Programming instructions

Chapter 5 Things You Should Know about Analog Input
LabVIEW Data Acquisition Basics Manual 5-10
© National Instruments Corporation
In general, a differential measurement system is preferable because it
rejects not only ground loop-induced errors, but also the noise picked up in
the environment to a certain degree. Use differential measurement systems
when all input signals meet the following criteria:
• Low-level signals (i.e., less than 1 V)
• Long or non-shielded cabling/wiring traveling through a noisy
environment
• Any of the input signals require a separate ground-reference point or
return signal
An ideal differential measurement system reads only the potential
difference between its two terminals—the (+) and (–) inputs. Any
voltage present at the instrumentation amplifier inputs with respect to the
amplifier ground is called a
common-mode voltage
. An ideal differential
measurement system completely rejects (does not measure) common-mode
voltage, as shown in Figure 5-8.
Figure 5-8.
Common-Mode Voltage
While a differential measurement system is often the best choice, a
single-ended configuration uses twice as many measurement channels.
A single-ended measurement system is acceptable when the magnitude of
the induced errors is smaller than the required accuracy of the data.
+
+
+
–
+
Vs
Vcm
Instrumentation Amplifier
Measured Voltage
Grounded Signal Source
Common Mode Voltage,
Ground Potential, Noise, etc.
–
–
–