Deciding Between Single-Ended Or Differential Measurements - Campbell CR300 Series Product Manual

17.11.1 Deciding between single-ended or differential
measurements
Deciding whether a differential or single-ended measurement is appropriate is usually, by far, the
most important consideration when addressing voltage measurement quality. The decision
requires trade-offs of accuracy and precision, noise cancellation, measurement speed, available
measurement hardware, and fiscal constraints.
In broad terms, analog voltage is best measured differentially because these measurements
include the following noise reduction features that are not included in single-ended
measurements.
Passive Noise Rejection
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No voltage reference offset
o
Common-mode noise rejection, which filters capacitively coupled noise
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Reasons for using single-ended measurements, however, include:
Not enough differential terminals are available. Differential measurements use twice as
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many analog input terminals as do single-ended measurements.
Rapid sampling is required. Single-ended measurement time is about half that of
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differential measurement time.
Sensor is not designed for differential measurements. Some Campbell Scientific sensors are
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not designed for differential measurement, but the drawbacks 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.
Because a single-ended measurement is referenced to data logger ground, any difference in
ground potential between the sensor and the data logger will result in an error in the
measurement. For more information on grounds, see
potential differences (p. 139).
Grounds (p. 10) and Minimizing ground
17. Tips and troubleshooting 138