Section 8. Operation
1. Switches to the measurement terminals
2. Sets the excitation, and then settle, and then measure
3. Reverse the excitation, and then settles, and then measure
4. Reverse the excitation, reverse the input terminals, settle, measure
5. Reverse the excitation, settle, measure
There are four delays per measure. The CR800 processes the four sub-
measurements into the reported measurement. In cases of excitation reversal,
excitation time for each polarity is exactly the same to ensure that ionic sensors do
not polarize with repetitive measurements.
Read More A white paper entitled "The Benefits of Input Reversal and
Excitation Reversal for Voltage Measurements" is available at
www.campbellsci.com.
Ground Reference Offset Voltage
When MeasOff is enabled (= True), the CR800 measures the offset voltage of the
ground reference prior to each VoltSe() or TCSe() measurement. This offset
voltage is subtracted from the subsequent measurement.
From Auto Self-Calibration
If RevDiff, RevEx, or MeasOff is disabled (= False), offset voltage compensation
is continues to be automatically performed, albeit less effectively, by using
measurements from the auto self-calibration. Disabling RevDiff, RevEx, or
MeasOff speeds up measurement time; however, the increase in speed comes at
the cost of accuracy because of the following:
1 RevDiff, RevEx, and MeasOff are more effective.
2 Auto self-calibrations are performed only periodically, so more time skew
occurs between the auto self-calibration offsets and the measurements to
which they are applied.
Note When measurement duration must be minimal to maximize
measurement frequency, consider disabling RevDiff, RevEx, and
MeasOff when CR800 module temperatures and return currents are slow
to change.
Time Skew Between Measurements
Time skew between consecutive voltage measurements is a function of settling
and integration times, A-to-D conversion, and the number entered into the Reps
parameter of the VoltDiff() or VoltSE() instruction. A close approximation is:
time skew = settling time + integration time + A-to-D conversion time + reps
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