National Instruments NI 6120 - Manual (Page 50)

Chapter 4 Connecting Signals
NI 6115/6120 User Manual 4-12 ni.com
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.
Common-Mode Signal Rejection Considerations
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
+
–