RMS and average errors resulting from low pass filter characteristics in instrumentation systems

Rectangular, steplike waveforms, occurring in many physical systems, are passed through sensors or signal conditioners having, in many cases, low-pass filter characteristics. The filtered waveforms are sampled by A/D (analog/digital) converters and the average or RMS (root-mean-square) values calculated. The author describes the effects of the low-pass filter characteristics on the average and RMS values of the waveforms and the analysis method used to determine the effects. It is shown that once the data-acquisition-system component under consideration has been modeled by an s-domain transfer function and the input waveform is known, the solution process is mechanical. By the way of example, the method is applied to a measurement problem and the results provided. The effects of assuming unit step excitation rather than physically realizable exponential step inputs are also discussed