Minimizing Offset Voltages - Campbell CR300 Series Product Manual

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

17.11.7 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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Remedies include:
Connecting power grounds to power ground terminals (G).
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Automatic offset compensation for single-ended measurements when MeasOff =
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False.
Using MeasOff = True for better offset compensation.
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Programming longer settling times.
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Single-ended measurements are susceptible to voltage drop at the ground terminal caused by
return currents from another device that is powered from the data logger wiring panel, such as
another manufacturer's communications modem, or a sensor that requires a lot of power.
VoltDiff()
Minimizing ground potential differences (p. 139).
17. Tips and troubleshooting 147