Ni-Daq 6251 Settling time/time Between Samples

[LSUgrad85]

I'm trying to calculate the expected error for an experiment I'm doing and I'm not sure if I've done enough to determine the "settling time" or time between samples.

We're using 8 differential channels at the maximum sample rate the card can do (1.25 MS/s). If I understand correctly, the the minimum time between each sample should be 1/1.25M, or 800 nanoseconds, such that the maximum time between sampling channel 1 and channel 16 would be on the order of 12uS (800nS * 15). If the expected voltage settings are the same for each channel (+/- 10v), should there still be a settling time? If so, how long?

Also, if one channel is acting up, and its voltages have a DC shift of about 5 or 6 volts, should that severely increase settling time if all the other channels average around +/-1v?

I apologize for the basic question, I couldn't find a straightforward answer in the documentation.

[NIquist]

The 6251 can only achieve a 1.25M sample rate when it's NOT multiplexing (i.e. only 1 channel).  As soon as you multiplex multiple channels you get 1MS/s aggregate.

If your signals are all the same general level (i.e. you DON"T have to adjust the input range) you should be able to achieve the max rate of 1,000,000S/#channels/sec.

Your 6V signal should mix OK with the 1V signals if you keep the range of all at 10V but at high speeds keep aware of the possibilty of crosstalk due to settling time of the Sample and Hold circuit.

[NIquist]

BTW,  https://www.ni.com/docs/en-US/bundle/pci-pcie-pxi-pxie-usb-62xx-features/resource/m-series-user-manu...

This is a link to the M Series manual.  Section 4-8 covers the issues you're facing.

[LSUgrad85]

Thanks for the correction on the max S/s.

I read section 4-8, but there isn't any sort of concrete number for settling times. It just says "Some devices can take many microseconds for the circuitry to settle this much." It isn't a make or break if there's an extra microsecond or two between channels, if it's "many" microseconds (>10 per channel, I don't know,  "many" is very vague), then that becomes a big deal.

[NIquist]

The manual can't be much more specific without becoming confusing as it tries to explain every possible scenario.  It would be nice if there was a table showing the PGIA settling times for all possible range changes (hint, hint, NI). Basically, you want to avoid switching a channel that's measuring a signal at the 10V range to a channel that's measuring in a <1V range.  If you do, you need to be aware that some of the larger signal might still be present at the ADC input and show up in the subsequent channel (ghosting).  You can see this effect empirically so if you need to you can set up an experiment with your 6251 to test its performance with different scanning parameters.  All this info may be available in a white paper somewhere but I haven't seen it.  Good Luck!

[Dustin_D · Accepted solution]

Hi LSUgrad85,

When looking for specific device information the Detailed Specification for that device will usually provide the specifics you are looking for.  Post back if you have questions about the information in the Detailed Spec.

Hope this helps!

Regards,

[LSUgrad85]

Wow, that was extremely helpful. I missed that page before. Thanks!