Campbell CR800 Series - Range-Code Option C Over-Voltages

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