National Instruments Data Acquisition Device E Series User Manual
Page 58
Chapter 2
Analog Input
© National Instruments Corporation
2-25
The previous figure shows two bias resistors connected in parallel with the
signal leads of a floating signal source. If you do not use the resistors and
the source is truly floating, the source is not likely to remain within the
common-mode signal range of the PGIA. The PGIA then saturates, causing
erroneous readings.
You must reference the source to AI GND. The easiest way to make this
reference 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 AI GND as well as
to the negative input of the PGIA, without using 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 DIFF
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 DIFF-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 AI GND, connect
the negative line to AI GND 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.
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 AI GND, as shown in this figure.
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 kΩ, 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 AI GND. 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 MΩ). In this
case, connect the negative input directly to AI GND. 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.