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.
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