Chapter 4
Connecting Signals
©
National Instruments Corporation
4-13
You must reference the source to AIGND. The easiest way is to connect
the positive side of the signal to the positive input of the PGIA and connect
the negative side of the signal to AIGND as well as to the negative input
of the PGIA, without any resistors. 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 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
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 4-5.
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
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, 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.
