National Instruments VXI-MIO Series

Chapter 4 Signal Connections

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National Instruments Corporation 4-17 VXI-MIO Series User Manual

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, and 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 series combination (sum) of the two resistors. If, for example,

the source impedance is 2 kΩ and each of the two resistors is 100 kΩ,

the resistors load down the source with 200 kΩ and produce a -1% gain

error.

Both PGIA inputs 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 kΩ to 1 MΩ). 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.

Refer to Application Note 025, Field Wiring and Noise Considerations

for Analog Signals, for more information.