National Instruments VXI-MIO Series User Manual page 63

Chapter 4 Signal Connections
any resistors at all. This connection works well for DC-coupled sources
with low source impedance (less than 100 ).
However, for larger source impedances, this connection leaves the
differential signal path significantly out of 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
differential-mode signal instead of a common-mode signal, and so the
PGIA does not reject it. In this case, instead of directly connecting the
negative line to AIGND, connect it to AIGND 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. Also, this configuration does not load
down the source (other than the very high input impedance of the
PGIA).
You can fully balance the signal path by connecting another resistor of
the same value between the positive input and AIGND, as shown in
Figure 4-5. 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 and each of the two resistors is 100 k ,
the resistors load down the source with 200 k and produce a -1% gain
error.
Both PGIA inputs require a DC path to ground in order for the PGIA to
work. If the source is AC-coupled (capacitively coupled), the PGIA
needs a resistor between the positive input and AIGND. 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 to 1 M ). In this case, you can tie the negative input
directly to AIGND. If the source has high output impedance, you should
balance the signal path as previously described using the same value
resistor on both the positive and negative inputs; you should be aware
that there is some gain error from loading down the source.
Refer to Application Note 025, Field Wiring and Noise Considerations
for Analog Signals, for more information.
National Instruments Corporation 4-17 VXI-MIO Series User Manual