DAQ timing specifications explained (PXI 6281)

[alysko]

Dear NI Tech Support,

On the example of PXI 6281 specifications I tried to characterise the A-D converter (see below). Could you please correct where I am wrong and add what is missing in the effects and their explanations.

A. Timing accuracy ......................... 50 ppm of sample rate

a)      Setting the sampling rate to 10 kS/s will result in 10kS/s*50ppm= 0.5 Hz uncertainty in the true frequency of a sinusoid generated or acquired.

For applications relying on FFT, this means that there is no point trying to improve the frequency resolution by observing for longer than 1/0.05=20s.

b)      Important for phase-sensitive applications utilising non-synchronised oscillators: phase drift of 50 ppm may eventually change the phase of a signal by as much as 180 degrees (loss of coherence). Sampling at the rate of 10 kS/s will result in 1 seconds of coherence time.

c)   Jitter of 1/(10kS/s)*50e-6=5ns  - see http://en.wikipedia.org/wiki/Analog-to-digital_converter - not good for digitizing 44.1 kHz or even 1 kHz at 18bits with PXI 6281. Here I am unsure about the applicability of the term JITTER. Please comment.

B. Timing resolution ....................... 50 ns

In phase-sensitive applications, this will create a small error in measured amplitude due to the phase difference. Assuming signal frequency of 1 kHz, for co-phase channel (I), the error is limited to 1-cos(2*p*1000*50e-9)=5e-8 (equiv 24 bits), whilst for quadrature channel (Q), the error is limited to sin(2*p*1000*50e-9)=0.0003. The latter corresponds to resolution of 11.7 bits. Both are the worst case scenarious and the actual error is due to the distribution between these values?
Jitter of 1/(10kS/s)*50e-6=5ns  - see http://en.wikipedia.org/wiki/Analog-to-digital_converter - not good for digitizing 44.1 kHz or even 1 kHz at 18bits with PXI 6281 (e.g. 1.2ns jitter required for digitizing 1kHz signal with 18 bits). Here I am unsure about the applicability of the term JITTER. Please comment.

C. How are these parameters (see above) related to the reference oscillator (which also has an external input, giving the opportunity to improve the frequency stability)?

Thanking you in advance.

Sincerely
Albert

[ChrisR.]

Hi, Albert. I'll try to clear up your confusion.

A. Timing accuracy = 50ppm of sample rate. This means the sample rate could be off by up to 50ppm.

a) There could be a 50ppm error in the true frequency of a generated sinusoid, and measured frequencies could be off by 50ppm. For example, a 1Hz sine wave could measure anywhere between 0.99995Hz and 1.00005Hz. The actual sample rate you use is irrelevant, and you will always improve frequency resolution by observing for a longer time.

b) I don't understand this point. There is only one oscillator on the PXI-6281. If you have more than one, they can both be phase-locked to the PXI backplane clock. (This might even be the default behavior; I'm not sure.)

c) 50ppm is the accuracy spec, not the jitter spec. Even thought it isn't specified, I'm sure jitter is MUCH better than 5ns!

B. Timing resolution = 50 nsec. This means your sample interval can be adjusted in 50nsec increments. You're talking about measuring an I and Q channel. If you're using just one board to do this, the interchannel delay has to be at least 2usec to allow for settling, but that phase error can be corrected very accurately using software. If you're using 2 boards, I believe the synchronization can be made much better than 50nsec, but I've never done it myself, so I'm not sure exactly how good it is.

C. Reference oscillator. All timing on the 6281 is derived from a single oscillator, which can be locked to the backplane PXI_CLK10 if desired. This in turn can be provided by a card in slot 2 (e.g. a PXI-6608) if better accuracy is required.

I hope this helps.
Chris

[Speedy_Badger]

Duplicate post. Please refer to the following discussion forum.

Regards,