National Instruments 6023E, 6024E, 6025E

Chapter4 SignalConnections

6023E/6024E/6025EUser Manual 4-16 ni.com

Youmust reference the source to AIGND. The easiest wayi sto conn ectthe

positiveside of the signal to the positive input of the PGIA and connect the

negativeside of the signal to AIGND as well as to the negative input of the

PGIA, without any resistors at all. This connection works well for

DC-coupled sources with lowsource impedan ce(less than 100 Ω).

However,for larger source impedances, this connection leaves the DIFF

signalpath significantly out of balance. Noise that couples electrostatically

onto the positiveline does not couple onto t he negativeline because it is

connected to ground. Hence, this noise appears as a DIFF-mode signal

insteadof 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 isa bout 100 times the equivalentsource

impedance.The resistor puts the signal path nearly in balance, sothat about

the same amount of noise couples onto both connections, yielding better

rejection of electrostatically coupled noise. Also, this configuration does

notlo ad downthe source (oth er than the very high input impedance of the

PGIA).

Youcan fully balance the signal path by connecting another resistor of the

samevalue between the positive input and AIGND, as shown in Figure 4-6.

This fully balanced configurationoffers slightly better noise rejection but

has the disadvantageof loading the source down with the series

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

impedance is 2 kΩand each of the two resistors is 100 kΩ,the resistors

loaddown the source with 200 kΩand produce a –1% gain error.

Bothinputs of the PGIA require a DC path to ground in o rder for the PGIA

towork. If the source is AC coupled (capacitively coupled),t he PGIA needs

aresistor betweenthe p ositiveinput and AIGND. If the source has low

impedance, choose a resistor that is large enough not to significantly load

thesource but small enough no tto producesignificant input offset voltage

asa result of input bias current (typically 100 kΩto1MΩ).In thiscase,

you can tiet he negativeinput directly to AIGND. If the source has high

outputimpedance, balance the signal path as previously described using the

samevalue resistor on both the positive and negative inputs; be aware that

therei ssome gain error from loading down the source.