National Instruments NI USB-621x Connecting Floating Signal Sources

Chapter 5 Connecting AI Signals on the USB-6210/6211 Devices

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).

DIFF input connections are recommended for greater signal integrity for any input signal that does not meet the preceding conditions.

© National Instruments Corporation 5-3 NI USB-621x User Manual