20-Bit, Pin-Programmable, Low Power Sigma-Delta ADC AD7781 FEATURES FUNCTIONAL BLOCK DIAGRAM AV GND GAIN REFIN(+) REFIN() Pin-programmable filter response DD Update rate: 10 Hz or 16.7 Hz Pin-programmable in-amp gain AIN(+) DOUT/RDY G = 1 20-BIT - Pin-programmable power-down and reset OR 128 ADC AIN() SCLK Status function DV DD Internal clock oscillator BPDSW INTERNAL CLOCK FILTER Internal bridge power-down switch AD7781 PDRST Current 115 A typical (gain = 1) Figure 1. 330 A typical (gain = 128) Table 1. Simultaneous 50 Hz/60 Hz rejection Power supply: 2.7 V to 5.25 V Parameter Gain = 128 Gain = 1 40C to +105C temperature range Output Data Rate 10 Hz 16.7 Hz 10 Hz 16.7 Hz Independent interface power supply RMS Noise Packages C Grade 44 nV 65 nV 2.4 V 2.7 V 14-lead, narrow body SOIC B Grade 55 nV 90 nV 2.4 V 2.7 V 16-lead TSSOP P-P Resolution 2-wire serial interface (read-only device) C Grade 17.6 17.1 18.8 18.7 SPI compatible B Grade 17.3 16.6 18.8 18.7 Schmitt trigger on SCLK Settling Time 300 ms 120 ms 300 ms 120 ms APPLICATIONS Weigh scales Pressure measurement Industrial process control Portable instrumentation GENERAL DESCRIPTION The AD7781 is a complete, low power front-end solution for The on-chip PGA has a gain of 1 or 128, supporting a full-scale bridge sensor products, including weigh scales, strain gages, differential input of 5 V or 39 mV. The device has two filter and pressure sensors. It contains a precision, low power, 20-bit response options. The filter response at the 16.7 Hz update rate sigma-delta (-) ADC, an on-chip, low noise programmable provides superior dynamic performance. The settling time is gain amplifier (PGA), and an on-chip oscillator. 120 ms at this update rate. At the 10 Hz update rate, the filter response provides better than 45 dB of stop-band attenuation. Consuming only 330 A, the AD7781 is particularly suitable for In load cell applications, this stop-band rejection is useful to portable or battery-operated products where very low power is reject low frequency mechanical vibrations of the load cell. The required. The AD7781 also has a power-down mode that allows settling time is 300 ms at this update rate. Simultaneous 50 Hz/ the user to switch off the power to the bridge sensor and power 60 Hz rejection occurs at both the 10 Hz and 16.7 Hz update rates. down the AD7781 when not converting, thus increasing the battery life of the product. The AD7781 operates with a power supply from 2.7 V to 5.25 V. It is available in a narrow body, 14-lead SOIC package and in a For ease of use, all the features of the AD7781 are controlled by 16-lead TSSOP package. dedicated pins. Each time that a data read occurs, eight status bits are appended to the 20-bit conversion. These status bits contain a pattern sequence that can be used to confirm the validity of the serial transfer. 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 2009 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. 08162-001AD7781 TABLE OF CONTENTS Features .............................................................................................. 1 Gain .............................................................................................. 11 Applications ....................................................................................... 1 Power-Down/Reset (PDRST) ................................................... 12 Functional Block Diagram .............................................................. 1 Analog Input Channel ............................................................... 12 General Description ......................................................................... 1 Bipolar Configuration ................................................................ 12 Revision History ............................................................................... 2 Data Output Coding .................................................................. 12 Specif icat ions ..................................................................................... 3 Reference ..................................................................................... 12 Timing Characteristics ................................................................ 5 Bridge Power-Down Switch ...................................................... 12 Absolute Maximum Ratings ............................................................ 6 Digital Interface .......................................................................... 13 Thermal Resistance ...................................................................... 6 Applications Information .............................................................. 14 ESD Caution .................................................................................. 6 Weigh Scales ................................................................................ 14 Pin Configurations and Function Descriptions ........................... 7 AD7781 Performance in a Weigh Scale System ......................... 14 Typical Performance Characteristics ............................................. 8 EMI Recommendations ............................................................. 14 Output Noise and Resolution........................................................ 10 Grounding and Layout .............................................................. 15 Theory of Operation ...................................................................... 11 Outline Dimensions ....................................................................... 16 Filter, Data Rate, and Settling Time ......................................... 11 Ordering Guide .......................................................................... 16 REVISION HISTORY 5/09Revision 0: Initial Version Rev. 0 Page 2 of 16