Multiple waveforms acquisition

[lyn]

i want to acquire 2 analog signals using PCI-6025E,

signal 1 (ACH0): sinewave +/-10V
signal 2 (ACH1): series of spikes +/- 1V

and signal 2 need to be synchronised to signal 1 because we want to measure the phases of the spikes with respect to signal 1 hence i need to acquire them simultaneously. I have tried to use acquire waveforms and buffered acquision but both give me the following problem

i) ACH0 is acquired properly, but ACH1 have a sinewave superimposed on it. Is it a form of crosstalk from ACH0 and how can it be resolved?

ii) The signal with lower magnitude is always affected by the signal with the higher magnitude(In my case, signal 2 of 1V is affected by signal 1 of 10V). How can the 2 signals be acquired without affe
cting one another yet at the same time synchronised?

iii) Have tried using Buffered acquisition example as well as Acquire Waveforms.vi but both gave me the same problem mentioned above. If the problem cannot be solved, are there any other VIs that can meet my objective?

we had found out that, if i acquire the +/-10V sinewave only using ACH0, the same waveform can be seen in the other channel even when they are not connected. suspect this is the cause of our problem.

[chutla]

Be sure you short out all your unused channel inputs. The multiplexer Will show crosstalk if you fail to do this. You can set the software trigger of a selected channel to maintain synchronization. Preferably the square wave channel.
Also, check the obvious, like your DAQ configuration...is it differential, single ended, etc.

Hope this helps!

Chutla

[RIO-Todd]

It sounds like you are experiencing a phenomenon known as "cross-talk." Cross-talk is caused when working with sources that have a high input impedance and when sampling at a high rate. When sampling at a high rate, the high input impedance does not allow the instrumentation amplifier on the device to settle. The sinewave source is probably the culprit that has the high source impedance.

There are a variety of ways to eliminate "cross-talk." I have included links below to documentation that goes through ways to get rid of this problem. My suggestion would be to look at decreasing the input impedance of the sine-wave source with a unity-gain buffer (an input impedance over 1 Kohm is too high). I hope this helps.

How Do I Create a Buffer to Decrease the Sou
rce Impedance of My Analog Input Signal?
http://digital.ni.com/public.nsf/websearch/CF83426BC3AC514A86256C10005A4771?OpenDocument

Data Acquisition: Troubleshooting Unexpected Voltages or Cross-talk in Analog Input Channels
http://digital.ni.com/public.nsf/websearch/B9BCDFD960C06B9186256A37007490CD?OpenDocument

Todd D.
Applications Engineer
National Instruments