Campbell Scientific CR800 and CR850 Measurement and Control Systems - Manual (Page 278)

Section 8. Operation
278
8.1.2.6 Offset Voltage Compensation
Analog measurement circuitry in the CR800 may introduce a small offset voltage
to a measurement. Depending on the magnitude of the signal, this offset voltage
may introduce significant error. For example, an offset of 3 μV on a 2500-mV
signal introduces an error of only 0.00012%; however, the same offset on a 0.25-
mV signal introduces an error of 1.2%.
The primary source of offset voltage is the Seebeck effect, which arises at the
junctions of differing metals in electronic circuits. Secondary sources of offset
voltages are return currents incident to powering external devices through the
CR800. Return currents create voltage drop at the ground terminals that may be
used as signal references.
CR800 measurement instructions incorporate techniques to cancel these unwanted
offsets. The table Analog Measurement Instructions and Offset Voltage
Compensation Options
(p. 278)
lists available options.
Table 53. Analog Measurements and Offset Voltage Compensation
CRBasic
Voltage
Measurement
Instruction
Input
Reversal
( RevDiff =
True )
Excitation
Reversal
( RevEx =
True )
Measure
Ground
Reference
Offset
( MeasOff =
True )
Background
Calibration
(RevDiff = False)
(RevEx = False)
(MeasOff = False)
VoltDiff()
* *
VoltSe()
* *
TCDiff()
* *
TCSe()
* *
BrHalf()
* *
BrHalf3W()
* *
Therm107()
* *
Therm108()
* *
Therm109()
* *
BrHalf4W()
* * *
BrFull()
* * *
BrFull6W()
* * *
AM25T()
* * *
8.1.2.6.1 Input and Excitation Reversal
Reversing inputs (differential measurements) or reversing polarity of excitation
voltage (bridge measurements) cancels stray voltage offsets. For example, if there
is a 3-
μV offset in the measurement circuitry, a 5-mV signal is measured as 5.003
mV. When the input or excitation is reversed, the measurement is -4.997 mV.
Subtracting the second measurement from the first and dividing by two cancels
the offset:
5.003 mV - (-4.997 mV) = 10.000 mV
10.000 mV / 2 = 5.000 mV.