Measurement Accuracy Example; Minimizing Offset Voltages - Campbell CR300 Series Product Manual

22.11.6.1 Measurement accuracy example

The following example illustrates the effect percent-of-reading and offset have on measurement
accuracy. The effect of offset is usually negligible on large signals.
Example:
Sensor-signal voltage: approximately 2500 mV
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CRBasic measurement instruction:
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Programmed input-voltage range (Range) : mV 2 500 (±-100 to 2500 mV)
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Input measurement reversal (RevDiff): True
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Data logger circuitry temperature: 10° C
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Accuracy of the measurement is calculated as follows:
accuracy = percent-of-reading + offset
where
percent-of-reading = 2500 mV • ±0.04%
=±1 mV
and
offset = ±20 µV
Therefore,
accuracy = ±(1 mV + 20 µV) = ±1.02 mV

22.11.7 Minimizing offset voltages

Voltage offset can be the source of significant error. For example, an offset of 3 μV on a 2500 mV
signal causes an error of only 0.00012%, but the same offset on a 0.25 mV signal causes an error
of 1.2%. Measurement offset voltages are unavoidable, but can be minimized. Offset voltages
originate with:
Ground currents. See
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Seebeck effect
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Residual voltage from a previous measurement
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VoltDiff()
Minimizing ground potential differences (p. 190).
22. Tips and troubleshooting 199