Campbell Scientific CR6 CR6 Measurement and Control System - Page 313
Single-Ended or Differential?
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Section 8. Operation The following topics discuss methods of generally improving voltage measurements. Related information for special case voltage measurements (thermocouples (p. 328), current loops, resistance (p. 329), and strain (p. 336)) is located in sections for those measurements. Single-Ended or Differential? 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 cancelation, measurement speed, available measurement hardware, and fiscal constraints. In broad terms, analog voltage is best measured differentially because these measurements include noise reduction features, listed below, that are not included in single-ended measurements. • Passive Noise Rejection o No voltage reference offset o Common-mode noise rejection ̶ Rejects capacitively coupled noise • Active Noise Rejection o Input reversal ̶ Review Input and Excitation Reversal (p. 325) for details ̶ Increases by twice the input reversal signal integration time Reasons for using single-ended measurements, however, include: • Not enough differential terminals available. Differential measurements use twice as many U 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. 596) for information concerning available multiplexers. Because a single-ended measurement is referenced to CR6 ground, any difference in ground potential between the sensor and the CR6 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 313
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