Using Digital Potentiometers to Design Low-Pass Adjustable Filters

The most common filter found in a data acquisition system signal path is a low-pass filter. This type of filter is usually used to reduce A/D Converter (ADC) aliasing errors. If there is more than one signal that is applied to the A/D converter through a multiplexer, each signal source may have its own set of filter requirements (i.e., settling time, fast transition region, etc.). Consequently, a variety of filters may be required in the circuit prior to the multiplexer. Usually these filters are implemented with operational amplifiers (op amps) in combination with fixed resistors and capacitors. An alternative filter design solution is to have one filter following the multiplexer. In this circuit, the low-pass filter would need to be programmable. The obvious advantage of having the filter serve many analog inputs is that there is a reduction in chip count.