DC restorer for EEG amplifier
- Thread starter cyril666
- Start date
I have never seen an ECG amp with DC restoration. Why do you need it?
can anyone give me some opinion or suggestion?
Don't filter it out to begin with.
When I built an ECG, I filtered out the DC with a simple filter, but to remove the 60 Hz interference (which I assume must be why you have any filter at all) I instead sampled the signal at 60 Hz, synchronized with the zero-crossing point of the AC, such that the AC interference became a DC issue, and so it was effectively invisible. (...but ultimately still there, which I guess might be a problem for you. My AC interference wasn't in phase with the household AC for whatever reason. It was also far from sinusoidal.)
I eventually decided 60 Hz wasn't enough, so I went up to 240 Hz, but kept the synchronization. Then I split the samples into sets of four, and samples at the same index in each set had the same error introduced by the AC, and so I simply applied a seperate DC software filter to each of the samples in each set of four. It mostly worked out quite well, and I'd guess you could do that if instead of applying a DC filter to each sample in each set, you instead applied a filter which adjusted the DC offsets of three of the samples to match the DC offset of whichever sample is most correct.
I honestly don't have a clue how one would do it any other way. Some of the detail of the signal is well around 60 Hz and so I would think that any 60 Hz filter would distort the signal, even if only in a way that isn't easy to see.
Don't filter it out to begin with.
When I built an ECG, I filtered out the DC with a simple filter, but to remove the 60 Hz interference (which I assume must be why you have any filter at all) I instead sampled the signal at 60 Hz, synchronized with the zero-crossing point of the AC, such that the AC interference became a DC issue, and so it was effectively invisible. (...but ultimately still there, which I guess might be a problem for you. My AC interference wasn't in phase with the household AC for whatever reason. It was also far from sinusoidal.)
I eventually decided 60 Hz wasn't enough, so I went up to 240 Hz, but kept the synchronization. Then I split the samples into sets of four, and samples at the same index in each set had the same error introduced by the AC, and so I simply applied a seperate DC software filter to each of the samples in each set of four. It mostly worked out quite well, and I'd guess you could do that if instead of applying a DC filter to each sample in each set, you instead applied a filter which adjusted the DC offsets of three of the samples to match the DC offset of whichever sample is most correct.
I honestly don't have a clue how one would do it any other way. Some of the detail of the signal is well around 60 Hz and so I would think that any 60 Hz filter would distort the signal, even if only in a way that isn't easy to see.
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