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NI-9217 RTD Sampling Speed, High-Resolution vs High-Speed

Hello.  I am using a NI-9217 RTD module in a CDAQ-9178 chassis. What is the difference between High-Resolution and High-Speed on the NI-9217?


In my setup, I am sampling all four channels and realize the specified 800ms total for all four channels in high-resolution mode and the 10mS when in high-speed.  The acquired signal does have more noticeable noise in the high-speed mode.  


My question is what is happening when I select either mode?  Is a different low pass filter used in high-resolution mode?  Is high-resolution mode simply taking many samples and averaging?  Is high-speed mode dropping low order bits?  Just curious to what is going on [grin].  The NI-9217 manual is not clear to me what the tradeoffs are.


I need more speed and was curious if I could achieve similar results to high-resolution mode by using high-speed mode and average the data.





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It appears that the difference between the two modes is that high-speed is taking single values directly, while high-resolution mode is averaging up to 80 values at a time. This is separate from the digital filter on the ADC, which is only meant to filter the high-frequency noise induced by the converter. The name high-resolution is a bit misleading - high-precision probably describes the behavior more accurately. Basically, averaging the values is acting as an additional low pass filter.


You should be able to get similar values if you average the values from high-speed mode yourself, so that would be a good trade-off between the two if you need better than 200 ms without allowing too much noise.


Why exactly do you need to sample faster than 5 Hz? The low sample rate on cards like these is due to the fact that temperatures typically don't change quick enough to justify a faster rate.



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Thanks for your input.  My suspicion is that the difference is simply averaging but would like confirmation.  In regards to your question about my need for more speed, you are correct in that I do not really need faster conversion.  My problem is that I am scanning some hardware inputs that have transients that I need to catch.  When in high-resolution mode, a read of all 4 inputs takes 800mS and I am lost in that function for the duration and miss my transient signals.  


Actually, I have two NI-9217 modules and reading 6 inputs so I am hung up for 1.2S (800mS+400mS).  I am going to try high-speed mode and average and see if I get similar results to high-resolution.  


Really need to try to figure out how to perform an external hardware interrupt.  This solution has eluded me.






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So you really only really need one temperature reading, but you need it to occur directly after a different signal occurs? That sounds like a single reading with the transient signal as a trigger would be a more reliable method, would that be doable? You may have some trouble finding a line you can trigger off of since that module only has RTD inputs though. If you could find a way to make that work, that would probably be a more reliable method though.



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I have switched to the high-speed mode and implemented a running average of 4 samples.  With the running average of 4, the resultant data looks similar to the high-resolution mode (it does have a little more noise but not much more).  If I get some free time, I'll post some results.



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