App note: A reference design for high-performance, low-lost weigh scales

in app notes by DP | 3 comments

Bearmos tipped us about this reference design  from Analog Devices for high-performance, low cost weigh scales. The design uses load-cell, and a load-cell ADC AD7799.

The trend in weigh scales towards higher accuracy and lower cost has produced an increased demand for high-performance analog signal processing at low cost. The scope of this requirement is not obvious; most weigh scales output the final weight value at a resolution of 1:3,000 or 1:10,000, which is easily met (apparently) by a 12-bit to 14-bit ADC (analog-to-digital converter). However, a closer examination of weigh scales shows that meeting the resolution requirement is not that easily accomplished; in fact, the ADC accuracy needs to be closer to 20 bits. In this article, we discuss some of the system specifications of weigh scales and deal with considerations for designing and building a weigh-scale system. The main areas considered are peak-to-peak-noise resolution, A/D-converter dynamic range, gain drift, and filtering. We compare measured data from a real load cell to inputs from a stable voltage reference, using a weigh-scale reference design as an evaluation board.

Via the forum.

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Comments

  1. Hardcorefs says:

    If you actually look inside ‘cheap’ scales for weighing people, very few have load cells.
    I even found a couple that build an oscillator using 8051 compatible microprocessor, then vary the osc, by the weight on some ceramics.
    Not very accurate but cheap.

  2. Drone says:

    I have a cheap (< $10) digital scale I use for weighing miscellaneous things. I also have a cheap calibration weight set I bought on ebaY that checked out OK on an expensive scale with an up-to-date cal sticker.

    The cheap scale uses a load cell similar to that shown in the Web page referenced by Arup. But the "chip" used by my scale is hidden under a blob of black epoxy on a cheap phenolic PCB. There are hardly any other parts on the PCB and what few parts there are are passive through hole-parts that are poorly hand soldered (typical Chinese construction).

    The AD799 mentioned in this App Note is relatively hard to find (like most ADI parts) and costs over $8 in unit quantity (as expected ADI = Ka-Ching!) So I seriously doubt the words "low-cost" in the app note's title bear much "weight".

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