Chapter 5 Connecting AI Signals on the USB-6210/6211 Devices

inputs; be aware that there is some gain error from loading down the source, as shown in Figure 5-4.

 

AC Coupling

AC Coupled

AI+

 

Floating

+

Signal

Vs

Source

–

 

AI–

 

AI SENSE

 

AI GND

Figure 5-4.Differential Connections for AC Coupled Floating Sources

with Balanced Bias Resistors

Using Non-Referenced Single-Ended (NRSE) Connections for Floating Signal Sources

It is important to connect the negative lead of a floating signals source to AI GND (either directly or through a resistor). Otherwise the source may float out of the valid input range of the NI-PGIA and the DAQ device returns erroneous data.

Figure 5-5shows a floating source connected to the DAQ device in NRSE mode.

AI

Floating +

Signal V

Source –s

AI SENSE

R

AI GND

Figure 5-5.NRSE Connections for Floating Signal Sources

All of the bias resistor configurations discussed in the Using Differential Connections for Floating Signal Sources section apply to the NRSE bias resistors as well. Replace AI– with AI SENSE in Figures 5-1,5-2,5-3,and 5-4for configurations with zero to two bias resistors. The noise rejection of NRSE mode is better than RSE mode because the AI SENSE connection is made remotely near the source. However, the noise rejection of NRSE mode is worse than DIFF mode because the AI SENSE