Filter problem... Very urgent pls..

[audioguru]

Hi Optikon,
The inamp will see any slight difference between its inputs as signal, and amplify it umpteen-thousands times.
The same amount of common mode interference that appears at both inputs is rejected by the inamp's superb CMRR.

Hi Ron,
Thanks for the sim. I thought that it would have more ringing. Now we are on the right track. This will show the author that he doesn't need a notch filter.
I wouldn't call the 60Hz "common-mode", it is an unbalanced interference that is amplified by the inamp, just like desired signal. See above.

 

[Optikon]

RON H: That was a nice example of Gibbs pehnomenon, Thanks. I am aware of the the theoretical part of it. I think though this filtering problem is more of a distortion question rather than a transient response question??

Do you mean harmonic distortion, such as that caused by nonlinear amplifiers? I can't see a difference between transient response and accompanying waveform distortion.
I ran a typical EKG pulse (1v pk) with 60Hz common mode (2v p-p) added to it through an 8 pole Butterworth with Fc=30Hz. It looks pretty darned good. See below. I think Audioguru is spot on.

Ron, in your sim, is the time delay due to the "group" delay you mentioned and the small distortions due to the abrupt chopping of harmonics(gibbs)?

Also, the author still needs the inamp on the front end yes? for CMRR as everyone has mentioned in some context. I mean, one still wants to get rid of the pickup before filtering.

So this 8th order Butterworth can go right into where the author had drawn an attempt at a filter. I think if the filter can work that well, he can nevermind digital filtering...

The probes and microvolt signal form what looks like a high impedance voltage source right? well, I Still think he should drive his inner shield at signal potential to avoid forming a voltage divider due to leakage current.


Maybe, if the author is still around he can post a cleaned up version of all these ideas, I would be interested in seeing the latest conglamoration.. where is he anyway???

 

[Roff]

Edit: Deleted erroneous info about filter delay. I was full of crap.

When I was talking about group delay previously I should have been saying group delay peaking. In a filter that has no right-half plane zeroes (no allpass networks), the phase shift (and group delay, which is defined as the negative of the derivative of the phase shift with respect to frequency, i.e., -dφ/dw) can always be predicted from the frequency response. The sharper the rolloff, the higher the group delay peak near cutoff. What this means is that frequencies near cutoff are delayed much more than frequencies that are well below cutoff.
I wish I could show a plot here, but I'm at home, and the software I did the 8-pole Butterworth sims on is at work.
He still needs the inamp, and he still needs the shield bootstrapping, but I doubt that impedances are high enough to worry about leakage currents.
I agree that digital filtering is probably not necessary. A cardiologist might not be happy with the transient response of the filter, but it should be fine for non-life-critical applications.

 

[audioguru]

Hi Guys,
Maybe our author got his ideas from this DIY Scientfic American magazine ECG project. It uses a very crude LPF and no shielding whatsoever!
I think the author of that project used it in a field or on the moon, well away from the mains.