Chapter 4 Signal ConnectionsNational Instruments Corporation4-17VXI-MIO Series User Manualany resistors at all. This connection works well for DC-coupled sourceswith low source impedance (less than 100 Ω).However, for larger source impedances, this connection leaves thedifferential signal path significantly out of balance. Noise that coupleselectrostatically onto the positive line does not couple onto the negativeline because it is connected to ground. Hence, this noise appears as adifferential-mode signal instead of a common-mode signal, and so thePGIA does not reject it. In this case, instead of directly connecting thenegative line to AIGND, connect it to AIGND through a resistor that isabout 100 times the equivalent source impedance. The resistor puts thesignal path nearly in balance, so that about the same amount of noisecouples onto both connections, yielding better rejection ofelectrostatically coupled noise. Also, this configuration does not loaddown the source (other than the very high input impedance of thePGIA).You can fully balance the signal path by connecting another resistor ofthe same value between the positive input and AIGND, as shown inFigure 4-5. This fully-balanced configuration offers slightly betternoise rejection but has the disadvantage of loading the source downwith 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% gainerror.Both PGIA inputs require a DC path to ground in order for the PGIA towork. If the source is AC-coupled (capacitively coupled), the PGIAneeds a resistor between the positive input and AIGND. If the sourcehas low impedance, choose a resistor that is large enough not tosignificantly load the source but small enough not to producesignificant input offset voltage as a result of input bias current(typically 100 kΩ to 1 MΩ). In this case, you can tie the negative inputdirectly to AIGND. If the source has high output impedance, you shouldbalance the signal path as previously described using the same valueresistor on both the positive and negative inputs; you should be awarethat there is some gain error from loading down the source.Refer to Application Note 025, Field Wiring and Noise Considerationsfor Analog Signals, for more information.
