How to disable antialias filter with PXI5124

[Julien_BERNU]

Hi,

I am digitizing data at 200MS/s and require a very flat response as high as possible. The embeded antialiasing filter quicks too early (60 MHz) and creates lots of ripples ion the useful band. I can try to compensate for it by applying digital filter on the digitized data, but would rather prefer to disable it in the hardware (and use my own antialiasing filter Low pass 80MHz or 100MHz).

So in clear, how can I disable the HW antialiasing filter?

Cheers,

Thomas

[Nick_Storton]

Hi Thomas,

If you are using LabVIEW then you need to use the Vertical:Maximum Input Frequency property and wire a -1 to it which will give you full bandwidth. If you are using CVI then use the NISCOPE_ATTR_MAX_INPUT_FREQUENCY attribute and again pass a -1.0 into the attribute.

Hope this helps,

Nick

[Julien_BERNU]

Thanks for such a quick reply. I manage to disable filter by setting max_frequency to 100MHz. I'll try your method. I think this option should be a bit less confusing (like disable_antialising_filter for example).

Cheers,

Thomas

[IronFritz]

Hi,

I have the same problem of original post, but I have to work with SignalExpress and NI-Scope Acquire step.

How could I disable the antialiasing filter of PXI 5124 using SignalExpress?

Thanks a lot,

Federico

[psisterhen]

On the Configuration tab in the NI-Scope step, the Bandwidth (Hz) control at the bottom right sets the same attribute mentioned above (Maximum Input Frequency).

Set that attribute to 150 MHz to disable the filter.  (Although anything over 70 MHz will work, too.)

[Shiying]

Hi.  I use NI-Scope 5124 to measure the ripple and noise peak-to-peak voltage in 20 MHz bandwidth.  I am going to use your suggestion - use the Vertical:Maximum Input Frequency property and then wire it to 20 MHz. But can you tell me what is default Maximum Input Frequency?  Thank you.

Shiying

[JoeFriedchicken]

Hi Shiying,

This information is found in the documentation for the 5124.

The 5124 has two filters: a 20 MHz low-pass noise filter and a 60 MHz low-pass anti-alias filter. Only one filter can be enabled any given time, but the anti-alias filter is enabled by default [1].

The NI-Scope VI "niScope Configure Chan Characteristics.vi" allows to enable or disable these filters by wiring the appropriate "max input frequency" to the VI. The frequency choices are: full, default, 20 MHz, 100 MHz [2].

[1] NI 5124 Specifications, page 4
http://www.ni.com/pdf/manuals/371135g.pdf

[2] NISCOPE_ATTR_MAX_INPUT_FREQUENCY
NI High-Speed Digitizers Help » Programming » Reference » NI-Scope Function Reference Help » Functions » Attributes » Vertical

[Shiying]

Hi Joe,

I did add "niScope Configure Chan Characteristics.vi" with "max input frequency = 20MHz" and "input impenance = 1Mohm" in the scope configuration as follow:

niScope Configure Chan Characteristics.vi --> niScope Configure Vertical.vi --> niScope Configure Horizontal.vi

But I still get the same result.  How do I know the 20MHz Noise Filter is presented?

Shiying

[Julien_BERNU]

Hi,
You could try to send a signal at a frequency higher than 20 MHz (Let's say for example 30 MHz), and check that it doesn't alias, or fold back, in the region below 20 Mhz (10 MHz in this case). This is a bit rough, but you could then characterise the 20MHz low pass filter.
Hope this help,
Thomas

[Shiying]

Tom,

Does it relate the "min sample rate" setup in niScope Configure Horizontal?  What are the correct settings for both Horizontal and Vertical Configure if the application is for measuring the peak to peak noise (AC coupling) with in 20 MHz bandwidth?

Shiying