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How to design an accurate bandpass filter for Electrocardiography application ?

Hello Guys,

I need to design and build a bandpass filter amplifier for my ECG (ElectroCardiography) project. The band pass filter is to remove noise. Heart signals are between 0.5 and 150 Hz, therefore the bandpass filter should remove noise below 0.5 Hz and above 150 Hz.

I have designed a simple bandpass filter but it doesn't work as it should. It doesn't filter out signals of 200 or even 300 Hz, nor even 500, something is clearly wrong. I've designed it in MultiSim (by National Instruments). I've attached a snapshot of the circuit for you to have a look.

The equation I used was frequency = 1 / 2 * PI * Resistance * Capacitance

I've also attached screenshots for the output at 150 Hz and at 200 Hz , at 200 Hz the output should be reduced if not made zero.

I've used the LF351D because it has high-input-noise impedance so please in your advice try to consider the fact that I'm gonna be using that op-amp.

The question is, is what I did correct ? How can I design an advance bandpass filter which is accurate and reliabel and suitable for the ECG application ? Please help and advise

Thanks

SOLUTION

Dave Baldwin

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SOLUTION

d-glitch

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adkry

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Hello DaveBaldwin,

Thank you so much for your great reply and helpful tips. I need to ask a few questions though. Could you please clarify what did you mean by "Unity gain stable op-amps" ? What did you mean by "You have to allow too much gain for the LF351 to work properly" ?

Thank you for your tip on separating the filter into a low-pass and a high-pass one. Also thanks for suggesting the LF353 however when I search, I found many types of it, which one is suitable for this ECG application ? http://www.datasheetcatalog.com/datasheets_pdf/L/F/3/5/LF353.shtml

You mentioned that my circuit won't get me a steep rolloff, so how can I achieve a steep roll-off ?

Thanks for your help much appreciated

Hello d-glitch,

Thank you so much for your great reply and helpful info. The attenuation I wish is very high attenuation in the stop bands (get the signal in the stop bands down to zero if possible), and no attenuation in the pass bands. What did you mean by ripple ?

I don't know the noise in my signal, I can't see a real signal unless I build the circuit, or should I build the circuit without the filter and then analyze the signal ? What do you mean by FFT ? and How can I do it ?

Are Sallen-Key filters the most suitable ? which type of it should I use ? i.e. Bessel ? Butterworth ? or Chebychev ?
Also I don't know what to fill in the fields you in that filter calculator. How many poles should use ? why 2 or 4 ?

Thanks for your suggestion on the book, I'll definitely check it out.

Sorry for the too many questions but if you can answer these questions I'll be very grateful since my understanding isn't exactly the best

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d-glitch

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adkry

ASKER

Hello D-glitch,

Thank you for your great answer. I will run an FFT and analyze it to see how much filtering do I need.

I believe I'll go for the Butterworth, since I'm later gonna use the output signal to apply signal processing on it, therefore it's better to have a consistent gain in the pass band.

Thanks for your help.

SOLUTION

Dave Baldwin

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adkry

ASKER

Thanks guys, your comments perfectly answered my questions :D

Dave Baldwin

You're welcome, glad to help.