Chapter 4 Signal Connections
National Instruments Corporation
4-17
DAQCard E Series User Manual
without any resistors at all. This connection works well for
DC-coupled sources with low source impedance (less than 100
Ω
).
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, 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 DAQCard 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.
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
Ω
to 1 M
Ω
). In this case, you can tie the negative input
directly to AIGND. 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.
