mW maximum @ 3 V
Standby current 8 μA maximum
16-lead PDIP, 16-lead SOIC, and 16-lead TSSOP packages
FUNCTIONAL BLOCK DIAGRAM
V
DD
REF IN(–)
REF IN(+)
AD7705/AD7706
CHARGE
BALANCING
A/D CONVERTER
ANALOG
INPUT
CHANNELS
MAX
BUFFER
PGA
A = 1
≈
128
DIGITAL FILTER
Σ
-Δ
MODULATOR
SERIAL INTERFACE
REGISTER BANK
MCLK IN
MCLK OUT
CLOCK
GENERATION
SCLK
CS
DIN
GENERAL DESCRIPTION
The AD7705/AD7706 are complete analog front ends for low
frequency measurement applications. These 2-/3-channel devices
can accept low level input signals directly from a transducer and
produce serial digital output. The devices employ a Σ-Δ
conversion technique to realize up to 16 bits of no missing codes
performance. The selected input signal is applied to a
proprietary, programmable-gain front end based around an
analog modulator. The modulator output is processed by an on-
chip digital filter. The first notch of this digital filter can be pro-
grammed via an on-chip control register, allowing adjustment of
the filter cutoff and output update rate.
The AD7705/AD7706 devices operate from a single 2.7 V to
3.3 V or 4.75 V to 5.25 V supply. The AD7705 features two fully
differential analog input channels; the AD7706 features three
pseudo differential input channels.
Both devices feature a differential reference input. Input signal
ranges of 0 mV to 20 mV through 0 V to 2.5 V can be
incorporated on both devices when operating with a V
DD
of 5 V
and a reference of 2.5 V. They can also handle bipolar input
signal ranges of ±20 mV through ±2.5 V, which are referenced to
the AIN(−) inputs on the AD7705 and to the COMMON input
on the AD7706.
DOUT
01166-001
GND
DRDY
RESET
Figure 1.
The AD7705/AD7706 devices, with a 3 V supply and a 1.225 V
reference, can handle unipolar input signal ranges of 0 mV to
10 mV through 0 V to 1.225 V. The devices can accept bipolar
input ranges of ±10 mV through ±1.225 V. Therefore, the
AD7705/AD7706 devices perform all signal conditioning and
conversion for a 2-channel or 3-channel system.
The AD7705/AD7706 are ideal for use in smart, microcontroller,
or DSP-based systems. The devices feature a serial interface that
can be configured for 3-wire operation. Gain settings, signal
polarity, and update rate selection can be configured in software
using the input serial port. The parts contains self-calibration and
system calibration options to eliminate gain and offset errors on
the part itself or in the system. CMOS construction ensures very
low power dissipation, and the power-down mode reduces the
standby power consumption to 20 μW typ.
These parts are available in a 16-lead, wide body (0.3 inch),
plastic dual in-line package (DIP); a 16-lead, wide body
(0.3 inch), standard small outline (SOIC) package; and a low
profile, 16-lead, thin shrink small outline package (TSSOP).
Rev. C
Information furnished by Analog Devices is believed to be accurate and reliable. However, no
responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other
rights of third parties that may result from its use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarks and registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
www.analog.com
Fax: 781.461.3113
©2006 Analog Devices, Inc. All rights reserved.
AD7705/AD7706
TABLE OF CONTENTS
Features .............................................................................................. 1
General Description ......................................................................... 1
Functional Block Diagram .............................................................. 1
Revision History ............................................................................... 3
Product Highlights ........................................................................... 4
Specifications..................................................................................... 5
Timing Characteristics ................................................................ 8
Absolute Maximum Ratings............................................................ 9
ESD Caution.................................................................................. 9
Pin Configurations and Function Descriptions ......................... 10
Output Noise (5 V Operation)...................................................... 12
Output Noise (3 V Operation)...................................................... 13
Typical Performance Characteristics ........................................... 14
On-Chip Registers .......................................................................... 16
Communication Register (RS2, RS1, RS0 = 0, 0, 0)............... 16
Setup Register (RS2, RS1, RS0 = 0, 0, 1); Power-On/Reset
Status: 01 Hexadecimal .............................................................. 17
Clock Register (RS2, RS1, RS0 = 0, 1, 0); Power-On/Reset
Status: 05 Hexadecimal .............................................................. 19
Data Register (RS2, RS1, RS0 = 0, 1, 1) ................................... 20
Test Register (RS2, RS1, RS0 = 1, 0, 0); Power-On/Reset
Status: 00 Hexadecimal .............................................................. 20
Zero-Scale Calibration Register (RS2, RS1, RS0 = 1, 1, 0);
Power-On/Reset Status: 1F4000 Hexadecimal ........................... 20
Full-Scale Calibration Register (RS2, RS1, RS0 = 1, 1, 1);
Power-On/Reset Status: 5761AB HexaDecimal......................... 20
Circuit Description......................................................................... 21
Analog Input ............................................................................... 22
Bipolar/Unipolar Input .............................................................. 22
Reference Input........................................................................... 23
Digital Filtering........................................................................... 23
Analog Filtering.......................................................................... 25
Calibration................................................................................... 25
Theory of Operation ...................................................................... 28
Clocking and Oscillator Circuit ............................................... 28
System Synchronization ............................................................ 28
RESET Input ............................................................................... 29
Standby Mode ............................................................................. 29
Accuracy ...................................................................................... 29
Drift Considerations .................................................................. 29
Power Supplies ............................................................................ 30
Supply Current............................................................................ 30
Grounding and Layout .............................................................. 30
Evaluating the Performance...................................................... 31
Digital Interface.......................................................................... 31
Configuring the AD7705/AD7706 .......................................... 33
Microcomputer/Microprocessor Interfacing ......................... 34
Code For Setting Up the AD7705/AD7706............................ 35
Applications..................................................................................... 38
Pressure Measurement............................................................... 38
Temperature Measurement ....................................................... 39
Smart Transmitters..................................................................... 40
Battery Monitoring .................................................................... 41
Outline Dimensions ....................................................................... 42
Ordering Guide .......................................................................... 43
Rev. C | Page 2 of 44
AD7705/AD7706
REVISION HISTORY
5/06—Rev. B to Rev. C
Updated Format.................................................................. Universal
Changes to Table 1 ............................................................................3
Updated Outline Dimensions........................................................42
Changes to Ordering Guide...........................................................43
6/05—Rev. A to Rev. B
Updated Format.................................................................. Universal
Changed Range of Absolute Voltage on Analog Inputs Universal
Changes to Table 19 ........................................................................21
Updated Outline Dimensions........................................................42
Changes to Ordering Guide...........................................................43
11/98—Rev. 0 to Rev. A
Revision 0: Initial Version
Rev. C | Page 3 of 44
AD7705/AD7706
PRODUCT HIGHLIGHTS
1.
The AD7705/AD7706 devices consume less than 1 mW at
3 V supplies and 1 MHz master clock, making them ideal
for use in low power systems. Standby current is less than 8
μA.
The programmable gain input allows the AD7705/AD7706
to accept input signals directly from a strain gage or
transducer, removing a considerable amount of signal
conditioning.
3.
The AD7705/AD7706 are ideal for microcontroller or DSP
processor applications with a 3-wire serial interface,
reducing the number of interconnect lines and reducing
the number of opto-couplers required in isolated systems.
The parts feature excellent static performance
specifications with 16 bits, no missing codes, ±0.003%
accuracy, and low rms noise (<600 nV). Endpoint errors
and the effects of temperature drift are eliminated by on-
chip calibration options, which remove zero-scale and full-
scale errors.
2.
4.
Rev. C | Page 4 of 44
AD7705/AD7706
SPECIFICATIONS
V
DD
= 3 V or 5 V, REF IN(+) = 1.225 V with V
DD
= 3 V, and 2.5 V with V
DD
= 5 V; REF IN(−) = GND; MCLK IN = 2.4576 MHz, unless
otherwise noted. All specifications T
MIN
to T
MAX
, unless otherwise noted.
Table 1.
Parameter
STATIC PERFORMANCE
No Missing Codes
Output Noise
Integral Nonlinearity
2
Unipolar Offset Error
3
Unipolar Offset Drift
4
Bipolar Zero Error
3
Bipolar Zero Drift
4
Positive Full-Scale Error
3, 5
Full-Scale Drift
4, 6
Gain Error
3, 7
Gain Drift
4, 8
Bipolar Negative Full-Scale Error
2
Bipolar Negative Full-Scale Drift
4
ANALOG INPUTS/REFERENCE INPUTS
Common-Mode Rejection (CMR)
2
V
DD
= 5 V
Gain = 1
Gain = 2
Gain = 4
Gain = 8 to 128
V
DD
= 3 V
Gain = 1
Gain = 2
Gain = 4
Gain = 8 to 128
Normal-Mode 50 Hz Rejection
2
Normal-Mode 60 Hz Rejection
2
Common-Mode 50 Hz Rejection
2
Common-Mode 60 Hz Rejection
2
Absolute/Common-Mode REF IN
Voltage
2
Absolute/Common-Mode AIN
Voltage
2, 9, 10
Absolute/Common-Mode AIN
Voltage
2, 9
AIN DC Input Current
AIN Sampling Capacitance
2
AIN Differential Voltage Range
11
2
B Version
1
16
See Table 5 and
Table 7
±0.003
0.5
0.5
0.1
0.5
0.5
±0.003
1
0.6
Unit
Bits min
Conditions/Comments
Guaranteed by design, filter notch < 60 Hz
Depends on filter cutoffs and selected gain
Filter notch < 60 Hz, typically ±0.0003%
% of FSR max
μV/°C typ
μV/°C typ
μV/°C typ
μV/°C typ
ppm of FSR/°C
typ
% of FSR typ
μV/°C typ
μV/°C typ
For gains 1, 2, and 4
For gains 8, 16, 32, 64, and 128
Typically ±0.001%
For gains of 1 to 4
For gains of 8 to 128
Specifications for AIN and REF IN, unless otherwise
noted
96
105
110
130
105
110
120
130
98
98
150
150
GND to V
DD
GND − 100 mV
V
DD
+ 30 mV
GND + 50 mV
V
DD
− 1.5 V
1
10
0 to +V
REF
/gain
12
±V
REF
/gain
dB typ
dB typ
dB typ
dB typ
dB typ
dB typ
dB typ
dB typ
dB typ
dB typ
dB typ
dB typ
V min to V max
V min
V max
V min
V max
nA max
pF max
nom
nom
For filter notches of 25 Hz, 50 Hz, ±0.02 × f
NOTCH
For filter notches of 20 Hz, 60 Hz, ±0.02 × f
NOTCH
For filter notches of 25 Hz, 50 Hz, ±0.02 × f
NOTCH
For filter notches of 20 Hz, 60 Hz, ±0.02 × f
NOTCH
BUF bit of setup register = 0
BUF bit of setup register = 1
Unipolar input range (B/U bit of setup register = 1)
Bipolar input range (B/U bit of setup register = 0)
Rev. C | Page 5 of 44
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