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the designer of the board (or other EE) would be able to translate that into a systems
specification, most A/D board owners are confused or mislead by such specs.
6.3 COMMON MISUNDERSTANDINGS
The CMR specification of a differential input is often confused with an isolation
specification, which it is not. CMR of 10 volts is not the same as 10 volts of isolation.
The graph above shows why.
Also, failure to specify the common mode plus signal system specification leads
people to believe that a DC offset equal to the component CMR can be rejected
regardless of the input signal voltage. It cannot as the graph above illustrates.
When is a differential input useful? The answer is whenever electromagnetic
interference (EMI) or radio frequency interference (RFI) may be present in the path of
the signal wires. EMI and RFI can induce voltages on both signal wires and the effect
on single ended inputs is generally a voltage fluctuation between signal high and
signal ground.
A differential input is not affected in that way. When the signal high and signal low of
a differential input have EMI or RFI voltage induced on them, that common mode
voltage is rejected, subject to the system constraint that common mode plus signal not
exceed the A/D board's CSR specification.
6.4 GROUND LOOPS
Ground loops are circuits (E=I*R) created when the signal ground and the PC ground
are not the same. Ground loop inducing voltage differential may be a few volts of
hundreds of volts. They may be constant or transient (spikes). A differential input
will prevent a ground loop as long as the CSR specifications is not exceeded.
If ground differences greater than the CMR are encountered, isolation is required.
6.5 LOW PASS FILTERS
A low pass filter is placed on the signal wires between a signal and an A/D board. It
stops frequencies greater than the cut off frequency from entering the A/D board's
analog or digital inputs.
The key term in a low pass filter circuit is cut-off frequency. The cut-off frequency is
that frequency above which no variation of voltage can enter the circuit. For example,
if a low pass filter had a cut-off frequency of 30 Hz, the kind of interference
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