PXIE-4302 PXI Analog Input Module

A floating signal source is not connected to the building ground system, but has an isolated ground-reference point through the ground of the device. Some examples of floating signal sources are outputs of transformers, thermocouples, battery-powered devices, optical isolators, and isolation amplifiers. An instrument or device that has an isolated output is a floating signal source.
It is important to connect the negative lead of a floating source to AIGND (either directly or through a bias resistor). Otherwise, the source may float out of the maximum working voltage range and return erroneous data. The easiest way to reference the source to AIGND is to connect the positive side of the signal to AI+ and the negative side of the signal to both AIGND and AIwithout using resistors, as shown in Figure 2-1. This connection works well for DC-coupled sources with source impedances of less than 100 Ω

For source impedances ≥100 Ω, this connection leaves the differential signal path significantly off balance. Noise that couples onto the positive line does not couple onto the negative line because it is connected to ground. This noise appears as a differential mode signal instead of a common-mode signal, and therefore appears in your data. In this case, instead of directly connecting the negative line to AIGND, connect the negative line to AIGND through a resistor that is about 100 times the equivalent source impedance, as shown in Figure 2-2. 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 coupled noise. Since the bias resistor is between the negative line of the floating source and AIGND, this configuration does not load down the floating source output.

PXIE-4302