National Instruments NI 6120 用户手册

Figure 4-5.  Differential Input Connections on the NI 6120 for Nonreferenced Signals

Figures 4-4 and 4-5 show a bias resistor connected between ACH0– and the 
floating signal source ground. This resistor provides a return path for the 
±200 pA bias current. A value of 10 k

Ω to 100 kΩ is usually sufficient. 

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 saturates, causing erroneous readings. You must reference 
the source to the respective channel ground.

Common-mode rejection might be improved by using another bias resistor 
from the ACH0+ input to ACH0GND. This connection gives a slight 
measurement error due to the voltage divider formed with the output 
impedance of the floating source, but it also gives a more balanced input for 
better common-mode rejection.

Figures 4-2 and  4-3 show connections for signal sources that are already 
referenced to some ground point with respect to the NI 6115/6120. In 
theory, the PGIA can reject any voltage caused by ground-potential 
differences between the signal source and the device. In addition, with 
pseudodifferential input connections, the PGIA can reject common-mode 
noise pickup in the leads connecting the signal sources to the device.

+

–

V

m

100 pF*

1 M*

50 

Ω

0.1 

µF

High-Frequency
Common Mode Choke

ACH0+

ACH0–

ACH0GND

PGIA

Instrumentation

Amplifier

*10 k

Ω

40 pf 

for ranges > ±10 V

Floating

Signal

Source

AC Coupling

Measured
Voltage

V

s

+

–

Bias
Resistor 
(see text)

I/O Connector

Bias

Current

Return

Paths

+

–