Measurement-computing CIO-INT-32 Manuel d'utilisateur télécharger pdf (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 *

* R * C

Where

= 3.14...

R = ohms

R = 1

C = Farads

* C * Fc

1 2 ... 14 15 16 17 18 19 20 21 22 23 24

Pas de commentaire

Measurement-computing CIO-INT-32 Manuel d'utilisateur Page 19