Measurement Computing PC104-DIO48 - Manual (Page 21)

5.4 LOW PASS FILTERS DE-BOUNCE INPUTS
A low pass filter is placed on the signal wires between a signal and an
DIO board. It attenuates frequencies greater than the cut-off frequency
preventing them from entering the digital inputs.
The key term in a low pass filter circuit is cut-off frequency. The cut-off
frequency is that frequency above which no variation of voltage with
respect to time may enter the circuit. For example, if a low pass filter
had a cut-off frequency of 30 Hz, interference associated with line
voltage (60 Hz) would be filtered out but a signal of 25 Hz would be
allowed to pass.
Also, in a digital circuit, a low pass filter might be used to “de-bounce”
(filter) an input from a switch or external relay. (Unless switch/relay
contacts are mercury-whetted, they tend to bounce briefly on closure,
generating a pulsating noise signal. This can easily lead to erroneous
counts unless filtered out.)
Figure 5-3. Low-Pass Filter
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