National Instruments SCB-68 - Manual (Page 35)

Chapter 3 Connecting Signals
© National Instruments Corporation 3-7 SCB-68 Shielded Connector Block User Manual
Differential Connections for Nonreferenced or
Floating Signal Sources
Figure 3-3 shows how to connect a floating signal source to a channel on
the DAQ device configured in DIFF input mode.
Figure 3-3. Differential Input Connections for Nonreferenced Signals
Using Bias Resistors
Figure 3-3 shows a bias resistor connected between ACH– or ACH<i+8>,
and AIGND. 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.
Measurement Device Configured in DIFF Input Mode
PGIA
–
+
–
+
–
+
Floating
Signal
Source
V
s
Bias
Resistor
(see text)
I/O Connector
AISENSE*
AIGND
Instrumentation
Amplifier
Measured
Voltage
V
m
ACH+ or ACH< i>
ACH– or ACH< i+8>
*AISENSE is not present on all devices.