National Instruments DAQ PCI E Series User Manual page 62

Chapter 4 Signal Connections
signal to AIGND as well as to the negative input of the PGIA, without
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 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 4-6. 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 inputs of the PGIA 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.
© National Instruments Corporation 4-19 PCI E Series User Manual