National-instruments Data Acquisition Device NI USB-621x Manual de usuario descargar pdf (Pagina 58)

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 Referenced Single-Ended (RSE)

Connections with Floating Signal Sources

Only use RSE 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 RSE.

• 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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National-instruments Data Acquisition Device NI USB-621x Manual de usuario Pagina 58