AD7705 and AD7706 are two 16-bit Sigma Delta ADCs. Equipped with on-chip digital filters and programmable gain front ends, these chips are ideal for low frequency multi-channel signal measurements. The main difference between AD7705 and AD7706 is that AD7705 has two fully differential input channels while AD7706 has three pseudo differential input channels.

This library I created interfaces with Arduino using the standard SPI bus. Most of the complexity is hidden behind the scene, and only a single parameter — the reference voltage — is needed to initialize the library since the reference voltage can vary depending on the particular implementation.

The following code snippet illustrates how to use this library. In this example, channel 1 of AD7706 is setup in bipolar mode with a unity gain. The common pin of the pseudo differential inputs (AIN1 to AIN3) is tied to the 2.5V voltage reference. This gives a measurement range of -2.5 V to +2.5 V. Pin SCK, CS, MOSI, MISO correspond to Arduino pin 13, 10, 11 and 12 respectively.

#include <AD770X.h>

//set reference voltage to 2.5 V
AD770X ad7706(2.5);
double v;

void setup()
 //initializes channel 1

void loop()
  //read the converted results (in volts)
  v = ad7706.readADResult(AD770X::CHN_AIN1);

Here is the schematics I used for the sample program above:


For a supply voltage of 5V, the recommended voltage reference is AD780/REF192. I used two 1.25 V precision shunt voltage references ADR1581 to form a 2.5V reference instead since those are the components I have on hand.

The library also provides an overloaded init method to setup channel, gain and update rate:

void init(byte channel, byte gain, byte updRate);

The available gain settings are defined as:

  static const byte GAIN_1 = 0x0;
  static const byte GAIN_2 = 0x1;
  static const byte GAIN_4 = 0x2;
  static const byte GAIN_8 = 0x3;
  static const byte GAIN_16 = 0x4;
  static const byte GAIN_32 = 0x5;
  static const byte GAIN_64 = 0x6;
  static const byte GAIN_128 = 0x7;

And the update rate settings can be chosen from:

  static const byte UPDATE_RATE_20 = 0x0; // 20 Hz
  static const byte UPDATE_RATE_25 = 0x1; // 25 Hz
  static const byte UPDATE_RATE_100 = 0x2; // 100 Hz
  static const byte UPDATE_RATE_200 = 0x3; // 200 Hz
  static const byte UPDATE_RATE_50 = 0x4; // 50 Hz
  static const byte UPDATE_RATE_60 = 0x5; // 60 Hz
  static const byte UPDATE_RATE_250 = 0x6; // 250 Hz
  static const byte UPDATE_RATE_500 = 0x7; // 500 Hz

Other settings can be accessed via the setup register directly using the writeSetupRegister method defined below:

//Setup Register
//  7     6     5     4     3      2      1      0
//MD10) MD0(0) G2(0) G1(0) G0(0) B/U(0) BUF(0) FSYNC(1)
void AD770X::writeSetupRegister(byte operationMode, byte gain, byte unipolar, byte buffered, byte fsync)

For more details, please refer to the datasheet and the code in the download section.

AD7706 Test Setup
AD7706 Test Setup

Download AD770X.tar.gz (Obsolete)
4/18/2012: AD770X1.1.tar.gz (see this post for changes since the previous version)

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