Chapter 4 Signal Connections
National Instruments Corporation
4-11
VXI-MIO Series User Manual
ground. Your VXI-MIO Series module ADC measures this output
voltage when it performs A/D conversions.
You must reference all signals to ground either at the signal source or
at the module. If you have a floating source, reference the signal to
ground by using the RSE input mode or the DIFF input configuration
with bias resistors (see the
Differential Connections for Nonreferenced
or Floating Signal Sources
section later in this chapter). If you have a
grounded source, you should not reference the signal to AIGND. You
can avoid this reference by using DIFF or NRSE input configurations.
Types of Signal Sources
When configuring the input channels and making signal connections,
you must first determine whether the signal sources are floating or
ground-referenced. The following sections describe these two types of
signals.
Floating Signal Sources
A floating signal source is one that is not connected in any way to earth
ground but, rather, has an isolated ground-reference point. Some
examples of floating signal sources are outputs of transformers,
thermocouples, battery-powered devices, optical isolated outputs, and
isolation amplifiers. Tie the ground reference of a floating signal to
your VXI-MIO Series module analog input ground to establish a local
or onboard reference for the signal. Otherwise, the measured input
signal varies as the source floats out of the common-mode input range.
Ground-Referenced Signal Sources
A ground-referenced signal source is one that is connected in some way
to the same ground reference as the VXI-MIO Series module. An
example of this type of signal is a nonisolated output of a signal
generator which is powered from the same power strip as the VXIbus
system.
The difference in ground potential between two instruments connected
to the same power distribution system is typically between 1 and
100 mV but can be much higher if power distribution circuits are not
properly connected. If a grounded signal source is improperly
measured, this difference may appear as an error in the measurement.
The connection instructions for grounded signal sources are designed to
eliminate this ground potential difference from the measured signal.
