Section 8. Operation
Summary
Measurement offset voltages are unavoidable, but can be minimized.
Offset voltages originate with:
• Ground currents
• Seebeck effect
• Residual voltage from a previous measurement
Remedies include:
• Connect power grounds to power ground terminals (G)
• Use input reveral (RevDiff = True) with differential measurements
• Automatic offset compensation for differential measurements when
RevDiff = False
• Automatic offset compensation for single-ended measurements when
MeasOff = False
• Better offset compensation when MeasOff = True
• Excitation reversal (RevEx = True)
• Longer settling times
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%. The primary sources of offset voltage
are ground currents and the Seebeck effect.
Single-ended measurements are susceptible to voltage drop at the ground terminal
caused by return currents from another device that is powered from the CR800
wiring panel, such as another manufacturer's comms modem, or a sensor that
requires a lot of power. Currents >5 mA are usually undesirable. The error can
be avoided by routing power grounds from these other devices to a power ground
G terminal on the CR800 wiring panel, rather than using a signal ground ( )
terminal. Ground currents can be caused by the excitation of resistive-bridge
sensors, but these do not usually cause offset error. These currents typically only
flow when a voltage excitation is applied. Return currents associated with
voltage excitation cannot influence other single-ended measurements because the
excitation is usually turned off before the CR800 moves to the next measurement.
However, if the CRBasic program is written in such a way that an excitation
terminal is enabled during an unrelated measurement of a small voltage, an offset
error may occur.
The Seebeck effect results in small thermally induced voltages across junctions of
dissimilar metals as are common in electronic devices. Differential
measurements are more immune to these than are single-ended measurements
because of passive voltage cancelation occurring between matched high and low
pairs such as 1H/1L. So use differential measurements when measuring critical
low-level voltages, especially those below 200 mV, such as are output from
pyranometers and thermocouples. Differential measurements also have the
advantage of an input reversal option, RevDiff. When RevDiff is True, two
differential measurements are made, the first with a positive polarity and the
second reversed. Subtraction of opposite polarity measurements cancels some
offset voltages associated with the measurement.
Single-ended and differential measurements without input reversal use an offset
voltage measurement with the PGIA inputs grounded. For differential
measurements without input reversal, this offset voltage measurement is
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