20 |
Figure 3.2 differential input mode for direct connection
However, for a floating signal source with large internal resistance, the above connection will lead
to the imbalance of differential signal, and the common-mode noise will be coupled to the signal
of AI+ while not to AI-, so that the common-mode noise will appear in the measured results.
Therefore, for such a signal source, you can use a bias resistor approximately 100 times the
internal resistance of the signal source to connect to AI- and AI-GND ports, as shown in Figure
3.3. This can make the differential signal close to equilibrium, hence couple the same amount of
noise at both ends of the signal to enable better common-mode noise suppression.
Figure 3.3 differential input mode with single bias resistor
For a floating signal source with a large internal resistance, you can use the differential input mode
with two bias resistors, as shown in Figure 3.3. The fully balanced bias resistor connection in this
way can provide a slightly better noise suppression, but it can reduce the load on the signal source
and result in gain errors. For example, suppose the internal resistance of the signal source is
2kΩ,
and two equilibrium resistors are
100kΩ
each, then the signal source load is 200
kΩ
, which results
in a 1% gain error.