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2.5.1 Single-Ended Inputs
There are three single-ended A/D converter inputs on the Digital I/O Card. A 5 V refer-
ence voltage is used for the single-ended inputs.
Two single-ended inputs that handle 0–10 V are filtered by a network of resistors and a
capacitor. The resistors form a 2:1 attenuator, and the capacitor protects the A/D converter
input against electrostatic transients. The input impedance is approximately 200 k ? .
The third single-ended input handles 0–1 V and is filtered by a network of resistors and a
capacitor, which stabilize the circuit against oscillation and electrostatic transients. The
op-amp and feedback resistors form a ×5 amplifier. The input impedance is of the order of
several megohms.
2.5.2 Differential Inputs
Differential measurements actually require two analog inputs. As the name differential
implies, the difference in voltage between the two inputs is measured rather than the
difference between the input and ground.
A 2.5 V reference voltage is used for the differential inputs to shift the center point of
operation to 2.5 V. The input impedance of each differential input is at least 1 M ? .
The differential input circuit of the Digital I/O Card was designed to measure voltages over a
range of ±250 mV. If the negative input is -250 mV, and the positive input is +250 mV, the
output to the A/D converter will be 5 V. If both inputs are tied together and to ground, the
output to the A/D converter will be 2.5 V. The output to the A/D converter will drop to 0 V
when the negative input reaches +250 mV and the positive input reaches -250 mV.
2.5.3 Calibrating the Analog Inputs
Manufacturing tolerances for resistors, bias currents, offset voltages, gain, and the like
introduce errors into the A/D conversions. Ideally there would be a one-to-one straight-
line relationship between the input voltage and the output of the A/D converter, and a
graph of such a line would have a slope of 1 and would pass through the (0,0) coordinate.
However, the errors arising from manufacturing tolerances introduce a deviation between
the input voltage that is actually applied and the voltage measurement that is output by the
A/D converter. The actual plot of voltage in vs. the voltage out from A/D converter is not
actually a straight line. However, a straight line is a very good first-order approximation,
and the calibration routines provided for the Digital I/O Card are based on a straight line
with a slope of 1 and an offset from (0,0). The calibration routines use two known mea-
surement points on the voltage-in vs. voltage-out line as the basis to calculate calibration
constants that will be used to adjust for the slope of the line and the offset from (0,0). The
calibration routines typically use input voltage points that are 10% of the voltage range
less than the maximum and 10% of the voltage range more than the minimum readings
possible for the A/D converter on any given range.
User ’s Manual
27
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