Trouble with myDAQ

[Majo]

Hi guys,

 

why when I generate triangle output signal through the myDAQ I have received something else.

I am attaching my block scheme and oscilloscope picture.

 

Thank you very much for your help 

[todd]

What happens if you change the number of samples from 50k to 100k?

[Majo]

What happens if you change the number of samples from 50k to 100k?

Absolutely nothing. It is the same. 

[gleichman]

It looks as expected for a 50K frequency waveform sampled at 200k samples/s. Four samples per triangle wave (which is what I observe in your graph - math still works!) will not give you a nice graph. You either need to sample MUCH faster or reduce your output frequency. http://en.wikipedia.org/wiki/Aliasing

Edited October 13, 2013 by gleichman

[Majo]

It looks as expected for a 50K frequency waveform sampled at 200k samples/s. Four samples per triangle wave (which is what I observe in your graph - math still works!) will not give you a nice graph. You either need to sample MUCH faster or reduce your output frequency. http://en.wikipedia.org/wiki/Aliasing

 

Well, I would like to sample much faster, but mydaq device is able sample only 200k samples/s. And I can´t reduce the frequency because this frequency (50kHz) is very important because I use this frequency on bio-electrical impedance analysis.  

[todd]

Then you need a reconstruction filter in hardware.

http://en.wikipedia.org/wiki/Reconstruction_filter

[gleichman]

It looks like Majo has two problems. The myDAQ is too slow to properly generate or read a 50KHz triangle wave. The analog out channels of the myDAQ have a max update rate of 200KHz, so you will need the additional hardware as todd suggest and to verify the waveform a faster DAQ device is needed like a real scope.

[Majo]

It looks like Majo has two problems. The myDAQ is too slow to properly generate or read a 50KHz triangle wave. The analog out channels of the myDAQ have a max update rate of 200KHz, so you will need the additional hardware as todd suggest and to verify the waveform a faster DAQ device is needed like a real scope.

OK, first of all I will try to measure output signal from myDAQ on real scope and I will see what happen. The worst thing is that I was hopping that I can use this device on generate and measure 100kHz and 150kHz. Is it a way how to increase the sample frequency or can I use frequency multiplier? 

[todd]

Please read about the Nyquist Theorem.

http://en.wikipedia.org/wiki/Nyquist_frequency

https://www.google.com/search?q=nyquist+theorem&rlz=1C1LENP_enUS552US552&espv=210&es_sm=93&tbm=isch&tbo=u&source=univ&sa=X&ei=Th1cUo6NCOOJjAL934C4BA&ved=0CFAQsAQ&biw=1920&bih=995

 

If the MyDAQ can only update at 200kHz, it can not create a 150kHz signal.

[hooovahh]

The myDAQ states that the analog output rate is 200K samples per second.  This means the output can be change up to 200,000 times in one second.  This seems like alot but generally to create a sine wave you must output 10 times in one cycle to get a clear picture of what the signal looks like (back to the Nyquist).  

 

So the fastest sine wave you should ever generate on the myDAQ is 20KHz.  You can output faster but you will have issues like what you are seeing.  Alternatives is to purchase hardware that supports outputs that are faster.  This USB X Series 6351 can output at 2.8MHz, so this can generate a sine wave at 280KHz without any problem.  I've never used this particular hardware, and I don't know your application so I can't say if this meets your needs, or has too much functionality.

[Mark Smith]

If all you need is a sine wave (or other standard functions), a fairly inexpensive alternative is to use a function generator instead of a analog out device capable of arbitrary wave forms. Maybe something like this

 

http://www.amazon.com/Velleman-PCGU1000-2MHz-FUNCTION-GENERATOR/dp/B0011E5RNO/ref=sr_1_1?s=hi&ie=UTF8&qid=1381783997&sr=1-1&keywords=pcgu1000

 

$165 on Amazon