Measurement Computing PC104-DIO48 User Manual

Page 20

Dropping the voltage proportionally is called attenuation. The formula
for attenuation is:

For a given attenuation, pick a handy resisitor
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 may also 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 the 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
1K. Remember that a TTL input is 'on' when the input voltage is greater
than 2.5V but less than 5.0V.

IMPORTANT NOTE: The resistors, R1 and R2, are
going to dissipate power in the divider circuit according
to the equation, Current = Voltage / Resistance And
power (watts) is current-squared times resistance
(W=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.