Campbell CR800 Series, Operator'S Manual

Page: 363 / 598

Section 8. Operation
363
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 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. 326)
lists some of the tools
available to minimize the effects of offset voltages.