Difference between Analog output of DSA cards and Normal DAQ card

[tintin_99]

Hi 

I would like to generate a sine wave at any frequency with any sampling rate . For example generating 10 kHz using the following sampling rates  101356 S/s or 101455 Hz or 102345 Hz

 I see using a DSA card I can generate this sine wave and the sine wave looks very clean . But when I use a normal DAQ card like 6368 the signal looks choppy and distorted . Could you please let me know what is the difference between 2 cards and why the DSA card can lock to that sampling rate?

Thanks

[Al.C]

Hi tintin_99,

What exactly do you mean by 'choppy and distorted', and which DSA card are you using?

I'm sure that you've considered this, but it's worth pointing out that if you were to compare, for example, the 6368 with the 4461 (DSA PXI card), the AO resolution of the DAQ card is 16bit vs. 24bit for the DSA, which contributes to much finer graduations.

[Nava]

I know this is a really old topic, but I thought a bit more clarity may be useful.

With the DAC on the 6368 you would need to increase the update rate considerably in order to get a clean looking sine wave. The update rate of ~100k would only get you 10 points per period which when digitized would look choppy. If you were to update at least 20 or more points per period there would be marked improvement in the way the signal looks. The 6368 is capable of 3.3MS/s update rate so it could be used at frequencies higher than 10k with smooth looking sine wave signals. 

Now to the second part of your question. Why does a DSA card at this update rate look smooth and clean? It is because the DAC is updating at a rate much faster than the one you put in. They are basically a Sigma-Delta design in reverse. There are many details online for more detailed looks into this design.  

Looking at the list below, at ~100k update rate the DAC has a 128 oversampling. It is also higher resolution as what was mentioned in a previous post.

AO characteristics of the 4461 board.

DAC modulator oversample rate

1.0 kS/s ≤ fs ≤ 1.6 kS/s ................8,192 fs

1.6 kS/s < fs ≤ 3.2 kS/s................4,096 fs

3.2 kS/s < fs ≤ 6.4 kS/s................2,048 fs

6.4 kS/s < fs ≤ 12.8 kS/s..............1,024 fs

12.8 kS/s < fs ≤ 25.6 kS/s ............512 fs

25.6 kS/s < fs ≤ 51.2 kS/s ............256 fs

51.2 kS/s < fs ≤ 102.4 kS/s..........128 fs

102.4 kS/s < fs ≤ 204.8 kS/s........64 fs