External Clock for DAQmx Sampling when using Sigma Delta ADCs

[RagMan]

As stated in PXIe-4302 User Manual (NI Document 377003a):

External Clock
The NI PXIe-4302/4303 ADCs cannot be clocked from external sources such as encoders or
tachometers. However, signal processing features in the Sound and Vibration Measurement
Suite often provide an excellent alternative to external clocking in encoder and tachometer
applications. Visit ni.com/soundandvibration for more information about the Sound and
Vibration Measurement Suite.

Is this true of all Sigma Delta ADCs?

If this is true, are there any other hardware methods one can use have an encoder be the external clock the AI sampling?

[RagMan]

Sorry, I posted this in the wrong board. Will re-post in PXI forums.

[Kevin_Price]

To the best of my knowledge, no you cannot use an encoder as an external sample clock with a Sigma-Delta (a.k.a. Delta-Sigma) converter.

Do you have any other DAQ devices available?   I'd be looking for an approach where I export the sample clock *out* of the PXIe-4302 so that another device with counters can correlate it with the external encoder signal.  There are several possible methods, but the simplest is probably position measurement with an external sample clock.

Keep in mind the there's filter delay built into the 4302.  I'm not sure if the exported hardware sample clock signal will inherently include this delay or not.  You may need to do some testing to be sure.  Then you may also need to shift or interpolate your respective data sets to line them back up in time properly.

-Kevin P

[RagMan]

Thanks for the reply. I also have two PXIe-4340 (LVDT boards) in the chassis. But they are also Delta Sigma ADCs.

I may have to either use the S&V tools VIs for converting the signals from Time based to Angle based using the Tachometer processing VIs. Or I can do the parsing in my own VIs, which I think would be easier.

[Kevin_Price]

If you've got access to any budget, I wouldn't rule out buying suitable hardware, perhaps a PXIe-6341 that offers a wide range of multifunction capabilities.

Capturing the encoder signal(s) with an AI task requires post-processing, may require a higher sample rate (and thus more data bandwidth to deal with), and opens you up to potentially non-trivial amounts of debug and troubleshooting time.

Whether $ or time, either way it's gonna cost.  A lot tends to depend on whose $ and whose time we're talking about...

-Kevin P

[RagMan]

Yes I agree. I have used Multi-function cards to do this kind of DAQ before. But for this system, we have LVDT sensors so PXIe-4340 is the only choice.

Since i only need to acquire data for < 10 seconds (4 revolutions of a hand-crank), using software to parse the data is not bad. In fact, I think I can double the resolution since I can parse data on both rising edge and falling edge of the encoder. I played with it this afternoon using Simulated data (using the Waveform Generation VIs to simulate the encoder, and the other 9 sensors). It came out ok.

The top graph shows the signals as acquired (so it is time based). The bottom graph shows the signals at each rising or falling edge of the encoder (so angle based). I deliberately changed the "hand-crank speed" throughout the test to simulate a sloppy human turning it. But the software parsing took care of the variations and rendered true angle based signals.

I processed the data in-line so that person doing the test will see the bottom graph live update during the test.

[Kevin_Price]

Ok, looks like you're in good shape, no need for further advice, well ahead of the average poster here...

-Kevin P