How do I select the filter on PXI-5122 from NI-Scope or NI-DAQ?

[menchar]

The PXI-5122 has both an anti-aliasing and a noise filter.

How can I directly enable/disable a specific filter from NI-Scope or NI-DAQ? I see some input BW attributes, but I should think it's possible to directly select a filter?

I am using CVI 7.1.1.

Thanks in advance.

[Brooks W.]

Hi Menchar,
You will need to use the niScope_ConfigureChanCharacteristics function and set the max sampling rate higher than 35 Mhz. The NI 5122 offers two filters that limit the bandwidth of the signal path a 20 MHz noise filter and a 35 MHz antialias filter. If you set the max sampling rate higher than 35 Mhz these filters will be disabled.

Hope this helps!

Brooks W.
National Instruments

[menchar]

Thanks Brooks, but I still don't have a good answer. I fully understand that there are two filters available, and that they are exclusive - both can't be simultaneously enabled.

What if I want to sample at, say, a 10 MSample/Sec rate (or a 5 MHz BW, take your pick) with one of the following conditions:

1. Noise filter only on
2. Anti-aliasing filter only on
3. Neither filter on.

These conditions overlap: simply specifying a sampling rate won't unambiguously select a filter. Simply specifying a channel bandwidth won't unambiguously select a filter condition either.

[Brooks W.]

Hi Menchar,

There is no other way to "directly enable/disable a specific filter from NI-Scope or NI-DAQ". You will need to use the "niScope_ConfigureChanCharacteristics" function.

This means that if you don't call the function "niScope_ConfigureChanCharacteristics", you can expect the default behavior (filter on). Also, if you use the function above and send a "0" for the parameter "maxInputFrequency", you can expect the default behavior. For the 5122, you can expect the following behavior for different inputs to the "maxInputFrequency" parameter:
1to 20M = 20 MHz noise filter on
21M to 35M = 35 MHz anti-alias filter on
36M to 100M = Full bandwidth/no filters (100 MHz)

Best Regards,
Brooks W.
National Instruments

[menchar]

Hi Brooks -

So if I configure an input channel with the function you suggest to indicate a max input frequency of say 25 MHz, can I then select a sample rate arbitrarily low (say 1 MS/sec) and still be certain that the antialias filter didn't get turned off and the noise filter turned on? That is, does the max input frequency parameter exclusively govern filter selection?

[Brooks W.]

Hi Menchar,
Yes, the max input frequency parameter does exclusively govern channel bandwidth (and indirectly filter selection). Yes, the anti-alias filter will be on at 25Mhz and you can specify a low "minimum sample rate" (say 1 Mhz). You have got the right idea...

Best Regards,
Brooks W.
National Instruments

[Westlinwind]

The spec sheet states the anti-aliasing filter as the default filter. How can I assure that  for sampling rates below 20 MHz, the noise filter is on and not the anti-aliasing filter? I really need the noise filter to be on.

[Brooks W.]

Hi Westlinwind,
You will need to use the "niScope_ConfigureChanCharacteristics" function or niScope Configure Chan Characteristics.vi.

Set the "maxInputFrequency" to something between 1 to 20 Mhz.  This will enable the noise filter.

Best Regards,
Brooks W.
National Instruments