Measurement Computing CIO-INT-32 - Manual (Page 19)

5.3 LOW PASS FILTERS DE-BOUNCE INPUTS
A low pass filter is placed on the signal wires between a signal and an A/D board. It
stops frequencies greater than the cut off frequency from entering the A/D board's
analog or 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, the kind
of interference associated with line voltage (60Hz) would be filtered out but a signal
of 25Hz would be allowed to pass.
Also, in a digital circuit, a low pass filter might be used to de-bounce an input from a
momentary contact button pushed by a person.
A low pass filter may be constructed from one resistor (R) and one capacitor (C). The
cut off frequency is determined according to the formula:
Fc = 1
2 *
π * R * C
Where π = 3.14...
R = ohms
R = 1 C = Farads
2* π * C * Fc
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