Chapter 4
Analog Input
©
National Instruments Corporation
4-11
•
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 k
Ω
and each of the two resistors is 100 k
Ω
, the resistors load down
the source with 200 k
Ω
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 k
Ω
to 1 M
Ω
). 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.
Field Wiring Considerations
Environmental noise can seriously affect the measurement accuracy of the
S 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.
