One question about "Low-Frequency Alias Rejection"

[dyesheep]

Hi,

In "Low-Frequency Alias Rejection" function, the sampling rate is forced to be never below 25.6kS/s, and the redundant data is discarded to keep the sampling rate user selects.

However, the cut-off frequency of the digital filter also becomes larger.

For example, with "Low-Frequency Alias Rejection" disabled, if the sampling rate is set as 10 kS/s, the cut-off frequency of the digital filter will be 5 kHz.

With "Low-Frequency Alias Rejection" enabled, the sampling rate will be advanced to 40 kS/s, and the cut-off frequency of the digital filter will becomes 20 kHz.

Therefore, signal with frequency 5 ~ 20 kHz will be introduced if "Low-Frequency Alias Rejection" is enabled.

As a result, for low frequency signal measurement, you can only either enable "Low-Frequency Alias Rejection" to filter high frequency noise and sustain the additional signal (5 ~ 20 kHz) introduced, or disable it to prevent this additional signal and sustain the high frequency noise.

Am I right?

Thanks.

[DaveWei]

Hello~

Please refer to the page (Low Frequency Alias Rejection with the NI 446x, NI 447x, and NI 449x):

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

Due to the analog filter mentioned above, high frequency noise will not be a problem.

The concept of Enhanced Alias Rejection is to increase the sampling rate (by multiplying a Decimation Factor) while the user sets the sampling rate below a specific rate (ex:25.6kS/s).  As it is mentioned in that link: "The benefit of Enhanced Alias Rejection at sample rates below 25.6KS/s is that no noise contained in Fs multiples reaches the delta sigma converter, which means there are no aliased images." By increasing the sampling rate, the frequency of aliased images has been increased also. Then, it could be filtered by the analog filter.

[dyesheep]

Hi,

Thank you for replying, but you didn't get my point.

I know "Low Frequency Alias Rejection" increases the sampling rate and moves the alias image farther from the cut-off frequency of the analog filter.

What I concern is that when increasing the sampling rate, the cut-off frequency of the digital filter (inside sigma-delta ADC), which is automatically set to half of the sampling rate, also increases accordingly.

Therefore, some low frequency component which could be filtered by the digital filter will not be filtered when "Low Frequency Alias Rejection" is enabled.

For example, I want to measure a signal with frequency 1 KHz, but there is unwanted signals with frequency 11 KHz presents. I set the sampling rate as 20 kS/s, which should filter signal with frequency larger than 10 kHz.

However, when "Low Freuqency Alias Rejection" is enable, the sampling rate will be forced to be 40 kS/s, and the cut-off frequency becomes 20 kHz, which cannot filter the unwanted 11 kHz signal.

I want to check if this is true.

Thanks.