National Instruments AT E Series

Chapter4 ConnectingSignals

©NationalInstruments Corporation 4-23 ATE Series User Manual

asto the negative input of the PGIA, without any resistors at all. This

connectionworks well for DC-coupled sources with low source impedance

(less than 100 Ω).

However,for larger source impedances, this connection leaves the

differentialsignal path significantly out of balance. Noise that couples

electrostatically onto the positiveline does not couple onto the negative

line because it is connected to ground. Hence, this noise appears as a

differential-modesignal instead of a com mon-mode signal, and so the

PGIA does not rejectit. In this case, instead of directly connecting the

negativeline to AIGND, connect it to AIGND through a resistor th at is

about 100 times the equivalentsource impedance. The resistor puts the

signalpath nearly in balance, so that about the same amount of noise

couplesonto both connections, yielding better rejection of electrostatically

coupled noise. Also, this configuration does not load downthe source

(other than the veryhigh inp ut 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 4-7.

This fully balanced configurationoffers slightly better noise rejection

buthas the disadvantage of 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 between the positiveinput 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

output impedance, you should balance the signal path as previously

described using the same valueresistor on both the positive and negative

inputs;you should be aware that there is some gain error from loading down

the source.