Campbell CR6 Series, Руководство оператора

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Section 5. Overview
74
A single-ended measurement measures the difference in voltage between the
terminal configured for single-ended input and the reference ground. While
differential measurements are usually preferred, a single-ended measurement is
often adequate in applications wherein some types of noise are not present and
care is taken to avoid problems caused by
ground currents
(p. 585).
Examples of
applications wherein a single-ended measurement may be preferred include:
• Not enough differential terminals available. Differential measurements
use twice as many U terminals as do single-ended measurements.
•
Sensor is not designed for differential measurements. Many Campbell
Scientific sensors are not designed for differential measurement, but the
draw backs of a single-ended measurement are usually mitigated by large
programmed excitation and/or sensor output voltages.
However, be aware that because a single-ended measurement is referenced to CR6
ground, any difference in ground potential between the sensor and the CR6 will
result in error, as emphasized in the following examples:
•
If the measuring junction of a thermocouple used to measure soil
temperature is not insulated, and the potential of earth ground is greater
at the sensor than at the point where the CR6 is grounded, a measurement
error will result. For example, if the difference in grounds is 1 mV, with
a copper-constantan thermocouple, the error will be approximately 25
°C.
• If signal conditioning circuitry, such as might be found in a gas analyzer,
and the CR6 use a common power supply, differences in current drain
and lead resistance often result in different ground potentials at the two
instruments despite the use of a common ground. A differential
measurement should be made on the analog output from the external
signal conditioner to avoid error.
5.2.2.1.2 Differential Measurements — Overview
Related Topics:
•
Differential Measurements — Overview
(p. 74)
•
Differential Measurements — Details
(p. 422)
Summary Use a differential configuration when making voltage
measurements, unless constrained to do otherwise.
A differential measurement measures the difference in voltage between two input
terminals. Its autosequence is characterized by multiple measurements, the
results of which are autoaveraged before the final value is reported. For example,
the sequence on a differential measurement using the
VoltDiff()
instruction
involves two measurements — first with the high input referenced to the low, then
with the inputs reversed. Reversing the inputs before the second measurement
cancels noise common to both leads as well as small errors caused by junctions of
different metals that are throughout the measurement electronics.