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flowing between two grounds, which generates common-mode noise for the mea-
surement. This is shown in Figure 2.5.
Figure 2.5 Ground loop effect.
If differential (ungrounded) input configuration is used instead, the high input imped-
ance of the negative input terminal prevents ground loop current from flowing, and
therefore rejects the common-mode noise.
2.4.4.3
Measuring an Ungrounded (Floating) Signal Source
For an ungrounded (floating) signal source, both differential input configuration and
single-ended input configuration are suitable.
When using differential input configuration, however, care must be taken to ensure
the input common-mode voltage level remains in the allowable range of the measur-
ing device. Due to the lack of DC path to the ground, the input bias current of input
stage amplifier may move the common-mode voltage level of the ungrounded signal
source out of the allowable range of the measuring device. When this happens, the
measured result will be incorrect or saturated (positive full-scale or negative full-
scale). Resistors with equal resistance value connecting between each input terminal
and ground can be used to alleviate this issue as shown in Figure 2.6. These resis-
tors are called bias resistors.
Figure 2.6 Differential input configuration with bias resistors.
The resistance value should be large enough that it does not load the signal source
and keep it remain floating, but small enough to make input common-mode voltage
level stay in the allowable range. Typically, resistance value between 10 kΩ to 100 kΩ
work well with low-impedance sources such as thermocouples and signal condition-
ing module outputs. When using bias resistor, the measured voltage will be attenu-
ated by the voltage divider formed by source resistance of the signal source and bias
resistors. This is shown in the following equation.
If the source impedance of the signal source is low, only one resistor connecting
between the negative input terminal and the ground is required to prevent input com-
mon-mode voltage level issue. However, this will lead to an unbalanced system if the
source impedance is relatively high. A balanced system is desirable from a noise
immunity point of view.
15
PCI-1716_Series User Manual
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