We want to use the ADS1281 in the modulator data stream output mode. We will be doing the post filtering in our FPGA, after the calculation of.y[n] as specified in the datasheet.
Our post filter will be a straight averaging and decimation filter. I need to understand the noise characteristics of the data stream after the y[n] calculation, so I can calculate the ENOB based on different final sample rates.
Any help on this would be greatly appreciated.
Thanx,
Yehuda
Hi Yehuda,
If you know the noise bandwidth of your filter AND if it is less than ~7kHz, you should be able to estimate the RMS broadband noise levels after your post-processing filter:
If you look at the ADS1281's modulator output spectrum, you'll see that broadband noise is fairly flat up to about ~7 kHz:
In this region (< 7 kHz), the noise spectral density (NSD) is flat and has an average value of about 31 nVrms/sqrt(Hz). This average value comes from knowing the SNR and bandwidths (BW) of the ADS1281 for each for the digital filter settings. I did the following calculations in Excel and found the average NSD:
...Knowing this information, simply take the square root of the bandwidth of your digital filter and multiply it by 31. nVrms/sqrt(Hz) to estimate the RMS noise that will pass through your digital filter.
Best Regards,
Chris
