Datasheet AD7785 (Analog Devices)

3-Channel, Low Noise, Low Power, 20-Bit ∑-Δ ADC with On-Chip In-Amp and Reference AD7785 FEATURES Industrial process control Up to 20 bits effective resolution Instrumentation RMS noise Portable instrumentation 40 nV @ 4.17 Hz Blood analysis 85 nV @ 16.7 Hz Smart transmitters Current: 400 μA typical Liquid/gas chromatography Power-down: 1 μA maximum 6-digit DVM 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 ∑-Δ 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 Power supply: 2.7 V to 5.25 V
The device contains a precision low noise, low drift internal
–40°C to +105°C temperature range
band gap reference and can accept an external differential
Independent interface power supply
reference. Other on-chip features include programmable
16-lead TSSOP package
excitation current sources, burnout currents, and a bias voltage
Interface
generator. The bias voltage generator sets the common-mode
3-wire serial
voltage of a channel to AVDD/2.
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
APPLICATIONS
software-programmable and can be varied from 4.17 Hz to 470 Hz. The device operates with a power supply from 2.7 V to
Thermocouple measurements
5.25 V. It consumes a current of 400 μA typical and is housed in
RTD measurements
a 16-lead TSSOP package.
Thermistor measurements Gas analysis FUNCTIONAL BLOCK DIAGRAM GND AVDD REFIN(+)/AIN3(+) REFIN(–)/AIN3(–) VBIAS BAND GAP REFERENCE GND AVDD AIN1(+) DOUT/RDY AIN1(–) SERIAL MUX INTERFACE DIN AIN2(+) Σ-Δ BUF IN-AMP AND ADC AIN2(–) SCLK CONTROL LOGIC CS AVDD GND DVDD IOUT1 INTERNAL CLOCK AD7785 IOUT2
01 0 1- 72
CLK
06 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 license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Tel: 781.329.4700 www.analog.com Trademarks and registered trademarks are the property of their respective owners. Fax: 781.461.3113 ©2007 Analog Devices, Inc. All rights reserved.
Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS TIMING CHARACTERISTICS TIMING DIAGRAMS ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS OUTPUT NOISE AND RESOLUTION SPECIFICATIONS EXTERNAL REFERENCE INTERNAL REFERENCE TYPICAL PERFORMANCE CHARACTERISTICS ON-CHIP REGISTERS COMMUNICATIONS REGISTER RS2, RS1, RS0 = 0, 0, 0 STATUS REGISTER RS2, RS1, RS0 = 0, 0, 0; Power-On/Reset = 0x88 MODE REGISTER RS2, RS1, RS0 = 0, 0, 1; Power-On/Reset = 0x000A CONFIGURATION REGISTER RS2, RS1, RS0 = 0, 1, 0; Power-On/Reset = 0x0710 DATA REGISTER RS2, RS1, RS0 = 0, 1, 1; Power-On/Reset = 0x00000F ID REGISTER RS2, RS1, RS0 = 1, 0, 0; Power-On/Reset = 0xXB IO REGISTER RS2, RS1, RS0 = 1, 0, 1; Power-On/Reset = 0x00 OFFSET REGISTER RS2, RS1, RS0 = 1, 1, 0; Power-On/Reset = 0x800000 FULL-SCALE REGISTER RS2, RS1, RS0 = 1, 1, 1; Power-On/Reset = 0x5XXX00 ADC CIRCUIT INFORMATION OVERVIEW DIGITAL INTERFACE Single Conversion Mode Continuous Conversion Mode Continuous Read CIRCUIT DESCRIPTION ANALOG INPUT CHANNEL INSTRUMENTATION AMPLIFIER BIPOLAR/UNIPOLAR CONFIGURATION DATA OUTPUT CODING BURNOUT CURRENTS EXCITATION CURRENTS BIAS VOLTAGE GENERATOR REFERENCE RESET AVDD MONITOR CALIBRATION GROUNDING AND LAYOUT APPLICATIONS INFORMATION TEMPERATURE MEASUREMENT USING A THERMOCOUPLE TEMPERATURE MEASUREMENT USING AN RTD OUTLINE DIMENSIONS ORDERING GUIDE