Campbell CR3000 Micrologger, Руководство оператора

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Section 8. Operation
355
where
percent-of-
reading = 5000 mV • ±0.04%
= ±2
mV
and
offset = (1.5 • 167 µV) + 1 µV
= 0.252 mV
Therefore,
accuracy = ±2 mV + 0.251 mV
= ±2.252 mV
Electronic Noise
Electronic "noise" can cause significant error in a voltage measurement,
especially when measuring voltages less than 200 mV. So long as input
limitations are observed, the PGIA ignores voltages, including noise, that are
common to each side of a differential-input pair. This is the common-mode
voltage. Ignoring (rejecting or canceling) the common-mode voltage is an
essential feature of the differential input configuration that improves voltage
measurements.
Figure PGIA with Input Signal Decomposition
(p. 383),
illustrates the common-
mode component (V
cm
) and the differential-mode component (V
dm
) of a voltage
signal. V
cm
is the average of the voltages on the V+ and V– inputs. So, V
cm
=
(V+ + V–)/2 or the voltage remaining on the inputs when V
dm
= 0. The total
voltage on the V+ and V–
inputs is given as V+ = V
cm
+ V
dm
/2, and V
L
= V
cm
–
V
dm
/2, respectively.
8.1.2.2 Thermocouple Measurements — Details
Related Topics:
• Thermocouple Measurements — Details
• Thermocouple Measurements — Instructions
Thermocouple measurements are special case voltage measurements.
Note Thermocouples are inexpensive and easy to use. However, they
pose several challenges to the acquisition of accurate temperature data,
particularly when using external reference junctions. Campbell Scientific
strongly encourages you to carefully evaluate the section Thermocouple
Error Analysis
(p. 356).
An introduction to thermocouple measurements is
located in the section Tutorial: Measuring a Thermocouple
(p. 42).