ZWorld Exp-A/D12, User Manual

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XP8500 I/O Configurations s 29
Step 4. Bracket Input Range
To be sure of measuring signals accurately at the extremes of the range of
input voltages, be aware of the interaction between the 10 k
Ω
fixed
resistors, RP3–RP4, and the gain and bias resistors, R1–R8. Ideally, a
signal at the minimum input level would be output to the A/D converter’s
input at the maximum expected value of 2.5 V (remember that U2 is an
inverting op-amp).
But real-world resistor values vary within their rated tolerances. Thus, if
the fixed input resistor has a resistance lower than its nominal value, and
the installed resistors have a resistance slightly higher than their nominal
value, the actual input to the A/D converter chip would be greater than
2.5 V. A loss of accuracy then results because the A/D converter input
would reach its maximum input value before the true signal input reaches
the minimum expected input level.
Similarly, a deviation from nominal values in the bias network could skew
the A/D converter’s input voltage away from the theoretically computed
value. For example, a small positive or negative deviation of the bias
voltage arising from variances in the resistor divider would offset the A/D
converter’s input voltage. This offset would be positive or negative,
tracking the deviation’s sign, and would be equal to the bias deviation
multiplied by the amplifier’s gain plus one. Both of these effects could
occur in the same circuit.
Step 5. Pick Proper Tolerance
Use care when compensating for discrepancies. For example, if standard
5% resistors are used for R1–R8, the values are spaced approximately 10%
apart. If the gain is too high by just a small amount, then going to the next
smallest standard 5% value could decrease the gain, and there would be an
A/D converter excursion approaching 10%. The same caveat applies to
the bias network. Use 1% resistors to have a more precise choice of
values.
Figure E-2 shows the result of adjusting the resistor values such that the
input to the A/D converter stays within its specified 2.5 V range.
Step 6. Confirm Performance
For critical measurements, always check the setup after installing resistors
by measuring test signals at and near the input-voltage limits. See if the
U2 op-amp output voltages fall within the A/D converter’s input range or if
accuracy is lost because of over-excursions at the A/D converter input.
The resistance of the 10 k
Ω
fixed input resistors can be measured after
installing the gain and bias resistors by measuring the voltages at the op-
amps’ inputs and outputs. Using Channel 0 as an example, ground the
CH0 input at pin 2 of Wago connector H1.