Scales: Weighed down by high accuracy requirements?

Colm Slattery, application engineer, and Mariah Nie, field application engineer, Analog Devices Inc. -- Control Engineering, 5/1/2006

The AD7799 measured data shows how averaging improves the result, in this case a 2.3 bit improvement on the raw ADC output, yet it has no effect on LCD output update rate.

Today's trend is toward higher-accuracy, lower-cost weigh scales, creating greater demand for high-performance, analog signal-processing circuitry. Initially, the performance requirement is not obvious. Most weigh scales output final weight values with a resolution of 1:3,000 or 1:10,000; so a 12-bit to 14-bit ADC would seem to be sufficient. However, a closer examination of a weigh scale shows this is not the case, and that ADC accuracy needs to be closer to 20 bits. Weigh scales that require high accuracy can benefit from a reference design.

Typical weigh scale systems have user resolution—or count—ranging from a low of 1:3,000 to 1:10,000. For example, a weigh scale that can measure up to 5 kg with a count of 1:10,000 would have a 0.5 g weight resolution. This precision is generally referred to as the external count. To ensure that the external count accuracy is met, weigh scale manufacturers often require that the system's internal resolution be an order of accuracy greater. Some standards dictate that the internal system count be a factor of 20 times better than that of the external count, so, in this case, the internal count needs to be 1:200,000 accurate.

In weigh-scale applications, only a fraction of the ADC range is actually used. Typically, a load cell has a 6-mV full-scale output. Using a typical gain-of-128 stage on the front end, the ADC input will see about 770 mV at full scale. Thus, with a standard 2.5-V reference, under 1/3 of the ADC dynamic range is used. If the internal count needs 1:200,000 accuracy for a 770-mV full-scale range, the ADC should be in the order of 3-to-4 times better that this to meet the performance requirements. In this case, the ADC must be accurate to 1:800,000, or 19-to-20 bits. The complexity of the signal-processing requirement is now understandable.

An averaging filter is very good for reducing random white noise while maintaining the sharpest step response. The reference design's software uses a moving averaging algorithm. The filter uses an M-point moving-average window. The moving-average filter operates by averaging a number of points from the input signal to produce each point in the output signal. In equation form, this is written:

Performance of the ADuC847's integrated ADC vs. the AD7799 stand-alone ADC:For low-cost weigh scale design, theADuC847 can implement a single-chip weigh scale solution. ADuC847 integrates a 24-bit sigma-delta ADC with 8052-compatible microcontroller core. ADuC847 also includes a gain-of-128 PGA with differential analog inputs and reference inputs. The stand-alone AD7799 has better noise performance, suiting it for high-end applications.