Measurement Computing PC104-CTR10HD User Manual

Page 15

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 PC104-CTR digital inputs. The voltage must be dropped to 5 volts
maximum when on. The Attenuation is 24:5 or 4.8. Use the equation above to find an
appropriate R1 if R2 is 1K. 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 and Power = Current-squared x Resistance
(P

watts

= I

*R). 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-TERM100 has the circuitry on board to create custom voltage dividers. The
CIO-TERM100 is a 16" by 4" screw terminal board with two 37-pin D-type
connectors and 56 screw terminals (12 to 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.