Suitable hardware configuration for 24-bit >50kHz feedback control loop?

[maoz]

I am looking for a hardware solution that will allow me to implement a fast (>50kHz) feedback loop, with 24-bit DAC/ADCs.
The application is feedback control of an STM (Scanning Tunneling Microscope) tip.

I have seen the CompactRIO FPGA hardware, but it seems that there are no suitable high sampling rate 24-bit DAC/ADC modules on that platform. On the PXI platform, the PXI-4461 DAC/ADC looks like a viable interface, but this lacks the high performance of an FPGA in the control loop.

Is there a way to get the best of both worlds, and have high-bandwidth 24-bit I/O to an FPGA controller?

[bburress]

Hey maoz,
    You could possibly use the PXI chassis and card for 24-bit AtoD, and use a PXI Real-Time controller to achieve certain loop rates.  We have performed some in-house benchmarking with our 8176 and 8196 Real-Time controllers, and 50kHz is about the upper limit of the loop rates we were able to achieve when performing data acquisition.  There have not been any such benchmarking tests with the newest controller (8196), so that is an unknown factor.

[maoz]

I see the PXI-8196 RT controller is rated at 57kHz single PID loop in the catalog, and this may be sufficient for my purposes. However, I am worried that any extra workload will lower the loop rate, so I would like to have some "spare" CPU cycles.

The PXIe-8105 seems to have a more powerful, dual-core CPU (but no loop rate numbers are specified for the Windows-based controllers). In realistic conditions, would the 8105 controller be the better choice? Is there any option to have the real-time OS run on a more powerful CPU like the dual-core on the 8105?

And again, restating my original question, can an FPGA on one of the R-series PXI cards be programmed to perform IO via an A/D card on the same PXI chassis, without that data having to go through the Windows/RTOS-based PXI controller? If so, what sort of PID loop performance can be expected from such a system (compared with the RT PXI controller option stated above)?

[CKap]

Hello !!

Currently its not possible to have an FPGA board exchange conversion data from any other board directly. The data needs to pass through a host programm. But its possible to exchange timing and synchronization signal using the RTSI bus.

You best bet would be a CompactRIO system which has a FPGA (NI 9004) on the back-plane and also had a real-time controller (NI 9012). You could use the NI 9233 module which has 24 bits and 50K sampling. But it has IEPE connections. So that might not work for you.

Hope this help....Feel free to call one of our technical sales people. They should be able to understand your needs and advise you more appropriately.

Regards,

Chetan K

Application Engineer

National Instruments

[CKap]

There was a typo in my last post.

For the FPGA backplane I wanted you to look at NI 9104 and not 9004. I apologize for the mistake.

Regards,

Chetan