AD7781

PRODUCTION

20-Bit, Pin-Programmable, Ultralow Power Sigma-Delta ADC

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Overview

  • 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
  • Please see data sheet for additional features

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.

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.

Applications

  • Weigh scales
  • Pressure measurement
  • Industrial process control
  • Portable instrumentation

AD7781
20-Bit, Pin-Programmable, Ultralow Power Sigma-Delta ADC
AD7781 Functional Block Diagram AD7781 Pin Configuration AD7781 Pin Configuration
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Documentation

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Software Resources


Tools & Simulations

Sigma-Delta ADC Tutorial

An interactive illustration showing the behavior of an idealized sigma-delta A/D converter.

Open Tool

Evaluation Kits

EVAL-AD7781

AD7781 Evaluation Board

Features and Benefits

  • Full-featured evaluation board for the AD7781
  • Standalone USB interface
  • Various linking options
  • PC software for control of AD7781

Product Details

The EVAL-AD7781 is the evaluation board for the AD7781, which is a pin programmable, low power, 20-bit sigma-delta (Σ-Δ) ADC. The AD7781 is a complete analog front end for low frequency measurement applications such as bridge sensor systems. It contains one differential input and includes a low noise instrumentation amplifier. The output data rate can be programmed to 10 Hz or 16.7 Hz. The AD7781 also has an on-board clock, eliminating the need for an external clock. It employs a Σ-Δ conversion technique to realize up to 20 bits of no missing codes performance. The input signal is applied to an analog modulator. The modulator output is processed by an on-chip digital filter. The analog input channel of the AD7781 accepts analog input signals of ±VREF or ±VREF/128. With the update rate programmed to 10 Hz, the rms noise is 44 nV when the gain is 128. Simultaneous 50 Hz/60 Hz rejection is available at both output data rates.

The evaluation board interfaces to the USB port of an IBM compatible PC. Software is available with the evaluation board that allows users to easily communicate with the AD7781.

Please note: This Evaluation software does not run on Windows 8 or Windows 10 (USB driver is non-compliant with OS >Windows 7).

Note that the AD7781 evaluation board software should be installed before connecting the AD7781 evaluation board to the PC.

Another component on the AD7781 evaluation board is the ADP3303 high precision, low power, 3.3 V output voltage regulator, which is used to power the USB/SPI interface.

EVAL-AD7781
AD7781 Evaluation Board

Reference Designs

Weigh Scale System Using the AD7781
CN0108 Circuits from the lab

Weigh Scale Design Using the AD7781 20-Bit Sigma-Delta ADC with Internal PGA

Features and Benefits

  • 20-bit Weigh scale application circuit
  • Integrated PGA allows for wide array of weigh scales
  • Low power and low cost
  • Typically used in portable weigh scale applications
CN0108
Weigh Scale Design Using the AD7781 20-Bit Sigma-Delta ADC with Internal PGA
Weigh Scale System Using the AD7781

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