Voltage measurements

Voltage measurements are made using an Analog-to-Digital Converter (ClosedADC Analog to digital conversion. The process that translates analog voltage levels to digital values.). A high-impedance Programmable-Gain Amplifier (ClosedPGA Programmable Gain Amplifier) amplifies the signal. Internal multiplexers route individual terminals within the amplifier. The ClosedCRBasic Campbell Scientific's BASIC-like programming language that supports analog and digital measurements, data processing and analysis routines, hardware control, and many communications protocols. measurement instruction controls the ADC gain and configuration – either single-ended or differential input. Information on the differences between single-ended and differential measurements can be found here: Deciding between single-ended or differential measurements.

A voltage measurement proceeds as follows:

  1. Set PGA gain for the voltage range selected with the CRBasic measurement instruction parameter Range. Set the ADC for the first notch frequency selected with fN1.

  2. If used, such as with bridge measurements, turn on excitation to the level selected with ExmV.

  3. Multiplex selected terminals (SEChan or DiffChan).

  4. Delay for the entered settling time (SettlingTime).

  5. Perform the analog-to-digital conversion.

  6. Repeat for input reversal as determined by parameters RevEx and RevDiff.

  7. Apply multiplier (Mult) and offset (Offset) to measured result.

Conceptually, analog voltage sensors output two signals: high and low. For example, a sensor that outputs 1000 mV on the high signal and 0 mV on the low has an overall output of 1000 mV. A sensor that outputs 2000 mV on the high signal and 1000 mV on the low also has an overall output of 1000 mV. Sometimes, the low signal is simply sensor ground (0 mV). A single-ended measurement measures the high signal with reference to ground; the low signal is tied to ground. A differential measurement measures the high signal with reference to the low signal. Each configuration has a purpose, but the differential configuration is usually preferred.

In general, use the smallest input range that accommodates the full-scale output of the sensor. This results in the best measurement accuracy and resolution (see Analog measurement specifications for more information).

A set overhead reduces the chance of overrange. Overrange limits are available in the specifications. The data logger indicates a measurement overrange by returning a ClosedNAN Not a number. A data word indicating a measurement or processing error. Voltage overrange, SDI-12 sensor error, and undefined mathematical results can produce NAN. for the measurement.

WARNING:

Sustained voltages in excess of ±20 V applied to terminals configured for analog input will damage CR6 circuitry.

Single-ended measurements

A single-ended measurement measures the difference in voltage between the terminal configured for single-ended input and the reference ground. For example, single-ended channel 1 is comprised of terminals U1 and . For more information, see Wiring panel and terminal functions. The single-ended configuration is used with the following CRBasic instructions:

  • VoltSE()
  • BrHalf()
  • BrHalf3W()
  • TCSE()
  • Therm107()
  • Therm108()
  • Therm109()
  • Thermistor()

See the CRBasic Editor help for detailed instruction information and program examples: https://help.campbellsci.com/crbasic/cr6/ .

Differential measurements

A differential measurement measures the difference in voltage between two input terminals. For example, differential channel 1 is comprised of terminals U1 and U2, with U1 as high and U2 as low. For more information, see Wiring panel and terminal functions. The differential configuration is used with the following CRBasic instructions:

  • VoltDiff()
  • BrFull()
  • BrFull6W()
  • BrHalf4W()
  • TCDiff()

Reverse differential

Differential measurements have the advantage of an input reversal option, RevDiff. When RevDiff is set to True, two differential measurements are made, the first with a positive polarity and the second reversed. Subtraction of opposite polarity measurements cancels some offset voltages associated with the measurement.

For more information on voltage measurements, see Improving voltage measurement quality and Analog measurement specifications.