National Instruments DAQ X NI 634 Series, Руководство пользователя

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Chapter 4 Analog Input
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 DIFF 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.
DIFF signal connections reduce noise pickup and increase common-mode noise rejection. DIFF
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 Non-Referenced Single-Ended (NRSE)
Connections with Floating Signal Sources
Only use NRSE input connections if the input signal meets the following conditions:
• 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).
DIFF input connections are recommended for greater signal integrity for any input signal that
does not meet the preceding conditions.
In the single-ended modes, more electrostatic and magnetic noise couples into the signal
connections than in DIFF configurations. The coupling is the result of differences in the signal
path. Magnetic coupling is proportional to the area between the two signal conductors. Electrical
coupling is a function of how much the electric field differs between the two conductors.
With this type of connection, the NI-PGIA rejects both the common-mode noise in the signal and
the ground potential difference between the signal source and the device ground.
Refer to the Using Non-Referenced Single-Ended (NRSE) Connections for Floating Signal
Sources section for more information about NRSE connections.