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Chapter 4

Signal Connections

4-12

 National Instruments Corporation

Figure 4-4.  Differential Input Connections for Nonreferenced Signals

Figure 4-4 shows a bias resistor connected between ACH0 – and the 
floating signal source ground. If you do not use the resistor and the 
source is truly floating, the source is not likely to remain within the 
common-mode signal range of the PGIA, and the PGIA will saturate, 
causing erroneous readings. You must reference the source to the 
respective channel ground. 

Figure 4-3 shows connections for signal sources that are already 
referenced to some ground point with respect to the 611X E board. In 
this case, the PGIA can reject any voltage caused by ground potential 
differences between the signal source and the board. In addition, with 
differential input connections, the PGIA can reject common-mode noise 
pickup in the leads connecting the signal sources to the board. The 
PGIA can reject common-mode signals as long as V+

in

 and V-

in

 (input 

signals) are both within ±11 V of the channel ground, for gain 

≥

 1. For 

gain <1, the input signals, for ACHO +, can be within ±42 V of the 
channel ground.

+

-

+

Floating

Signal

Source

Instrumentation

Amplifier

V

m

Measured

Voltage

-

V

S

-

+

I/O Connector

ACH0GND

Bias

Current

Return

Paths

ACH0-

ACH0+

ACH0 Connections Shown

PGIA

1M

Ω

100pf

10nf

Bias
Resistor 
(see text)

PCI_E.book  Page 12  Thursday, June 25, 1998  12:55 PM