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OpenBCI / V3_Hardware_Design_Files

Contains files created with Design Spark PCB Capture Software
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why the anti aliasing filter is set so high? #12

Open R0dri opened 4 years ago

R0dri commented 4 years ago

I saw the schematics to find a 10K ohm resistor and a 1nF cap which means the cut off frequency is 72kHz! I guess you would have to sample at +150Hz to avoid aliasing right? but the default sampling frequency is set to 250Hz.

I saw the ganglion board had a low pass filter at ~150Hz, which to me seems like a better design from that point of view. Why it is different now?

JoshQuint commented 1 year ago

This has been discussed at the official forum as well see here for the basics and here for an in-depth explanation

In a nutshell the ADS1299 uses a Delta-Sigma analog to digital converter and features an built-in sinc-filter, which has the advantage of not requiring much pre-filtering. This internal filter has a bad frequency response starting at 1.024 MHz, where the external filter comes into play. It works in conjunction with the internal one as an EMI and "anti-aliasing" filter. Therefore the cut-off frequency is not the main factor, but the attenuation at this frequency, which is reasonably high with ~ -23db. Further filtering is mostly done by software to not obscure the original signal. This filter is also used with slightly different values at TI's ADS1299 developer board, that consisting of 4k99 Ohms and 4.7nF, resulting in a 6.7kHz cut-off freq.