Measurement Computing CIO-DAS16/M1 - Manual (Page 36)

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terminals (12 - 22 AWG). Designed for table top, wall or rack mounting, the board provides prototype,
divider circuit, filter circuit and pull-up resistor positions which you may complete with components for
your application.
6.7 LOW PASS FILTERS
A low pass filter is placed on the signal wires between a signal and an A/D board. It attenuates
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 largely filtered out but a signal of 25Hz would be allowed to pass.
In a digital circuit, a low-pass filter can be used to filter (“de-bounce”) an input from a switch or relay
contact.
SIGNAL HIGH
SIGNAL LOW
A/D BOARD
HIGH INPUT
A/D BOARD
LOW INPUT
SIGNAL
VOLTS
LOW PASS FILTER
R
C
F
C
=
2 *P i*R *C
1
Figure 6-4. Low-pass Filter
A low pass filter can be constructed from one resistor (R) and one capacitor (C). The cut-off frequency F
c
is determined according to the formula below:
(Pi = 3.14); R is in Ohms; C is in Farads; Fc is in cycles per second
Fc = 1/(2 * Pi * R * C)
R = 1/(2* Pi * C * Fc)