Appendix C
142
If it is available, use Amp Low as a remote ground sense when running in pseudo-differential
mode.
Differential Inputs
Differential inputs offer the maximum noise rejection at the expense of half your total channel
count. For the best results, shielded twisted pairs are a must. The shield must connect at one
end so that ground currents do not travel over the shield. In low-level voltage applications,
differential inputs reduce problems not only due to electrostatic and magnetic noise, but due
to cross-talk and thermal errors.
One problem to consider with differential inputs is the bias current error. The differential
impedance is usually hundreds of megaohms. With a very small bias current multiplied by
this high input impedance, the voltage produced is out of the common-mode input range of
the instrumentation amplifier.
You must provide an external resistor to return this bias current to the analog common of the
data acquisition board. This resistor is typically in the order of 1 kΩ to 100 kΩ from the input
low side to analog common. Alternatively, you can return the external common through a 10 Ω
to 100 kΩ resistor to analog common (it cannot be 0 Ω due to ground currents).
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