User manual

Chapter 4 Connecting Signals

Types of Signal Sources

When 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 not connected in any way to the building ground

system but, rather, has an isolated ground-reference point. Some examples

of floating signal sources are outputs of transformers, thermocouples,

battery-powered devices, optical isolator outputs, and isolation amplifiers.

An instrument or device that has an isolated output is a floating signal

source. You must tie the ground reference of a floating signal to the

NI 6034E/6035E/6036E device 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 connected in some way to the

building system ground and is, therefore, already connected to a common

ground point with respect to the NI 6034E/6035E/6036E device, assuming

that the computer is plugged into the same power system. Non-isolated

outputs of instruments and devices that plug into the building power system

fall into this category.

The difference in ground potential between two instruments connected

to the same building power system is typically between 1 and 100 mV,

but it 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.

Analog Input Modes

You can configure your device for one of three input modes: nonreferenced

single ended (NRSE), referenced single ended (RSE), and differential

(DIFF). With the different configurations, you can use the PGIA in

different ways. Figure 4-2 shows a diagram of your device PGIA.

UM.book Page 7 Monday, May 14, 2001 10:32 AM