Campbell Scientific CR9000X Measurement and Control System - Manual (Page 95)

Section 3. CR9000X Measurement Details
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.
Input Limit check, R option :
As previously mentioned, input voltages in which V
H
or V
L
are beyond the
±5V Input Limits may suffer from undetected measurement errors. The “R”
range code option (e.g., mV1000R) invokes SE measurements of both V
H
and
V
L
after the associated differential voltage measurement. If either V
H
or V
L
is
found to be outside the Input Limit range, then a NAN is returned for the
measured result instead of a possible erroneous value. To avoid misleading
data, either be sure that the inputs are within the Input Limits with respect to
the CR9000X analog ground, or use the voltage range R option to check
common mode range.
Example 3.1.2-3: If VH of a differential input is at 4.3 V and VL is at 3.4
V relative to CR9000X ground, a sound measurement can be made. A
measurement made on the CR9050 module using the mV1000 range code
option range will return 900 mV. However, if the high input is at 5.6 V
and the low input is at 4.8 V, the measurement result returned could either
be NAN or some erroneous numeric. If the mV1000R range code option
were utilized, it would force a result of NAN to be returned rather than
possibly allowing a bogus value to be returned.
“C” and “R” Range Combination
The “C” and “R” options can both be utilized for a given VoltDiff and TCDiff
instruction combined (e.g., mV200CR). For a “CR” range code option, the “C”
portion is first performed, followed by the associated differential voltage
measurement, followed by the “R” portion of the measurement. A NAN result
indicates either a sensor over range, an open input, or that V
H
and/or V
L
exceeded the ± 5 V Input Limits when using the “CR” range code option.
Problems with exceeding the Input Limits may be encountered when the
CR9000X 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
CR9000X. 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
CR9000X 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 CR9000X 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 in Section 3.1.1.1. 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.
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