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 The degree to which the result of a measurement, calculation, or specification conforms to the correct value or a standard. and precision The amount of agreement between repeated measurements of the same quantity (AKA repeatability)., 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
- No voltage reference offset
- Common-mode noise rejection, which filters capacitively coupled noise
Reasons for using single-ended measurements, however, include:
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Not enough differential terminals are available. Differential measurements use twice as many analog input terminals as do single-ended measurements.
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Rapid sampling is required. Single-ended measurement time is about half that of differential measurement time.
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Sensor is not designed for differential measurements. Some Campbell Scientific sensors are 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 The degree to which the result of a measurement, calculation, or specification conforms to the correct value or a standard. or precision The amount of agreement between repeated measurements of the same quantity (AKA repeatability)..
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 Grounds and Minimizing ground potential differences.