Noise

Hi all,

Since noise in one form or another seems to be a popular topic here at present, I have a question relating to it. This heart monitor I've been playing with lately is astonishingly good for the price; I really can't significantly fault it. But you can't expect perfection for five quid. I've noticed there is a small amount of noise riding on the output signal, depending on one's body posture for some reason. The noise closely resembles a triangle wave and its frequency is 200Hz. What's the best solution filter-wise for removing it? I obviously wish to avoid any attenuation of the positive peak of the QRS complex; the only sharp transient present on a normal ECG trace. Here's a diagram showing once whole cycle of a regular trace:

formatting link

So... Best flavour of filter for this particular application, please?

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Cursitor Doom
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Is it periodic? Harmonic of 50 Hz line?

Lowpass filter it.

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John Larkin         Highland Technology, Inc 
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John Larkin

If it is a supply ripple, add a bit of the reverse ripple to the output.

I did that successfully in an intercom with nasty supply noise.

Reply to
Sjouke Burry

It's not supply ripple. I've scoped the rails and they're dead flat.

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Cursitor Doom

If it is then it can only arise through induction from the domestic supply wiring, in which case I'd expect it to be 50Hz and not some harmonic of that.

Tricky bit is, 200Hz is already pretty low. :(

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Not sampling frequency is it? A tuned filter oughta help.

NT

Reply to
tabbypurr

Activated twin-T notch, one for 200 and one for 600, then lowpass.

Cheers

Phil Hobbs

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Phil Hobbs

Cursitor Doom wrote in news:q54d13$cq1$ snipped-for-privacy@dont-email.me:

How many probes does your kit have? for five quid.

Not 12 I can gaurantee.

Isn't that less than 10 bucks?

I have serious doubts about any analysis a 7 dollar toy makes. Oh... did I misspell 'tool'?

It likely generates the noise you are seeing.

Have you placed it into an anechoic box?

Or is it that your body is an antenna, and all the 'cancellation' attempts in the world will not attenuate the noise you are injecting into that probe contact?

Reply to
DecadentLinuxUserNumeroUno

If you have an oscilloscope, like trigger the scope and look at the signal. If it doesn't move horizontally, it's line sync'd.

Capacitive coupling from the AC line could have big harmonics. It's been a long time since the AC line was sinusoidal.

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John Larkin         Highland Technology, Inc 
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John Larkin

Or 150 Hz brickwall.

An elliptic lowpass with a notch at 200 would be good too.

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John Larkin         Highland Technology, Inc 
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John Larkin

The unit probably notch filters mains fundamental and the first couple of harmonics in order to work at all. You are seeing artefacts from the incomplete cancellation of higher frequency mains harmonics.

You could probably get away with a steep elliptic low pass filter or chebyshev type 2 with the null sat on 200Hz or just post processing of the data offline in Excel if superficial noise bothers you that much.

Chebyshev type 2 has the advantage of a flat pass band response although it is somewhat more tedious to implement well.

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Martin Brown

I found the answer to the problem. It was simply the lack of electrode lube on the skin contacts pads. Given a smear of this conductive lube on the sensors and the noise just vanished. I mean, it may still be there, although now invisible, but is *tiny* compared to the desired signal which is now as sharp as a razor. I'll try to post some pictures of the trace if time permits over the next couple of days. So 10 out of 10 for those industrious Chinese people. :)

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