accuracy of t0 from pxi4472

[graduate student 44]

the pxi4472 spec indicates a 25ppm sampling frequency accuracy. i wonder how to translate this to the accuracy of the t0's of waveforms returned by each call to 'AI Read.vi'? or what's the accuracy of the pxi4472 controller clock that these t0's are dependent on?

[Shea C]

Student,

Using DAQ functions to program the 4472 the card will grab the system clock the first time a sample is taken and each additional stamp will be computed using dt. So each time the AI Read.vi is called it grabs the system time once. Therefore the accuracy of the to's is dependent on many factors including how often you read from the board and the number of samples that you read each call to AI Read.

Shea C
Applications
NI

[graduate student 44]

thanks Shea. for system clock, do you mean the system clock on the host computer or the system clock on the pxi-4472 card (if there is a clock on the card)? if it's the latter case, what's the accuracy of the system clock on pxi-4472?

[Shea C]

Student,

That's a good question. DAQmx uses the windows timer to get the timestamp t0, and it tries to account for the exact time the first sample came in. The calculation goes something like this.

Lets say this is a finite read operation and all samples are read in 1 call to Read. t0 is calculated as

t0 = current system time when Read was called - (number of samples already acquired)* (sample rate)

I hope this makes things more clear!

Shea C
Applications
NI