Campbell Scientific CR6 CR6 Measurement and Control System - Page 324

Section 8. Operation 324 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 CR6 self-calibration. 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 integration. • 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. The table Offset-Voltage Compensation Options (p. 324) lists some of the tools available to minimize the effects of offset voltages.