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 CR1000 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 CR1000 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 Background 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 automatic background 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 Background calibrations are performed only periodically, so more time skew
occurs between the background 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 CR1000
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
1
+
reps
2
1 A-to-D conversion time, which equals 15 µs.
2
If Reps > 1 (multiple measurements by a single instruction), no additional time
is required. If Reps = 1 in consecutive voltage instructions, add 15 µs per
instruction.
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