Measurement Computing CIO-CTRxxHD - Manual (Page 14)

7 ELECTRONICS AND INTERFACING
7.1 VOLTAGE DIVIDERS
An alternative method of measuring a signal which varies over a range greater than the
input range of a digital input, is to use a voltage divider. When correctly designed, it
can drop the voltage of the input signal to a safe level the digital input can accept.
Ohm's law states:
Voltage = Current x Resistance
Kirkoff's law states:
The sum of the voltage drops around a circuit will be equal to the voltage
drop for the entire circuit.
In a voltage divider, the voltage across one resistor in a series circuit is proportional to
the total resistance divided by the one resistor (see formula below).
The object in a voltage divider is to choose two resistors having the proportions of
the maximum voltage of the input signal to the maximum allowed input voltage.
The formula for attenuation is:
For a given attenuation, pick a resistor and call it
R2, the use this formula to calculate R1. R1 = (A-1) x 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 (A) is 2:1 or just 2.
2 = 10K + 10K
10K
Attenuation = R1 + R2
R2
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.
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