PXIe-4497 lowpass filter

[drakkar]

According to documentation PXIe-449x modules have a built in lowpass filter for each analog input channel to help with aliasing. I can't find what the cut off frequncy is on that filter.  Also can that filter be switched on and off or is it on by default? 

I

[DiegoH214]

Hello Drakkar

I want to make sure this is what you are referring to.

I believe this document addresses your concern.

Low Frequency Alias Rejection with NI Dynamic Signal Acquisition Modules

http://digital.ni.com/public.nsf/allkb/8BD1190B07BDC43286256FD9006DC52F

Basically, the cutoff frequency is dependable of the sampling rate selected since it is not a Low pass filter but a similar behavior.

You can turn it off using the property node mentioned on the document attached.

You can find more information on the manual (p.11)

http://www.ni.com/pdf/manuals/372125f.pdf

Diego H

National Instruments 

[drakkar]

I am referring to this note from http://www.ni.com/pdf/manuals/371235h.pdf :

In addition to the ADC built-in digital filtering, DSA devices feature a fixed-frequency analog filter. The analog filter removes high-frequency components in the analog signal path before they reach the ADC. This filtering addresses the possibility of high-frequency aliasing from the narrow bands that are not covered by the digital filter. Each input channel on a DSA device is equipped with a multiple pole lowpass analog filter. While the frequency response of the digital filter directly scales with the sample rate, the analog filter –3 dB point is fixed. The analog filter response produces good high-frequency alias rejection while maintaining a flat in-band frequency response. Because the analog filters are not high-order systems, the filter roll-off is not extremely sharp. The filter provides effective alias rejection at higher sampling rates, where only very high frequencies in the previously mentioned susceptible areas can pass through the digital filter. Some DSA devices support enhanced low-frequency alias rejection. The NI USB-443x, NI 446x, NI 447x, and NI 449x devices have different filter response curves that are available in Appendix A, Device-Specific Information.

[DiegoH214]

Ok I see, then coming back to your question.

The cutoff frequency is .49 of the  sampling frequency. This ensure that the sampling frequency is always above the Nyquist frequency for the signals being measured.

http://www.ni.com/pdf/manuals/372125f.pdf