my problem about differential and RSE mode when testing PCB?

[gpibssx]

i am programming on Testing a PCB Of mainboard for PC. On the PCB, There exits a few test point. Most of i do is to test the Voltage on these test points.
Question 1): does these signal of the test point belongs to Floating Signal source or Grounder source?
Question 2): some of the voltage on the test points are about 3mv or so. If i use RSE , i can see a waveform on the Testpoint(A) and get the read voltage.But if i link Testpoint(A) to CH0 ,and i link CH8 to AGND of PCB BOARD. I cannot see the waveform on CH0 AND CH8. What i want to ask is that if i get Voltage using AIREAD.VI, which testpoint does the Voltage belongs to??CH0 OR CH8??OR just their subtract value?

[Russell]

Hello,

I think I understand your question. It's confusing that for Differential mode you actually use the CH8 pin for the negative reference of CH0. This measurement actually has nothing to do with channel 8 of the DAQ board. It's just using the pin that would be assigned to CH8 in RSE or NRSE mode.

Assuming you are in Differential mode, just reading channel 0 of the DAQ board will give you the voltage difference between the CH0 pin and CH8 pin. So you do not have to do any subtraction or anything.

Let me know if you have any question.

Russell
Applications Engineer
National Instruments
http://www.ni.com/support