scxi throughput and resolution

[Krajnik]

I have some questions regarding the capabilities of SCXI signal conditioning for my application, and some of the information I've found seems to be conflicting.

I currently have a PCI-6071E (12-bit, 1.25 MS/s), and am using a 16-channel 5B backplane and an 8-channel 5B backplane, for a total of 24 conditioned channels. I'm looking at upgrading the system. There are a number of things we'd like to accomplish:
-isolation
-adjustable (preferably programmable) antialiasing filter
-16-bit resolution
-10kHz sampling rate for the 24 channels (240kS/s total throughput)

Initially, I was under the understanding that the multiplexed SCXI bus was capable of total throughput of 333kS/s at 16-bit, so I was looking at upgrading to a 16-bit DAQ card, and using 3 SCXI-1120D modules cascaded to 3 SCXI-1141/2/3 modules.

However I've read some posts and KnowledgeBase entries that seem to indicate that with this configuration I will not be able to get my desired 240kS/s at 16-bit resolution. The KnowledgeBase entry at:
http://digital.ni.com/public.nsf/websearch/410A70C25A4D12B486256A1E0070BDAE?OpenDocument
suggests that with a 16-bit board, my max throughput will be limited to 100kS/s.

If I look at the documentation for the SCXI modules it indicates that the per-channel settling time for .0015% (16-bit accuracy) is 20microseconds, suggesting a max throughput of only 50kS/s.

Am I missing something? Can anyone help clear up my confusion on this matter? With the above configuration, what is my achievable throughput?

I'd appreciate any insights and/or help with this matter.

Andrew Krajnik

Message Edited by Krajnik on 06-15-2005 12:06 PM

[sergesl]

Hello K.

The entry you refered to said:"module that can go 333kS/s will be only able to go 100kS/s if a 100kS/s module is in the same chassis." If you check the datasheet, the 1120D and the 1141/2/3 all have a max sampling rate of 333kS/s. The sampling rate only goes down if one of the modules has a lower sampling rate.

[Krajnik]

Serges,

Thanks for your response. However, I'm still confused. In item 4 of the entry I referred to, it says:

"4. Determine the resolution of the DAQ board.
If the resolution of the DAQ board is 16 bits, then your maximum sample rate will be limited to 100KS/s because that is the fastest rate at which SCXI can settle analog signals to 16 bits of accuracy."

I also found another knowledge entry at: http://digital.ni.com/public.nsf/websearch/F6D3D99501D9893486256BF300838E7D?OpenDocument
This entry also supports the 10us settling time for 16-bit accuracy.

Now, if my understanding is correct, I have to settle to within .0015% of my signal range to have 16-bit accuracy (1/2^16 = 1.5E-5). Well according to the multiplexer performance data in the data sheet for the 1120D, (http://www.ni.com/pdf/products/us/4scxisc296_ETC_196.pdf) the scan interval required to settle within .0015% is 20us. This leads me to believe that I really only have 50kS/s of throughput for my multiplexed SCXI channels.

This concerns me, because there are plenty of DAQ cards with the capability to handle much higher throughput rates at 16-bit resolution, but with SCXI, that performance is severly limited, and I'd be forced to purchase multiple DAQ cards. If I were able to connect the SCXI madules in parallel mode directly to the channels on the same DAQ card, as discussed in this thread:
http://forums.ni.com/ni/board/message?board.id=300&message.id=1504
it seems I'd be able to make use of the full capability of the DAQ card. However, it looks like with that configuration I'd be sacrificing accuracy.

Is there any way, short of purchasing multiple DAQ cards, to get the performance I'm looking for?

[sergesl]

Hello Krajnik,

When multiplexing, you cannot expect to get the same performance speed wise that you would get in parallel mode. To run at MAX, parallel mode is the way to go. However, to increase your sampling speed without loosing in accuracy, you may consider sample-and-hold boards such as the SCXO-1140.
You may also want to purchase M series DAQ with double connector and connect to two SCXI modules in parallel mode. That's one DAQ connecting to 2 modules in parallel and these M series are low cost. Check out the following KnowledgeBase entry:
http://digital.ni.com/public.nsf/websearch/1936A86D5321444186256FF1004FF966?OpenDocument