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Appendix C
Figure 42: Even when using an A/D per channel, noise can contribute errors to your measurement
To minimize noise and ground loops, some newer systems offer isolation between the input
signal ground reference and the computer ground. This effectively separates the computer
ground from the measurement portion of the system. But still, there is no isolation between
input sensor channels, which is a common source of error and frustration for user
applications. Why?
The assumption is made that all signal sources have the same exact ground reference. After
all...ground is ground...isn't it? Often this is not the case. For example, thermocouples for
measuring temperature may be dispersed widely throughout an industrial setting, such as in
the manufacture of air frames or in curing ovens. Grounds for these sensors may differ by
several volts or even hundreds of volts. The resulting common mode voltage causes current to
flow in the signal path, producing serious errors, which are very hard to diagnose and correct.
Isolating Each Input
Oftentimes it is NOT apparent that ground references from various sensors such as
thermocouples, RTDs, strain gages, etc., are at different voltage potentials. Factors that can
contribute to these ground differences are extensive wiring from long runs, crosstalk from
motors or generators, or high source impedance from the signal source. Without recognizing
this extraneous voltage, the measurement system "sees" this noise or common mode voltage
as the actual signal. These unwanted noise sources lead to measurement errors (see
132
results.
Figure
43).
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