Chapter 3
Connecting Signals
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National Instruments Corporation
3-9
Differential Connection Considerations
Differential connections are those in which each DAQCard-700 AI signal
has its own reference signal or signal return path. These connections are
available when the DAQCard-700 is configured in the DIFF mode. Each
input signal is tied to the positive input of the instrumentation amplifier,
and its reference signal, or return, is tied to the negative input of the
instrumentation amplifier.
When the DAQCard-700 is configured for DIFF input mode, each signal
uses two of the multiplexer inputs—one for the signal and one for its
reference signal. Therefore, only eight AI channels are available when
using DIFF input mode. DIFF input mode should be used when any of the
following conditions are present:
•
Input signals are low-level (less than 1 V).
•
Leads connecting signals to the DAQCard-700 are greater than 15 ft.
•
Any of the input signals requires a separate ground reference point or
return signal.
•
The signal leads travel through noisy environments.
Differential signal connections reduce noise pickup and increase
common-mode noise rejection. With these connections, input signals
can float within the common-mode limits of the input instrumentation
amplifier.
