(...continued)   Further, it should be noted that the passband and stopband frequencies of digital filters differ from the cutoff frequency of the commonly used Bessel and Butterworth analog filters. The cutoff frequency of an analog filter, typically specified at an attenuation of 3 dB, usually lies in the transition band between the passband and stopband frequencies of comparable digital filters. Digital filters are a combination of mathematical algorithms and fast digital circuits which operate on a series of digital data acquired over a period of time. The necessity of using a series of data leads to a delay as the data passes through the filter. After each new sample is taken, the oldest data drops off the front of the series, the remaining data is moved forward in the series, and the data just acquired is added to the end of the series. Then the algorithm is applied to the series of data to obtain a calculated value for the filtered data. The delay, calculated as the time a particular sample takes to get midway through the series, is a function of the ADC sampling rate, the number of terms used in the series, and the passband frequency. Accordingly, the same digital filter should be selected for all measurement channels to ensure that all data acquired at the same time emerges from the digital filters at the same, but delayed, time. Page 8 of 24