The -3dB corner is determined by the 1MegΩ*1.5nF feedback network, and is at 106Hz.

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Ron

 

Are you sure about that? I would have thought it was the Cap and the 5kOhm resistor?

 

When the capacitive reactance of the 1.5nF capacitor equals 1M at 106Hz then the gain of the opamp is reduced -3dB.

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Uncle $crooge

 

Ok cool, thanks both of you for the clarification. So A3 acts as both an amplifier and a low-pass filter correct? Boosting the signal with a gain dependant on the ratio of the two resistors while attenuating the signal above a cutoff frequency dependant on the relationship between the feedback resistor and capacitor?

Does anyone have any idea what the purpose of this filtration is in this application? Anti-aliasing maybe?

 

Apparently 100Hz is a standard decided upon by the American Heart Association. Not everyone agrees that 100Hz is adequate. Search "ECG bandwidth".
Any bandwidth beyond what is "adequate" will just pass unwanted noise and transients which will tend to make reading the ECG more difficult.

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Ron

 

Great stuff - thanks a lot. I've stumbled across a few very interesting websites due to that search...

 

i want to detect the fetal heratbeat by using electrode per minute.Many people that i ask will say that this is crazy project. I just want to sure that it is correct that electrode cannot detect the fetal heartbeat? I want to use the instrumentation amplifier and low pass filter like the ECG circuit. Do you have any suggestion to me?

 

How will you attach the electrodes to the unborn baby?
They use ultrasound or a stethoscope.

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Uncle $crooge

 

I don't think it's crazy. Yes, start with that ECG circuit, seems like it ought to work. Why do people think it's crazy?

 

Can somebody please expalin me the working of this ECG amplifier circuit?? All opamps are TL074 and the bottom one which has B and C inputs is an AMP02.
Thank you....it's really urgent for me

 

i think opamp TL074 connected to A is a buffering amp just to buffer the ref point A of the human body
AMP02 connected to B and C seems to be an instrumentation amp that hv a large gain to amplify the small ECG signal with respect to the ref point/signal of A
Then, the last stage is a second amplifier stage and filtering stage before get the ECG signal
the separate opamp should be used to stabilize the virtual ground

I am not sure my explanation is correct or not
Maybe there is someone else can help u out more
My advise is to get the opamp datasheet to understand all the pins description

 

Your circuit does not make sense:
1) U5 is not an opamp. It is an intrumentation amp.
2) U2 does not do anything.
3) R5 does not do anything.
4) R6 does not do anything.
5) U1 is missing an input resistor.

U3 is a high gain amplifier and a very simple lowpass filter.

You should make the ECG circuit that is shown on the datasheets of instrumentation amps.

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Uncle $crooge

 

Post deleted - essentially a duplication of Audioguru's.

EDIT: U3 is also missing an input bias resistor. I suspect R5 is supposed to be connected to ground, from the junction of C1 and U3 + input.

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Ron

 

I agree with that comment to a certain degree, with a caveat. I absolutely LOVE discussing subjects outside of my expertise. In all honesty, my only areas of electronics expertise would be RF and antenna circuits and industrial control instrumentation, but fortunately these areas are broad enough to have familiarity with many aspects of electronics, enough to "speak intelligently" so to speak. By discussing them, I reinforce and often deepen my comprehension, and sometimes correct misconceptions.

So I think it is fully appropriate to comment, but be prepared to concede mistakes or else defend your position with supported facts, and most importantly clearly state your gaps of knowledge to avoid confusion. The last part is usually the most difficult to do.

 

I agree with that. The problem with offering "expertise" when you are lacking it is that, gone uncontested, it may lead the OP (and others) down the garden path.
I probably should have replaced "reply" with "offer advice".

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Ron