Measurement-computing PC104-CTR10HD Manuel d'utilisateur télécharger pdf (Page 16)

5.2 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 board's 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 (60 Hz) would be filtered out but a signal of

25 Hz would be allowed to pass.

In a digital circuit, a low-pass filter can be used to filter an input from a switch or

relay. See Figure 4-2 for the circuit configuration

Figure 4-2. Low-Pass Filter

A simple low-pass filter can be made from one resistor (R) and one capacitor (C).

The cut off frequency is determined according to the formula:

Fc = 1

2 *

* R * C

R = 1

* C * Fc

Where

= 3.14...

Fc = frequency in cycles per second

R = resistance in Ohms

C = capacitance in Farads

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Measurement-computing PC104-CTR10HD Manuel d'utilisateur Page 16