9.2.2 Detailed Design Procedure

A key consideration in the design of a bridge transducer for weigh applications is noise-free resolution. Noise-free resolution is defined by the ratio of full scale signal to the conversion noise of the ADC. Other considerations are data throughput rate and input signal settling time.

Table 1 shows the ADC conversion noise expressed as an input-referred quantity. The table shows various tradeoffs among gain, sample rate and sinc filter in order to optimize noise for a given design. For this example, the configuration of the ADC that yields the lowest noise while achieving the sample rate and settling time requirement is gain = 128, 10 SPS, filter order = sinc 1 and by using the chop mode. Use of the chop mode has the additional advantage of eliminating offset drift from the ADC.

Configuring the ADC for 10 SPS, the sinc4 filter order and disabling chop mode yields approximately the same noise performance compared to the target configuration (shown above) but do not satisfy the settling time requirement of 200 ms. The sinc4 filter order settles in four conversion periods, or 400 ms.

Noise-free counts are improved by increasing the signal output from the bridge. Increasing the signal output is possible by the use of a bridge with a higher gauge-factor, or by increasing the excitation voltage. Operation with an excitation voltage above 5 V requires voltage division of the bridge sense voltage before it is input to the ADC reference pins.

External filter components filter the signal and reference inputs of the ADC. The filters remove both differential and common-mode high-frequency noise. Component value mismatch in the common-mode filter converts common-mode noise into differential noise. To minimize the effect of the mismatch, the differential filter capacitor values (10 nF) are 10x higher value than the common-mode capacitors (1 nF). Increase the capacitor values to provides additional noise filtering. Maintain the resistors at low values to minimize thermal noise. For consistent noise performance, match the corner frequencies of the input and reference filters. More information is found in the RTD Ratiometric Measurements and Filtering Using the ADS1148 and ADS1248 Family of Devices Application Report.