actual analog output rate of NI-6351

[HoumollerJ]

Hi, 

I am trying to create an arbitrary waveform generator out of the two DAC channels on a NI-6351.

It all works fine - except for the fact that I am not sure what the actual analog output rate is. 

This is very crucial to me, as I calculate my waveform based on a discrete fast Fourier transform - so to get the correct frequencies, it is important to know the actual output rate.

The card has a AO update rate 2.86MS/s,. The timing property note - actual sample clock rate also outputs these 2.86MHz. 

When I then try to use this same frequency to generate two analog outputs, it still tells me it is using 2,86MHz, even though the specifications only says it can output 2MS/s when using two channels.

So my question is, how do I determine the actual output rate of the analog outputs? 

And what is the actual maximum output rate of the DACs?

Regards 

Jørgen

[JarmoLE]

Hello HoumollerJ,

It would be good to see how you are generating the waveforms. What program are you using to control the USB-6351?

This is because I think you are still using just one channel. Have you measured the output frequency with an external device?

The outputs are probably multiplexed so that the 2.0MS/s is acquired by sharing the clock between two outputs.

Best regards,

Jarmo Levo

Applications engineer

National Instruments

[HoumollerJ]

Hello,

I am using labview DAQmx api's in labview. 

The two outputs share the same clock. 

For generation of the two waveforms (one is just the negative of the other) I just generate 4095 points (FIFO length) and outputs these points from the onboard memmory at the rate of (apparently) 2.86MS/s.

[JarmoLE]

Hey again,

Would it be possible to share the code you're using?

Are you getting any errors? Are the error wires wired in the code?

If the limit is at 2MHz and you try configure in 2,86MHz the drivers should return an error.

- Jarmo

[HoumollerJ]

The actual code is hidden inside a couple of classes, but the overall structure is illustrated here. 

If I set the rate to 3MS/s the drivers warn me about a possible problem with the DAC conversion rate. 

Further more, I have tried to generate a 300KHz signal with both the 2.86MS/s and a 2MS/s rate, and my ociloscope reads 300KHz - so I think the rate is actually true?

[HoumollerJ · Accepted solution]

I just had a scope attached, while generating a sine function at 300kHz, with the rate of 2.86MS/s. 

The scope read 300kHz which must mean that the DAC actually runs at 2.86MS/s, else the resulting waveform would have been at nearly 200kHz. 

I will mark this as resolved, eventhough I still not folly understand why it is able to output at this frequency.