Campbell CR3000 Micrologger Operator'S Manual Download

Page: 398 / 644

Section 8. Operation

398

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
performed as part of the routine auto-calibration of the CR3000.    Single-ended
measurement instructions VoltSE() and TCSe() MeasOff parameter determines
whether the offset voltage measured is done at the beginning of measurement
instruction, or as part of self-calibration. This option provides you with the
opportunity to weigh measurement speed against measurement accuracy.    When
MeasOff

True, a measurement of the single-ended offset voltage is made at the

beginning of the VoltSE() instruction. When MeasOff

False, an offset voltage

measurement is made during self-calibration. For slowly fluctuating offset
voltages, choosing MeasOff

True for the VoltSE() instruction results in better

offset voltage performance.

Ratiometric measurements use an excitation voltage or current to excite the sensor
during the measurement process.    Reversing excitation polarity also reduces
offset voltage error.    Setting the RevEx parameter to True programs the
measurement for excitation reversal.    Excitation reversal results in a polarity
change of the measured voltage so that two measurements with opposite polarity
can be subtracted and divided by 2 for offset reduction similar to input reversal for
differential measurements.    Ratiometric differential measurement instructions
allow both RevDiff and RevEx to be set True.    This results in four measurement
sequences:

•

positive excitation polarity with positive differential input polarity

•

negative excitation polarity with positive differential input polarity

•

positive excitation polarity with negative differential input polarity

•

positive excitation polarity then negative excitation differential input
polarity

For ratiometric single-ended measurements, such as a BrHalf(), setting RevEx

True results in two measurements of opposite excitation polarity that are
subtracted and divided by 2 for offset voltage reduction. For RevEx

False for

ratiometric single-ended measurements, an offset-voltage measurement is made
during the self-calibration.

When analog voltage signals are measured in series by a single measurement
instruction, such as occurs when VoltSE() is programmed with Reps

2 or more,

measurements on subsequent terminals may be affected by an offset, the
magnitude of which is a function of the voltage from the previous measurement.
While this offset is usually small and negligible when measuring large signals,
significant error, or NAN, can occur when measuring very small signals. This

Campbell CR3000 Micrologger, Operator'S Manual

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