Measurement Computing CIO-INT-32 User Manual

Page 18

The trick to using a voltage divider is to choose two resistors with the proper
proportions relative to the full scale of the digital input and the maximum signal
voltage.

The phenomena 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 voltage max and the
full scale of the analog input.

Attenuation = R1 + R2

Digital inputs also make 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 the CIO-DIO digital inputs. The
voltage must be dropped to 5 volts max when on. The 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 the power in the divider circuit according to the
equation Current = 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 or less, no resistor should be less than 10K.

For Attenuation of greater than 5:1, no resistor should be less than
1K.

The CIO-TERMINAL has the circuitry on board to create custom voltage dividers.
The CIO-TERMINAL is a 16" by 4" screw terminal board with two 37-pin D-type
connectors and 56 screw terminals (12 - 22 AWG). Designed for table top, wall or
rack mounting, the board provides prototype, divider circuit, filter circuit and pull-up
resistor positions which you may complete with the proper value components for your
application.