Chapter 4
Connecting the Signals
©
National Instruments Corporation
4-15
However, for larger source impedances, this connection leaves the
differential 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. Therefore, 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 about 100 times the
equivalent source impedance. The resistor puts the signal path nearly in
balance, so similar amounts of noise couple onto both connections, yielding
better rejection of electrostatically coupled noise. Also, this configuration
does not load down the source (other than the 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 balanced configuration offers slightly better noise rejection, but it
loads down the source 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.
Both PGIA inputs require a DC path to ground 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, connect 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. Some gain error is
produced by loading down the source.