The following table lists some of the tools available to minimize the effects of offset voltages:
Table 6-2: Offset voltage compensation options
CRBasic
Measurement
Instruction
Input Reversal
(RevDiff=True)
Excitation
Reversal
(RevEx=True)
Measure Offset
During
Measurement
(MeasOff=True)
Measure Offset During
Background Calibration
(RevDiff=False)
(RevEx=False)
(MeasOff=False)
BrHalf()
ü
ü
BrHalf3W()
ü
ü
BrHalf4W()
ü
ü
ü
BrFull()
ü
ü
ü
BrFull6W()
ü
ü
ü
TCDiff()
ü
ü
TCSe()
ü
ü
VoltDiff()
ü
ü
VoltSe()
ü
ü
Compensating for offset voltage
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.
Ratiometric measurements use an excitation voltage 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.
For example, if 3 µV offset exists in the measurement circuitry, a 5 mV signal is measured as 5.003
mV. When the input or excitation is reversed, the second sub-measurement is –4.997 mV.
Subtracting the second sub-measurement from the first and then dividing by 2 cancels the offset:
5.003 mV – (–4.997 mV) = 10.000 mV
10.000 mV / 2 = 5.000 mV
6. Measurements
59
Содержание CR1000X
Страница 1: ...Revision 02 21 2019 Copyright 2000 2019 Campbell Scientific ...
Страница 2: ......
Страница 4: ......
Страница 6: ......
Страница 8: ......
Страница 16: ...Appendix A Glossary 176 Appendix B Index 214 Table of Contents viii ...
Страница 20: ...l Smart sensors o SDI 12 o RS 232 o Modbus o DNP3 o TCP IP o RS 485 1 Data acquisition system components 4 ...
Страница 192: ......
Страница 237: ...wiring panel 5 Appendix B Index 220 ...