Signal Conditioning before PXI-6133

[tingyulou]

I want to add the SCC before PXI-6133 and i chose SCXI-1125.
here is the question:

PXI-6133 is working with the max sampling rate 2.5MS/s.
SCXI-1125 is working with the max sampling rate only 333kS/s.

Is it means that i will lose some data during the measurements? I want to measure 5 channel signals with the frequency of 10kHz.

Anxious for your reply.

Regards.

[tingyulou]

maybe it is not the right way that i post the message here, sorry if so.

[Brent B.]

With the release of NI-DAQ 7.4, which will release on the web in the next week, you will be able to use any S Series board in parallel mode with SCXI. Parallel mode means that each channel on your SCXI module is in parallel with a channel on your data acquisition device, as opposed to every channel on an SCXI module being multiplexed to a single channel on your data acquisition device. In your case, you will be able to connect each PXI-6133 to the rear of each SCXI-1125 module. Once you have made the proper connections, you will have to set the PXI-6133/SCXI-1125 in parallel mode in Measurement and Automation Explorer.

[tingyulou]

That's great!
But how can scxi-1125's sampling rate be as fast as PXI-6133's?

[Stuart_G]

What Brent meant is that in parallel mode you will be able to go at 333kS/S for each channel, instead of 333kS/S divided by the number of channels you have (in your case 333/5). You are correct that you will never be able to go at the 2.5MS/s of the 6133.

StuartG

[Otis]

An SCXI system has a maximum rate of 333kS/s. Regardless of your DAQ board you will only be able to sample at 333kS/s. If you had a board that only sampled at 250kS/s, then that would be the limiting factor. But in your case the SCXI system is the limiting factor.

If you want to maximize your sampling rates then the solutions that Brent and Stuart recommended might be best. Otherwise the SCXI system will multiplex the 5 signals and you will get a rate of ~66kS/s per channel.

[Stuart_G]

It can't

StuartG

[Brent B.]

Please allow me to further clarify. When in multiplexed mode, meaning the analog channels entering each SCXI module are multiplexed through the SCXI backplane, the limits are 333 kS/s for 12-bit accuracy and 100 kS/s for 16-bit accuracy. Even in the multiplexed case, you can scan faster than the rates above, but you will your signal will not settle properly.

In parallel mode, the rates above are useless because the signals are not being routed through the multiplexed SCXI backplane. In the parallel mode case, each signal is sent, in parallel, through its own circuitry and is fed directly to the data acquisition device. When this happens, the settling time constraints are moved from the SCXI multiplexing backplane to the data acquisition device itself. For instance, if I am using a data acquisition device that is capable of 1 MSample/s, and I am using it in parallel mode, I can use the DAQ device, and the accompanying SCXI module, up to its full sampling rate (1 MS/s). Whereas, if I was using SCXI in multiplexed mode, I would be limited to 100kS/s or 333 kS/s, depending on the resolution of my daq device.

Whether you are multiplexing through the SCXI backplane or on the DAQ device itself, there are selling time restrictions and the analog to digital converter is shared amongst multiple channels. S Series devices offer an advantage over multiplexed data acquisition devices since they have an ADC (analog to digital converter) per channel. This is significant since the sample rate is not divided by the number of channels scanned and you get simultaenous sampling of all incoming signals.

When placing an SCXI module that is parallel-mode capable; such as the 1125, 1141, 1142, 1143 and 1520; in parallel with an S Series device, the bandwidth/scanning limit is typically the bandwidth of the SCXI device, not the scan rate of the DAQ device. For instance, if I use the 6133 (2.5 MS/s per channel) in parallel mode with the SCXI-1125 (maximum bandwidth of 10 kHz), I will be able to oversample the maximum frequency content of the signal by 250X. This is a bit of overkill.

When using the SCXI-1125 in parallel mode with S Series, I would suggest using the NI 6143. It is capable of 250 kS/s per channel and has the appropriate amount of resolution, 16-bits. Using the SCXI-1125 and NI 6143, the maximum frequency content of the signal will be oversampled by 25X. The 6143 is lower cost than the 6133 and offers better matched performance to SCXI parallel-mode capable modules.

Again, this capability is not availible until the release of NI-DAQmx 7.4 which will be availible on the web sometime next week. I hope that this clears it up for everyone.

Brent