Campbell CR3000 Micrologger, Руководство оператора

Страница: 387 / 644

Section 8. Operation
387
addressing voltage measurement quality:
• Preventing and Attacking Measurement Noise Problems
• Benefits of Input Reversal and Excitation Reversal for Voltage
Measurements
• Voltage Measurement Accuracy, Self- Calibration, and Ratiometric
Measurements
• Estimating Measurement Accuracy for Ratiometric Measurement
Instructions.
The following topics discuss methods of generally improving voltage
measurements. Related information for special case voltage measurements
(thermocouples
(p. 355),
current loops
(p. 380),
resistance
(p. 367),
and strain
(p. 378))
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, which filters capacitively coupled
noise
•
Active Noise Rejection
o
Input reversal
— Review Input and Excitation Reversal
(p. 350)
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 H/L 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.