Campbell Scientific CR3000 Micrologger - Manual (Page 288)

Section 8. Operation
288
Table 55. 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.
When the CR3000 reverses differential inputs or excitation polarity, it delays the
same settling time after the reversal as it does before the first measurement. So,
there are two delays per channel when either
RevDiff or RevEx is used. If both
RevDiff and RevEx are True , four measurements are performed; positive and
negative excitations with the inputs one way and positive and negative excitations
with the inputs reversed. To illustrate,
1. the CR3000 switches to the channel
2. sets the excitation, settles, measures,
3. reverses the excitation, settles, measures,
4. reverses the excitation, reverses the inputs, settles, measures,
5. reverses the excitation, settles, measures.
There are four delays per channel measured. The CR3000 processes the four sub-
measurements into a single reported value. In cases of excitation reversal,