Measurement Computing PC104-DI48 - Manual (Page 14)

Digital inputs often require the use of voltage dividers. For example, if you wish to
measure a digital signal that is at 0 volts when off and 24 volts when on, you cannot
connect that directly to a digital input. The voltage must be dropped to 5 volts max
when on. The attenuation is 24:5 or 4.8.
Using the equation above, if R2 is 1K, R1 = (4.8
−
1) x 1000 = 3.8K.
Remember that a TTL input is 'on' when the input voltage is greater than 2.5 volts.
NOTE
The resistors, R1 and R2, are going to dissipate power in the divider
circuit according to the equation W = I
2 x R; (Current (I) =
Voltage/Resistance). The higher the value of the resistance (R1 +
R2), the less power dissipated by the divider circuit. Here is a
simple rule:
For attenuation of <5:1, no resistor should be less than 10K.
For attenuation of > 5:1, no resistor should be less than 1K.
7.4 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
prevents frequencies greater than the cut-off frequency from entering the A/D board's
digital inputs.
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 mostly filtered out but a signal of 25 Hz would pass with less attenuation.
Also, in a digital circuit, a low-pass filter is often used to remove contact bounce noise
signals from a switch or a relay contacts.
A simple low-pass filter can be constructed from one resistor (R) and one capacitor
(C). The cut-off frequency is determined according to the formula:
Fc = 1 Where
π
= 3.14...
2
π
R C R = ohms
C = farads
R = 1 Fc = cut-off frequency in cycles/second.
2
π
C Fc
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