Measurement Computing CIO-DAS08-AOH - Manual (Page 29)

8.7 LOW PASS FILTERS
A low pass filter is placed on the signal wires between a signal source and an A/D board input. It greatly reduces frequencies
greater than the cut off frequency that are 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 digital circuits, low pass filters are often used remove to switch bounce noise.
A simple low pass filter can be made from one resistor (R) and one capacitor (C). The cut off frequency is determined by the
formula:
Fc = 1
2 * Pi * R * C
therefore,
R = 1
2* Pi * C * Fc
Fc = cycles/sec
Pi = 3.14...
R = Ohms
C = Farads
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