National Instruments NI 783xR Benutzerhandbuch

 

Figure 2-6.  Differential Input Connections for Nonreferenced Signals

Figure 2-6 shows two bias resistors connected in parallel with the signal 
leads of a floating signal source. If you do not use the resistors and the 
source is truly floating, the source might not remain within the 
common-mode signal range of the instrumentation amplifier, causing 
erroneous readings. You must reference the source to AIGND by 
connecting the positive side of the signal to the positive input of the 
instrumentation amplifier and connecting the negative side of the signal to 
AIGND and to the negative input of the instrumentation amplifier without 
resistors. This connection works well for DC-coupled sources with low 
source impedance, less than 100 

Ω.

For larger source impedances, this connection leaves the differential signal 
path significantly out of balance. Noise that couples electrostatically onto 
the positive line does not couple onto the negative line because it is 
connected to ground. Hence, this noise appears as a differential-mode 
signal instead of a common-mode signal, and the instrumentation amplifier 
does not reject it. In this case, instead of directly connecting the negative 
line to AIGND, connect it to AIGND through a resistor that is about 
100 times the equivalent source impedance. The resistor puts the signal 
path nearly in balance. About the same amount of noise couples onto both 
connections, which yields better rejection of electrostatically coupled 
noise. Also, this input mode does not load down the source, other than the 
very high-input impedance of the instrumentation amplifier.

Bias
Resistors
(see text)

DIFF Input Mode Selected

–

+

–

+

AISENSE

AIGND

V

m

Measured

Voltage

Instrumentation

Amplifier

AI+ 

AI–

–

+

Floating

Signal

Source

Bias

Current

Return

Paths

V

s

I/O Connector