Measurement Computing PCI-DIO48H/CTR15 - Manual (Page 24)

For a given attenuation, pick a handy
resistor and call it R2, then use this
formula to calculate R1.
R1 = (A
−
1) x R2
Digital inputs often require voltage dividers. For example, if you wish to detect a field
signal that is at 0 volts when OFF and 24 volts when ON, you cannot connect that
directly to the PCI-DIO48H/CTR15 digital inputs. The voltage must be dropped to
5 volts maximum when ON. The Attenuation required 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.
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. As 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.
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