Campbell Scientific CR5000 Measurement and Control Module - Manual (Page 56)

Section 3. CR5000 Measurement Details
3-2
An AutoRange measurement will return Not-A-Number if the voltage exceeds
the range picked by the first measurement. To avoid problems with a signal on
the edge of a range, AutoRange selects the next larger range when the signal
exceeds 90% of a range.
AutoRange is very good for a signal that occasionally exceeds a particular
range, for example, a Type J thermocouple that most of the time will be less
than 360
°
C (
±
20 mV range) but will occasionally see temperatures as high as
400
°
C (
±
50 mV range, Table 3.4-2). AutoRange should not be used for
rapidly fluctuating signals, particularly those whose signal traverses several
voltage ranges rapidly because of the possibility that the signal could change
ranges between the range check and the actual measurement.
3.1.2 Reversing Excitation or the Differential Input
Reversing the excitation polarity or the differential input are techniques to
cancel voltage offsets that are not part of the signal. For example, if there is a
+5
µ
V offset in the measurement circuitry, a 5 mV signal will be measured as
5.005 mV. When the input is reversed, the measurement will be
-4.995 mV. Subtracting the second measurement from the first and dividing
by 2 gives the correct answer: 5.005-(-4.995)=10, 10/2=5. Most offsets are
thermocouple effects caused by temperature gradients in the measurement
circuitry or wiring.
Reversing the excitation polarity cancels voltage offsets in the sensor, wiring,
and measurement circuitry. One measurement is made with the excitation
voltage with the polarity programmed and a second measurement is made with
the polarity reversed. The excitation "on time" for each polarity is exactly the
same to ensure that ionic sensors do not polarize with repetitive measurements.
Reversing the inputs of a differential measurement cancels offsets in the
CR5000 measurement circuitry and improves common-mode rejection. One
measurement is made with the high input referenced to the low input and a
second with the low referenced to the high.
3.1.3 Measuring Single-Ended Offset
The single-ended offset is a voltage offset on a single-ended input. It is
measured by internally switching the input to ground and measuring the
voltage. When a single-ended measurement is made this offset is corrected for
in the calibration. The offset can either be measured automatically as part of
the background calibration or as part of the measurement sequence each time
the measurement is made (adding to the time to make the measurement).
When the offset is measured in the measurement sequence, the offset is
measured once prior to completing all of the instruction reps.
The MeasOfs parameter in instructions that make single-ended voltage
measurements is used to force the offset measurement. In most cases the
background calibration is adequate. Additional accuracy can be gained by
making the offset measurement with each measurement instruction when the
offset is changing rapidly as it would during when the CR5000 is undergoing
rapid temperature swings.