National Instruments 700 - Manual (Page 25)

Signal Connections Chapter 3
DAQCard-700 User Manual 3-6 © National Instruments Corporation
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V in
V
V
-
+
Instrumentation
Amplifier
V = [ - ] GAIN
in
+
-
in
+
-
+
V in
m
Measured
Voltage Vm
*
Figure 3-2. DAQCard-700 Instrumentation Amplifier
The DAQCard-700 instrumentation amplifier applies common-mode voltage rejection and
presents a high-input impedance to the analog input signals connected to the DAQCard-700.
Signals are routed to the positive and negative inputs of the instrumentation amplifier through
input multiplexers on the DAQCard-700. The instrumentation amplifier converts two input
signals to a signal that is the difference between the two input signals multiplied by the gain
setting of the amplifier. The amplifier output voltage is referenced to the DAQCard-700 ground.
The DAQCard-700 ADC measures this output voltage when it performs A/D conversions.
Single-Ended Mode
Although the instrumentation amplifier on the DAQCard-700 is actually bypassed in single-
ended mode, the effect of this mode is the same as if signals connected to ACH<15..0> were
routed to the positive terminal of the instrumentation amplifier and the negative terminal of the
instrumentation amplifier were connected to the analog ground reference of the DAQCard-700.
Thus, the voltage measured by the DAQCard-700 in single-ended mode is the difference between
an input signal and the DAQCard-700 analog ground reference.
Differential Mode
In differential mode, signals connected to ACH<7..0> are routed to the positive input of the
instrumentation amplifier, and signals connected to ACH<15..8> are routed to the negative input
of the instrumentation amplifier. Thus, the voltage measured by the DAQCard-700 in
differential mode is the difference between two of the input signals.