Section 8. Operation
Reasons for using single-ended measurements, however, include:
•
Not enough differential terminals available. Differential measurements use
twice as many
H
/
L
] terminals as do single-ended measurements.
•
Rapid sampling is required. Single-ended measurement time is about half
that of differential measurement time.
•
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.
Sensors with a high signal-to-noise ratio, such as a relative-humidity sensor with a
full-scale output of 0 to 1000 mV, can normally be measured as single-ended
without a significant reduction in accuracy or precision.
Sensors with a low signal-to-noise ratio, such as thermocouples, should normally
be measured differentially. However, if the measurement to be made does not
require high accuracy or precision, such as thermocouples measuring brush-fire
temperatures, which can exceed 2500 °C, a single-ended measurement may be
appropriate. If sensors require differential measurement, but adequate input
terminals are not available, an analog multiplexer should be acquired to expand
differential input capacity. Refer to the appendix
Analog Multiplexers
(p. 646)
for
information concerning available multiplexers.
Because a single-ended measurement is referenced to CR1000 ground, any
difference in ground potential between the sensor and the CR1000 will result in an
error in the measurement. For example, if the measuring junction of a copper-
constantan thermocouple being used to measure soil temperature is not insulated,
and the potential of earth ground is 1 mV greater at the sensor than at the point
where the CR1000 is grounded, the measured voltage will be 1 mV greater than
the true thermocouple output, or report a temperature that is approximately 25 °C
too high. A common problem with ground-potential difference occurs in
applications wherein external, signal-conditioning circuitry is powered by the
same source as the CR1000, such as an ac mains power receptacle. Despite being
tied to the same ground, differences in current drain and lead resistance may result
in a different ground potential between the two instruments. So, as a precaution, a
differential measurement should be made on the analog output from an external
signal conditioner; differential measurements MUST be used when the low input
is known to be different from ground.
Electronic Noise
Electronic "noise" can cause significant error in a voltage measurement,
especially when measuring voltages less than 200 mV. So long as input
limitations are observed, the PGIA ignores voltages, including noise, that are
common to each side of a differential-input pair. This is the common-mode
voltage. Ignoring (rejecting or canceling) the common-mode voltage is an
essential feature of the differential input configuration that improves voltage
measurements.
Figure
PGIA with Input-Signal Decomposition
(p. 311),
illustrates the common-
mode component (V
cm
) and the differential-mode component (V
dm
) of a voltage
signal. V
cm
is the average of the voltages on the V+ and V– inputs. So, V
cm
=
312
Summary of Contents for CR1000
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