Question related to analog input using pci 6221 and scc 68

[rsd111]

I am using PCI 6221 and scc 68 to read analog voltage between the drain and source terminals of a transistor. The drain is at 0 volts and source is connected to -5 volts via a 100k resistor. The transistor is used as source follower and hence output is measured at the source terminal. When I measure the voltage using oscilloscope, I clearly see the change in output as a result of change in gate voltage. But, when I try to acquire the same using labview, I see nothing. I tried to change the signal to RSE, NRSE, but, nothing works. Seems to be something related to impedance mismatch. Can you please advice me on this issue? Thanks in advance

[Filippo_Persia · Accepted solution]

Hi rsd111,

I understand you are measuring a circuit similar to this one ( http://en.wikipedia.org/wiki/Common_drain )

I get also you're using an external power source with GND connected to Drain and -5V connected to Source with a 100k resistor.

You should be able to measure the Voltage between D and S configuring a differential measurement and connecting Ai0+ to D and Ai0- to S, actually you can youse the Ain you prefer.

You should be able to do the same measurement also in NRSE configuration connecting Ai0 to Drain and AiSense to Source.

The Input impedence of the board is bigger than 10GOhm, it should behave like the oscilloscope in High Impedence DC coupling mode. 

6221 Specifications :  http://digital.ni.com/manuals.nsf/websearch/8117DF4C5A29C95C862573020061023B 

Have a good day. 

Message Edited by Jon_Snow on 10-09-2008 04:51 AM