Campbell CR3000 Micrologger, Operator'S Manual

Page: 349 / 644

Section 8. Operation
349
•
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.
TABLE: Offset Voltage Compensation Options
(p. 349)
lists some of the tools
available to minimize the effects of offset voltages.
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)
AM25T()
  
BrHalf()
 
BrHalf3W()
 
BrHalf4W()
  
BrFull()
  