Chapter 4 Analog Input
© National Instruments Corporation 4-25 NI USB-621x User Manual
Connecting Floating Signal Sources
What Are Floating Signal Sources?
A floating signal source is not connected to the building ground system, but
has an isolated ground-reference point. Some examples of floating signal
sources are outputs of transformers, thermocouples, battery-powered
devices, optical isolators, and isolation amplifiers. An instrument or device
that has an isolated output is a floating signal source.
When to Use Differential Connections with Floating
Signal Sources
Use differential input connections for any channel that meets any of the
following conditions:
• The input signal is low level (less than 1 V).
• The leads connecting the signal to the device are greater than 3 m
(10 ft).
• The input signal requires a separate ground-reference point or return
signal.
• The signal leads travel through noisy environments.
• Two analog input channels, AI+ and AI–, are available for the signal.
Differential signal connections reduce noise pickup and increase
common-mode noise rejection. Differential signal connections also allow
input signals to float within the common-mode limits of the NI-PGIA.
Refer to the Using Differential Connections for Floating Signal Sources
section for more information about differential connections.
When to Use Referenced Single-Ended Connections
with Floating Signal Sources
Only use referenced single-ended input connections if the input signal
meets the following conditions:
• The input signal can share a common reference point, AI GND, with
other signals that use referenced single-ended.
• The input signal is high-level (greater than 1 V).
• The leads connecting the signal to the device are less than 3 m (10 ft).
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