ADC. AD7781 Datasheet

AD7781 Datasheet PDF


Part

AD7781

Description

Low Power Sigma-Delta ADC

Manufacture

Analog Devices

Page 16 Pages
Datasheet
Download AD7781 Datasheet


AD7781 Datasheet
FEATURES
Pin-programmable filter response
Update rate: 10 Hz or 16.7 Hz
Pin-programmable in-amp gain
Pin-programmable power-down and reset
Status function
Internal clock oscillator
Internal bridge power-down switch
Current
115 μA typical (gain = 1)
330 μA typical (gain = 128)
Simultaneous 50 Hz/60 Hz rejection
Power supply: 2.7 V to 5.25 V
−40°C to +105°C temperature range
Independent interface power supply
Packages
14-lead, narrow body SOIC
16-lead TSSOP
2-wire serial interface (read-only device)
SPI compatible
Schmitt trigger on SCLK
APPLICATIONS
Weigh scales
Pressure measurement
Industrial process control
Portable instrumentation
www.DaGtaESNhEeeRt4AUL.cDomESCRIPTION
The AD7781 is a complete, low power front-end solution for
bridge sensor products, including weigh scales, strain gages,
and pressure sensors. It contains a precision, low power, 20-bit
sigma-delta (Σ-Δ) ADC, an on-chip, low noise programmable
gain amplifier (PGA), and an on-chip oscillator.
Consuming only 330 μA, the AD7781 is particularly suitable for
portable or battery-operated products where very low power is
required. The AD7781 also has a power-down mode that allows
the user to switch off the power to the bridge sensor and power
down the AD7781 when not converting, thus increasing the
battery life of the product.
For ease of use, all the features of the AD7781 are controlled by
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
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.
Trademarksandregisteredtrademarksarethepropertyoftheirrespectiveowners.
20-Bit, Pin-Programmable,
Low Power Sigma-Delta ADC
AD7781
FUNCTIONAL BLOCK DIAGRAM
GND AVDD GAIN
REFIN(+) REFIN(–)
AIN(+)
AIN(–)
G=1
OR 128
20-BIT Σ-Δ
ADC
BPDSW
AD7781
INTERNAL
CLOCK
Figure 1.
DOUT/RDY
SCLK
DVDD
FILTER
PDRST
Table 1.
Parameter
Output Data Rate
RMS Noise
C Grade
B Grade
P-P Resolution
C Grade
B Grade
Settling Time
Gain = 128
10 Hz 16.7 Hz
44 nV
55 nV
65 nV
90 nV
17.6
17.3
300 ms
17.1
16.6
120 ms
Gain = 1
10 Hz 16.7 Hz
2.4 μV
2.4 μV
2.7 μV
2.7 μV
18.8
18.8
300 ms
18.7
18.7
120 ms
The on-chip PGA has a gain of 1 or 128, supporting a full-scale
differential input of ±5 V or ±39 mV. The device has two filter
response options. The filter response at the 16.7 Hz update rate
provides superior dynamic performance. The settling time is
120 ms at this update rate. At the 10 Hz update rate, the filter
response provides better than −45 dB of stop-band attenuation.
In load cell applications, this stop-band rejection is useful to
reject low frequency mechanical vibrations of the load cell. The
settling time is 300 ms at this update rate. Simultaneous 50 Hz/
60 Hz rejection occurs at both the 10 Hz and 16.7 Hz update rates.
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
16-lead TSSOP package.
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
©2009 Analog Devices, Inc. All rights reserved.

AD7781 Datasheet
AD7781
TABLE OF CONTENTS
Features .............................................................................................. 1 
Applications....................................................................................... 1 
Functional Block Diagram .............................................................. 1 
General Description ......................................................................... 1 
Revision History ............................................................................... 2 
Specifications..................................................................................... 3 
Timing Characteristics ................................................................ 5 
Absolute Maximum Ratings............................................................ 6 
Thermal Resistance ...................................................................... 6 
ESD Caution.................................................................................. 6 
Pin Configurations and Function Descriptions ........................... 7 
Typical Performance Characteristics ............................................. 8 
Output Noise and Resolution........................................................ 10 
Theory of Operation ...................................................................... 11 
Filter, Data Rate, and Settling Time ......................................... 11 
REVISION HISTORY
5/09—Revision 0: Initial Version
Gain.............................................................................................. 11 
Power-Down/Reset (PDRST) ................................................... 12 
Analog Input Channel ............................................................... 12 
Bipolar Configuration................................................................ 12 
Data Output Coding .................................................................. 12 
Reference ..................................................................................... 12 
Bridge Power-Down Switch...................................................... 12 
Digital Interface.......................................................................... 13 
Applications Information .............................................................. 14 
Weigh Scales................................................................................ 14 
AD7781 Performance in a Weigh Scale System......................... 14 
EMI Recommendations............................................................. 14 
Grounding and Layout .............................................................. 15 
Outline Dimensions ....................................................................... 16 
Ordering Guide .......................................................................... 16 
www.DataSheet4U.com
Rev. 0 | Page 2 of 16


Features Datasheet pdf 20-Bit, Pin-Programmable, Low Power Sigm a-Delta ADC AD7781 FEATURES Pin-program mable filter response Update rate: 10 H z or 16.7 Hz Pin-programmable in-amp ga in Pin-programmable power-down and rese t Status function Internal clock oscill ator Internal bridge power-down switch Current 115 μA typical (gain = 1) 330 μA typical (gain = 128) Simultaneous 5 0 Hz/60 Hz rejection Power supply: 2.7 V to 5.25 V −40°C to +105°C tempera ture range Independent interface power supply Packages 14-lead, narrow body SO IC 16-lead TSSOP 2-wire serial interfac e (read-only device) SPI compatible Sch mitt trigger on SCLK FUNCTIONAL BLOCK DIAGRAM GND AVDD GAIN REFIN(+) REFIN( ) AIN(+) G=1 OR 128 AIN(–) BPDSW IN TERNAL CLOCK 20-BIT Σ-Δ ADC DOUT/RDY SCLK DVDD 08162-001 FILTER PDRST AD7 781 Figure 1. Table 1. Parameter Outpu t Data Rate RMS Noise C Grade B Grade P -P Resolution C Grade B Grade Settling Time Gain = 128 10 Hz 16.7 Hz 44 nV 55 nV 17.6 17.3 300 ms 65 nV 90 nV 17.1 16.6 120 ms 10 Hz Gain = 1 16.7 Hz 2.7.
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