Measurement Computing CIO-DAS16/330 - Manual (Page 26)

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
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-AD 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 2.5 volts.
IMPORTANT NOTE
The resistors, R1 and R2, are going to dissipate all the power in the divider circuit according to the
equation W=I
2 x R (Power (Watts) = Current Squared times 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.
6.2 DIFFERENTIAL & SINGLE ENDED INPUTS
The two types of analog inputs commonly found on A/D boards are differential and single-ended.
COMMON MODE RANGE
Differential inputs have a common mode range (CMR). Single-ended inputs have no CMR . Common mode range is the
voltage range over which differences in the low side of the signal and A/D input ground have no impact on the A/D's
measurement of the signal voltage. A differential input can reject voltage differences between signal ground and PC
ground.
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