Campbell CR9000, Instruction Manual

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Section 3. CR9000 Measurement Details
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may return the over range value Not-A-Number (NAN) or a valid, but
incorrect, number. To avoid misleading data, either be sure the signal is
referenced to CR9000 ground or use the voltage range R option to check
common mode range as described below.
Sensors that have a floating output (the output is not referenced to ground
through a separate connection) may float out of common-mode range, causing
measurement problems. The voltage range C option described below can be
used to keep floating differential inputs within common-mode range. Another
solution is to connect one side of the differential input to ground to ensure the
signal remains within the common mode range.
There are several measurement options for differential voltage measurements
and differential voltage thermocouple measurements (VoltDiff and TCDiff),
specified in the range code, that are related to common mode:
Range Code C option : Before making the differential measurement, the H
and L inputs are briefly connected to internal voltages within the common
mode range allowing floating inputs to remain within common mode for the
differential measurement.
The C option has the added benefit of being able to detect an open input (e.g.,
broken thermocouple). The H input is connected to a voltage approximately
2.8 V above the L input so that an open input will result in an over range on
the ±200 mV and ±50 mV input ranges. With an open input the high and low
inputs are floating independently and remain close to the values they reached
while connected to the excitation, over ranging voltage ranges up to ±200 mV
and causing Not a Number (NAN) to be returned for the result.
Check common mode range, R, option (e.g., mV1000R) : After making the
differential measurement, appropriate single-ended measurements are made on
the H and L inputs to determine if the differential measurement was within
common-mode range. The result of the differential measurement is set to the
over range value (NAN) if the measurement was determined to be out of
common-mode range.
The options to pull into common mode before the differential measurement
and to check that the input remained in common mode with a single ended
measurement after the differential measurement can be combined (e.g.,
mV1000CR).
Problems with exceeding common mode range may be encountered when the
CR9000 is used to read the output of external signal conditioning circuitry if a
good ground connection does not exist between the external circuitry and the
CR9000. When operating where AC power is available, it is not always safe to
assume that a good ground connection exists through the AC wiring. If a
CR9000 is used to measure the output from a laboratory instrument (both
plugged into AC power and referencing ground to outlet ground), it is best to
run a ground wire between the CR9000 and the external circuitry. Even with
this ground connection, the ground potential of the two instruments may not be
at exactly the same level, which is why a differential measurement is desired.
A differential measurement has the option of reversing the inputs to cancel
offsets as described above. The maximum offset when the inputs are reversed
on a differential measurement offset is about one quarter what it is on a single
ended or one way differential.