PCI-6111 Valid Sampling Rates

[isaac000]

We have old PCI-6111 card which we are still using. A question came up regarding setting of the applicable sampling rate.

According to the manual, this thing should have base clock of 20 MHz and 100 kHz. It also has a 24-bit divider and (not sure about this) 2-bit multiplier. I assume these can be used for scaling the base clock rate appropriately.

What sampling rates can be set on this thing? It's a 5 MSPS spec, so that would be the upper limit. Can I set any arbitrary (integer) frequency under that? Or do I have to work out explicitly what it is capable of? With a 24-bit divider, it would imply there is a huge range of frequencies that "won't work" on this card.

What happens when I select a "bad" frequency? Does it round up, round down?

[Paul-D · Accepted solution]

Hello Isaac, this is Paul with Applications Engineering at NI.

This device offeres 2 simultaneously sampled analog inputs, 5 MS/s per channel - The 2 AI can sample up to 5Mhz per channel each.  You can only have 1 clock rate for your AI tasks, but this clock rate can be set from 1kHz up to 5Mhz, defined by you.  Note that most of our cards allow AI to go down to 1HZ, but this ADC is different because it uses a pipelined ADC convereter.  Additionally, because this card can sample simultaneously, this means that both analog channels are going to be read at the exact same time (in parallel).  Additionally, this AI value is a 12-bit number.

As far as your questions about clocking.  This card features either a 20MHz timebase or a 100kHz timebase.  Which is used is chosen by the driver after you've selected your sample clock rate.  As far as the division of the timebase, the timebase is divided by integers.  For instance you could select 5Mhz, 1Mhz, 250kHz, etc and those would be exact.  If you choose a frequency that is in between 2 integer divisors, then on the S-series (which is what this card is), it will round to the closest frequency.  So, if you chose 3Mhz, this would be between 20  / 7 ~= 2.857Mhz and 20 / 6  = 3.333Mhz, it would choose 2.857MHz as the closest number.

If you need to know in code which frequency has been chosen for you, you verify the sampling rate through the DAQmx Timing property node in LabVIEW.

Let us know if you have any more questions.

Regards,

Paul