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Chapter 4
Analog Input
to the negative input of the instrumentation amplifier, without using resistors. This
connection works well for DC-coupled sources with low source impedance (less than
100 Ω).
•
High Source Impedance—For larger source impedances, this connection leaves the DIFF
signal path significantly off balance. Noise that couples electrostatically onto the positive
line does not couple onto the negative line because it is connected to ground. Hence, this
noise appears as a DIFF-mode signal instead of a common-mode signal, and the
instrumentation amplifier does not reject it. In this case, instead of directly connecting the
negative line to AI GND, connect the negative line to AI GND through a resistor that is
about 100 times the equivalent source impedance. The resistor puts the signal path nearly
in balance, so that about the same amount of noise couples onto both connections, yielding
better rejection of electrostatically coupled noise. This configuration does not load down
the source (other than the very high input impedance of the instrumentation amplifier).
You can fully balance the signal path by connecting another resistor of the same value
between the positive input and AI GND. This fully balanced configuration offers slightly
better noise rejection but has the disadvantage of loading the source down with the series
combination (sum) of the two resistors. If, for example, the source impedance is 2 kΩ and
each of the two resistors is 100 kΩ, the resistors load down the source with 200 kΩ and
produce a -1% gain error.
AC-Coupled
Both inputs of the instrumentation amplifier require a DC path to ground in order for the
instrumentation amplifier to work. If the source is AC-coupled (capacitively coupled), the
instrumentation amplifier needs a resistor between the positive input and AI GND. If the source
has low-impedance, choose a resistor that is large enough not to significantly load the source,
but small enough not to produce significant input offset voltage as a result of input bias current
(typically 100 kΩ to 1 MΩ). In this case, connect the negative input directly to AI GND. If the
source has high output impedance, balance the signal path as previously described using the
same value resistor on both the positive and negative inputs; be aware that there is some gain
error from loading down the source.
Unused Channels
NI recommends connecting unused channel inputs AI+ and AI- to AIGND. This prevents the
inputs from floating outside of the Vcm rating, which can generate unecessary heat and
measurement drift. Additionally, open inputs can detect and amplify unwanted environmental
noise.
Field Wiring Considerations
Environmental noise can seriously affect the measurement accuracy of the Simultaneous
MIO X Series device if you do not take proper care when running signal wires between signal
sources and the device. The following recommendations apply mainly to AI signal routing,
although they also apply to signal routing in general.
4-44 | ni.com
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