Measurement Computing CIO-DI48 - Manual (Page 21)

The process of dropping the voltage proportionally is often called attenuation. The
formula for attenuation is:
For a given attenuation, pick a handy resistor and
call it R2, the use this formula to calculate R1. R1 = (A-1) * R2
For example, if the signal varies between 0 and 20
volts and you wish to measure that with an analog
input with a full scale range of 0 to 10 volts, the
attenuation is 2:1 or just 2.
2 = 10K + 10K
10K
The variable attenuation is the proportional
difference between the signal vo ltage max and the
full scale of the analog input.
Attenuation = R1 + R2
R2
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 the CIO-DI## digital inputs. The
voltage must be dropped t o 5 volts maximum when on. T he Attenuation is 24:5 or
4.8. Use the equation above to find an appropriate R1 if R2 is 10K. Remember that a
TTL input is 'on' when the input voltage is greater than 2.5 volts.
IMPORTANT NOTE
: The resistors, R1 and R2, are going to
dissipate all th e power in the divider circuit according to th e
equation Current = Voltage/Resistance and power is current
-squared times resistance (P
watts
= I 2 x R). T he higher the value of
the resistance (R1 + R 2) the less power dissipated by the divider
circuit. Here is a simple rule:
For Attenuation of 5:1 or less, no resistor should be less than 10K.
For Attenuation of greater than 5:1, no resistor should be less than
1K.
7.4 LOW-PASS FILTERS DE-BOUNCE INPUTS
A low-pass filter can be placed on the signal wires between a signal and an A/D
board. It attenuates frequencies higher than the cut-off frequency, preventing them
from entering the A/D board's digital inputs.
The key parameter in a lo w-pass filter circuit is the cut- off frequency. T he cut-off
frequency is that frequency above which no variation of voltage can enter the circuit.
For example, if a low pass filter had a cut off frequency of 30 Hz, the kind of
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