Hi, I am using a NI PCI-6281 trying to acquire signals in accordance with a sample clock. I am trying to acquire signals on two channels within this DAQ card, however the amplitudes that the DAQ card is measuring do not match the amplitude as measured by an oscilloscope and I am not sure what the problem could be (I've gone through the documentation and based my set-up on what I have read there) I have included diagrams of my signals, my sampling clock, and my trigger as measured by an oscilloscope. The first image, "BothSignals1M.jpg" Illustrates the magnitude of both signal as measured by an osciloscope, using highZ input impedance (1mega-Ohm). One signal is in yellow and the other in blue (sorry about the overlap). It can be seen here that the yellow signal (hereafter called the Reference) peaks at a value of about 1.7V, and that the blue signal (hereafter called the Probe) peaks at a value of about 1.4V Just to make sure, I have done the same for both signals using 50 Ohm Input impedance into the oscilloscope, and the signals in this case are shown in "BothSignals50Ohm.jpg". In this case the Probe (blue) peaks at about 0.7V and the Pump (yellow) at about 0.85V. The sampling clock is a TTL signal, which is shown in Green in "Sampling ClockTTL Overlap with Signals.jpg". In this case, everything is measured with highZ impedance. It can be seen that the sampling clock is timed such that a sample is collected at the signal peaks. The trigger I am using is shown in Magenta in "Trigger with Signals and sampling clock.jpg" and is also a TTL pulse. It can be seen that the trigger is a little bit after the peaks. Ok, so, assuming this setup, we now switch to using the DAQ instead of the oscilloscope. I have connected the reference and pump into a pair of analog inputs in the daq card, ai0 and ai1. When using internal sampling (instead of a sampling clock) I can observe that the signals are present, and so can conclude that these channels have been connected correctly. Next I have connected the trigger TTL signal into the PFI3 input of the daq card (corresponding to pins 42 and 9) and I have connected the sampling clock TTL signal into the PFI2 input of the daq card (corresponding to pins 43 and 44). Now, when acquiring a signal, I should expect to see only the peak values. However, when I run a test, the daq card is not recording accurate values. The recorded values by the daq card are shown in "Recorded values on the NI DAQ.jpg". My reference beam (this time shown in blue) is read by the DAQ card to be peaking at a value of around 1V, and my probe beam (in red) is shown to be peaking at a value of 0.4V. These do not match the peak values as recorded by the oscilloscope, and I don't know what I'm doing wrong. I thought maybe the sampling clock is not recording at the peaks or maybe the trigger signal is affecting when the DAQ card takes a measurment but I don't really have a very good idea. Can someone suggest possible problems with what I have done or otherwise explain why the values my DAQ card records don't match the values that my oscilloscope is recording? Thank you so much!

With a pulse of 7 ps, you are probably right that you are not sampling fast enough.

Multifunction DAQ

DAQ Card sampling clock voltage disagreement

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I have been indeed, just not with very good resolution on the DAQ card. reads fine in the scope.

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Author

Yes I can, though this does greatly limit signal acquisition time since it reads only 10000 samples or nothing, whereas I only want to read at the peak.

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MAybe just a question of the scope setup/trigger and how many points you actually read 😉

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Henrik

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