Section 8. Operation
performed as part of the routine auto-calibration of the CR800. 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
effect is caused by dielectric absorption of the integrator capacitor and cannot be
overcome by circuit design. Remedies include the following:
• Program longer settling times
• Use an individual instruction for each input terminal, the effect of which
is to reset the integrator circuit prior to filtering.
• Avoid preceding a very small voltage input with a very large voltage
input in a measurement sequence if a single measurement instruction
must be used.
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