Chapter 3
Connecting Signals
3-12
ni.com
A resistor from each input to ground, as shown in Figure 3-5, provides bias
current return paths for an AC-coupled input signal. This solution, although
necessary for AC-coupled signals, lowers the input impedance of the
AI channel. In addition, the input offset current of the instrumentation
amplifier contributes a DC offset voltage at the input. The amplifier has a
maximum input offset current of ±0.75 nA and a typical offset current drift
of ±1.5 pA/
°
C.
Multiplied by the 100 k
Ω
resistor, this current contributes a maximum
offset voltage of 75
µ
V and a typical offset voltage drift of 150 nV/
°
C at the
input. Thus, the offset is unlikely to be more than one LSB, so it can usually
be ignored.
If the input signal is DC coupled, only the resistor connecting the negative
signal input to ground is needed. This connection does not lower the input
impedance of the AI channel.
Common-Mode Signal Rejection Considerations
Figures 3-4 and 3-5 show connections for signal sources that are already
referenced to some ground point with respect to the DAQCard-700.
In these cases, the instrumentation amplifier can reject any voltage
due to ground potential differences between the signal source and the
DAQCard-700. In addition, with DIFF input connections, the
instrumentation amplifier can reject common-mode noise pickup in
the leads connecting the signal sources to the DAQCard-700.
The common-mode input range of the DAQCard-700 instrumentation
amplifier is defined as the magnitude of the greatest common-mode signal
that can be rejected. The DAQCard-700 can reject common-mode input
signals so long as V
+
in
and V
–
in
are both in the range ±9.5 V.
The common-mode input range for the DAQCard-700 depends on the size
of the differential input signal (V
diff
= V
+
in
– V
–
in
). The formula for the
permissible common-mode input range is as follows:
V
cm-max
= ± (9.5 V – V
diff
/2)
Thus, for a differential voltage as large as 10 V, the largest common-mode
voltage that can be rejected is ±4.5 V. However, if the differential signal is
2.5 V, ±8.25 V common-mode voltage can be rejected.