Voltage measurements
Voltage measurements are made using an Analog-to-Digital Converter (ADC Analog to digital conversion. The process that translates analog voltage levels to digital values.). A high-impedance Programmable-Gain Amplifier (PGA Programmable Gain Amplifier) amplifies the signal. Internal multiplexers route individual terminals within the amplifier. The CRBasic 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:
-
Set PGA gain for the voltage range selected with the CRBasic measurement instruction parameter
Range
. Set the ADC for the first notch frequency selected withfN1
. -
If used, such as with bridge measurements, turn on excitation to the level selected with
ExmV
. -
Multiplex selected terminals (
SEChan
orDiffChan
). -
Delay for the entered settling time (
SettlingTime
). -
Perform the analog-to-digital conversion.
-
Repeat for input reversal as determined by parameter
RevDiff
. -
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 NAN 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.
Sustained voltages in excess of
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
VoltSE()
BrHalf()
BrHalf3W()
TCSE()
Therm107()
Therm108()
Therm109()
See the CRBasic Editor help for detailed instruction information and program examples:
Differential measurements
A differential measurement measures the difference in voltage between two input terminals. For example, DIFF channel 1 is comprised of terminals 1H and 1L, with 1H as high and 1L as low.
VoltDiff()
BrFull()
BrFull6W()
BrHalf4W()
TCDiff()
For more information on voltage measurements, see Improving voltage measurement quality and Analog measurement specifications.