3-Channel, Low Noise, Low Power, 20-Bit - ADC with On-Chip In-Amp and Reference AD7785 FEATURES Industrial process control Instrumentation Up to 20 bits effective resolution Portable instrumentation RMS noise Blood analysis 40 nV 4.17 Hz Smart transmitters 85 nV 16.7 Hz Liquid/gas chromatography Current: 400 A typical 6-digit DVM Power-down: 1 A maximum Low noise programmable gain instrumentation amp GENERAL DESCRIPTION Band gap reference with 4 ppm/C drift typical The AD7785 is a low power, low noise, complete analog front Update rate: 4.17 Hz to 470 Hz end for high precision measurement applications. The AD7785 3 differential inputs contains a low noise 20-bit - ADC with three differential Internal clock oscillator analog inputs. The on-chip, low noise instrumentation amplifier Simultaneous 50 Hz/60 Hz rejection means that signals of small amplitude can be interfaced directly Programmable current sources to the ADC. With a gain setting of 64, the rms noise is 40 nV On-chip bias voltage generator when the update rate equals 4.17 Hz. Burnout currents The device contains a precision low noise, low drift internal Power supply: 2.7 V to 5.25 V band gap reference and can accept an external differential 40C to +105C temperature range reference. Other on-chip features include programmable Independent interface power supply excitation current sources, burnout currents, and a bias voltage 16-lead TSSOP package generator. The bias voltage generator sets the common-mode Interface voltage of a channel to AVDD/2. 3-wire serial SPI, QSPI, MICROWIRE, and DSP compatible The AD7785 can be operated with either the internal clock Schmitt trigger on SCLK or an external clock. The output data rate from the device is software-programmable and can be varied from 4.17 Hz to 470 APPLICATIONS Hz. The device operates with a power supply from 2.7 V to Thermocouple measurements 5.25 V. It consumes a current of 400 A typical and is housed in RTD measurements a 16-lead TSSOP package. Thermistor measurements Gas analysis FUNCTIONAL BLOCK DIAGRAM GND AV REFIN(+)/AIN3(+) REFIN()/AIN3() DD V BIAS BAND GAP REFERENCE GND AV DD AIN1(+) DOUT/RDY AIN1() SERIAL MUX INTERFACE DIN - AIN2(+) BUF IN-AMP AND ADC SCLK AIN2() CONTROL LOGIC CS AV DD GND DV DD IOUT1 INTERNAL AD7785 CLOCK IOUT2 CLK Figure 1. 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 One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. rights of third parties that may result from its use. Specifications subject to change without notice. No Tel: 781.329.4700 www.analog.com license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Fax: 781.461.3113 2007 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. 06721-001AD7785 TABLE OF CONTENTS Features .............................................................................................. 1 Offset Register ............................................................................ 19 Applications....................................................................................... 1 Full-Scale Register...................................................................... 19 General Description ......................................................................... 1 ADC Circuit Information.............................................................. 20 Functional Block Diagram .............................................................. 1 Overview ..................................................................................... 20 Revision History ............................................................................... 2 Digital Interface.......................................................................... 21 Specifications..................................................................................... 3 Circuit Description......................................................................... 25 Timing Characteristics..................................................................... 6 Analog Input Channel ............................................................... 25 Timing Diagrams.......................................................................... 7 Instrumentation Amplifier........................................................ 25 Absolute Maximum Ratings............................................................ 8 Bipolar/Unipolar Configuration .............................................. 25 ESD Caution.................................................................................. 8 Data Output Coding .................................................................. 25 Pin Configuration and Function Descriptions............................. 9 Burnout Currents ....................................................................... 26 Output Noise and Resolution Specifications .............................. 11 Excitation Currents .................................................................... 26 External Reference...................................................................... 11 Bias Voltage Generator .............................................................. 26 Internal Reference ...................................................................... 12 Reference ..................................................................................... 26 Typical Performance Characteristics ........................................... 13 Reset ............................................................................................. 26 On-Chip Registers.......................................................................... 14 AV Monitor ............................................................................. 27 DD Communications Register......................................................... 14 Calibration................................................................................... 27 Status Register............................................................................. 15 Grounding and Layout .............................................................. 27 Mode Register ............................................................................. 15 Applications Information .............................................................. 29 Configuration Register .............................................................. 17 Temperature Measurement using a Thermocouple............... 29 Data Register ............................................................................... 18 Temperature Measurement using an RTD.............................. 30 ID Register................................................................................... 18 Outline Dimensions....................................................................... 31 IO Register................................................................................... 18 Ordering Guide .......................................................................... 31 REVISION HISTORY 4/07Revision 0: Initial Version Rev. 0 Page 2 of 32