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Weigh Scale / Bridge Sensor

Industrial, Load Cell Based, Electronic Weigh Scales and Bridge Sensor Metering.

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Design Considerations

Weigh Scale / Bridge Sensor:

Electronic weigh scales are found in many industrial applications in some shape or form and are ubiquitous in today’s food industry. Manufacturers of electronic weigh scales occasionally develop proprietary ASICs to tailor the performance of the analog front end for highest accuracy and stability. However, there are design approaches using standard products offering up to 23 noise-free bits of resolution, which parallels the performance of customized Solutions.

A major challenge in designing weigh scales is the sampling of multiple load cells while offering extremely low input referred noise (RTI). The ADS1230, ADS1232, and ADS1234 offer the low input referred noise required in this application. Another important factor is the analog circuitry’s long-term stability with regard to offset drift and gain, over time and temperature. Especially since Real-world sensors have span and offset errors, ever changing over temperature. In addition, many bridge pressure sensors have a nonlinear output with applied force. Here the accuracy of the amplified input signal, either single-ended or differential, must be guaranteed over years of operation. Auto-zero amplifiers, such as the OPA335 and the INA326 instrumentation amplifier, easily meet these stringent requirements by achieving offset drifts of 0.05µV/°C (OPA335) and 0.4µV/°C (INA326).

Bridge sensors convert a physical value into a differential signal in the mV/V range and are referred to as metal thick-film, ceramic or piezo-resistive. However, these are very nonlinear, temperature dependent and have large offset and offset drift. Plus, they require attention to electronic calibration and compensation. The bridge excitation linearization circuit is optimized for bridge pressure nonlinearities with a parabolic shape. The linearization circuit is often digitally programmable, but the pure analog signal conditioning side is handled by the tuning of the analog signal chain. Otherwise, calibration parameters need to be stored in an external nonvolatile memory to eliminate manual trimming and achieve long-term stability.

Application Notes

  • Flash Programming Solutions for the TMS320F28xxx DSCs (spraal3.htm, 8 KB)
    19 Aug 2008 Abstract
  • MSP430F42x Single-Chip Weight Scale (slaa220.htm, 8 KB)
    15 Nov 2004 Abstract
    

Selection and Solution Guides

Solution Guides

  • Industrial Solutions Guide (Rev. C) (slab039c.pdf, 2.35 MB)
    15 Jan 2006 Download

Product Bulletin & White Papers

White Papers

  • Complete Front End Solutions for Weigh Scales (sbaa154.pdf, 336 KB)
    14 Jun 2007 Download

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