Generating and measuring signals w/ daq

[urh8]

Hello!

Im trying to generate sine signals up to 5000Hz and measuring them to ~0,5mV. But i have problems. Im doing this with Simulate signals box and two DAQ Assists-one for generation, one for measuring(image1).

Problems:

-i can "normally" measure only signals bigger than 10mV and of lower frequencies(~500Hz)

-generated signals over ~1000Hz are obviously broken

Configuration:

-input(image2)

-output(image4)

-signal generation(image5)

-generated and measured sig at 5000Hz(image3)

I would be very thankful to anyone who could give me some advice on this matter.

Have a nice day

urh8

[LAVA 1.0 Content]

You should have a look at the examples shipped with LabVIEW:

Hardware IO\DAQmx\Synchronization\Multi-Function\Multi-Function-Synch AI-AO.vi.

Express VIs are not suitable for such tasks.

You try to acquire for 90 seconds of data in one shot, this will (standard) not work since you have a timeout of 10 seconds.

Ton

[urh8]

Thanks for your help.

I tried your sugesion and looked a bit through examples, but i still cant produce signals lower than 1V. it seems like there is a strong noise source on the output.When i watch my output in labview it looks perfect but on the osciloscope i can barely recognize the signal.

Any idea?

[Tim_S]

QUOTE (urh8 @ Jul 22 2008, 06:33 AM)

it seems like there is a strong noise source on the output.When i watch my output in labview it looks perfect but on the osciloscope i can barely recognize the signal.

I would recommend two things:

1) Disconnect everything you can and connect only the oscilloscope to the analog output.

2) Disconnect the oscilloscope from ground.

The second can be very important. There are some oscilloscopes that can create a ground loop through the ground pin of the power plug -- at least with US-style three-prong plugs. I assume that other styles of plugs can also create this issue.

[Neville D]

QUOTE (urh8 @ Jul 22 2008, 03:33 AM)

Thanks for your help.

When i watch my output in labview it looks perfect but on the osciloscope i can barely recognize the signal.

Any idea?

What do you mean? Are you connecting the AO channel to an AI channel and observing the AI channel on a plot? If you are just looking at the AO samples being sent to the card, they will of course always be perfect.

Its been a while since I did any DAQ, but change the RSE (referenced single-ended) to Differential, and see if that makes a difference. I seem to remember you can't measure negative voltages in single-ended mode (I may be wrong here..) Check grounding connections for differential carefully (see manual).

Read the pdf manual that comes with your card (don't know if they ship that any more for the newer cards). It talks about how connections are to be made to the card and what the differences are between RSE, NRSE, and Differential.

Grounding is extremely important, or else you will get all sorts of problems.

Ground AI chans not being used (as a start) to see that those are not causing any issues.

What sort of breakout board are you using? If in doubt, check that the ground pins on the breakout are not fried. (Turn the board over and eyeball the traces or check continuity between the screw terminal and the connector pin on the other side).

Ground the AI chan connected and see that it reads 0V in LV and with the scope.

Now connect a battery and see that it reads a reasonable constant noise-free static value. Check with scope. Make sure your scope is on a 'reasonable' V/div setting

Now output a constant voltage with your AO chan, connect to the AI chan and repeat above to check.

Then generate a low freq waveform and confirm above.

Finally generate your required high freq waveform and check.

Report results of the above for further help.

Finally some of the E-series cards have a self-test utility somewhere on the NI site. Download and run (if applicable to your card) to check if there is a card malfunction (I doubt it, in all my years of DAQ, I have seen only a single "bad" card).

You can use NI-MAX for debugging or your own code, but MAX is a good place to start.

Another thought occured to me: if you are using a "low-cost" card, they usually have a single DMA channel, in which case it can be either used for high speed AI or AO, but not both.

Try slowing down your sample rate to see if the problem disappears with that. What card are you using?

Neville.