Campbell CR800 Series, Operator'S Manual

Page: 361 / 598

Section 8. Operation
361
Range-Code Option C Over-Voltages
Input Range (mV) Over-Voltage
±
2.5
± 7.5
±
25
±
250
300 mV
± 2500 C option with caveat
1
±5000
C option not available
1
C results in the H terminal being briefly connected to a voltage greater than
2500 mV, while the L terminal is connected to ground. The resulting
common-mode voltage is 1250 mV, which is not adequate to null residual
common-mode voltage, but is adequate to facilitate a type of open-input
detect. This requires inclusion of an If / Then / Else statement in the CRBasic
program to test the results of the measurement. For example:
•The result of a
VoltDiff() measurement using mV2500C as the Range code
can be tested for a result >2500 mV, which would indicate an open input.
•The result of the
BrHalf() measurement, X , using the mV2500C range code
can be tested for values > 1 . A result of X > 1 indicates an open input for the
primary measurement, V1, where X = V1/Vx and Vx is the excitation voltage.
A similar strategy can be used with other bridge measurements.
Offset Voltage Compensation
Related Topics
• Auto Self-Calibration — Overview
(p. 89)
•
Auto Self-Calibration — Details
(p. 337)
•
Auto Self-Calibration — Errors
(p. 475)
• Offset Voltage Compensation
(p. 323)
•
Factory Calibration
(p. 86)
•
Factory Calibration or Repair Procedure
(p. 461)
Summary
Measurement offset voltages are unavoidable, but can be minimized.
Offset voltages originate with:
• Ground currents
• Seebeck effect
• Residual voltage from a previous measurement
Remedies include:
• Connect power grounds to power ground terminals ( G )
• Use input reveral ( RevDiff = True ) with differential measurements
• Automatic offset compensation for differential measurements when
RevDiff
=
False
• Automatic offset compensation for single-ended measurements when
MeasOff = False
• Better offset compensation when MeasOff = True