National Instruments AT-MIO-64F-5, User Manual

Page: 45 / 266

Chapter 2 Configuration and Installation
© National Instruments Corporation 2-23 AT-MIO-64F-5 User Manual
Differential Connections for Nonreferenced or Floating Signal Sources
Figure 2-8 shows how to connect a floating signal source to an AT-MIO-64F-5 board configured
in the DIFF input mode. The AT-MIO-64F-5 analog input circuitry must be configured for DIFF
input to make these types of connections. Configuration instructions are included in Chapter 4,
Register Map and Descriptions.
Floating
Signal
Source
+
-
+
Input Multiplexers
AI SENSE
PGIA
V
m
Measured
Voltage
AI GND
-
V
s
-
+
I/O Connector
AT-MIO-64F-5 Board in the DIFF Input Configuration
Bias
Resistors
Bias
Current
Return
Paths
ACH<8..15>
ACH<0..7>
Gain
ACH<16..39>
ACH<40..63>
Figure 2-8. Differential Input Connections for Nonreferenced Signals
Figure 2-8 shows two bias resistors connected in parallel with the signal leads of a floating signal
source. If the source is truly floating, it 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 AI GND. The best way is simply to connect the positive side of the signal to the
positive (+) input of the PGIA and connect the negative side of the signal to AI GND as well as
to the negative (-) input of the PGIA. This works well for DC-coupled sources with low source
impedance (less than 100
W
). However, for larger source impedances, this connection leaves the
differential signal path significantly out of balance. Noise, which couples electrostatically onto
the positive (+) line, does not couple onto the negative (-) line because it is connected to ground.