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How do i remove noises in heartsound?

qkaifong

New Member
Hi all im doing a school project that requires me to capture heart sound, as close as it will be using a stethoscope.
Im using invesense MEMS microphones analog ICS 40180, and LM358AP Op-amp. (I have attached a diagram of my connection)

I read from quora the frequency for heartsound is from 20Hz to 500Hz, I didn't have to exact resistor/capacitor values, so I'm capturing from 16Hz to 482 Hz.
(Do let me know if this range is wrong)
I have attached a picture of my signal that i captured and the signal i'm trying to achieve.

The signal i captured introduced a lot of noises, i have been trying different methods like putting capacitor in the power line, but it still did not remove the noise.
(I'm using 3V Battery)

How can i reduce/completely remove the noises?
Any help will be much appreciated, thank you!!
 

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ronsimpson

Well-Known Member
Most Helpful Member
What do you want this circuit to do?
It is not doing what you think it should.
Gain=? 100x maybe
Frequency response? 20 to 500hz if I remember right.
What do you want the DC bias to be? About 1V and why 1V?
119397
 

qkaifong

New Member
I'm making a non inverting amplifier that give a gain 100, because the mic pickup signal is too low.
If i did not include DC bias of 0.9V, whatever signal i picked up that fall below negative will be cut off.

Thank you so much for your fast reply!
 

ronsimpson

Well-Known Member
Most Helpful Member
How about this:
1) make Vref better. I think it should come from a real 1V voltage reference but 1k and 429 ohms is better than 1meg.
Added C3 to remove high frequency. I did not work out the right cap. (in my head)
119398

119399
 

qkaifong

New Member
Thank you for the detailed diagram, i will try it out.
Btw i see you have changed to ADTL084, will it still work if i were to use LM358AP still?

After output from opamp, capture signal and process it on Matlab.
 

ronsimpson

Well-Known Member
Most Helpful Member
i see you have changed to ADTL084
I did not have a LM358 to try. So I just picked something that I have a library for.
I would make Vref=1/2 supply or 1.5V.
The LM358; 3V supply is the smallest voltage it should work at. (warning)
capture signal
how? that is the question.

You probably understand C1R1 sets the low frequency cut off point.
C3R2 sets the high frequency point.
 

qkaifong

New Member
There is an app written for it for this project that allow me to record it in a tablet, then i will have to open up the text file in matlab to hear the recording.

I'm very new to working with Op-amp, and trying to learn as i go along.
What's the "warning" for?

And what is R4 for?
 
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ronsimpson

Well-Known Member
Most Helpful Member
What's the "warning" for?
"supply voltage range 3V to 30V" Ok but you are on the edge.
"input voltage range" 0 to Vcc-1.7V So your Vref of 0.9V might be better than 1.5V as I said.
"output voltage range" with 10k load. Vcc-1.5V to near 30mV. Just saying you will not get a output near the 3V supply.
You might look at a "R-R output amp" They will work from supply to supply. R-R is rail to rail.
 

audioguru

Well-Known Member
Most Helpful Member
The LM358 is not used for low level audio because it is noisy (the hiss your circuit produces), it has crossover distortion and it has poor high audio frequencies output. I made an electronic stethoscope 15 years ago using a TL072 audio dual opamp and an LM386 audio power amplifier all powered from two 9V batteries. I used an ordinary electret mic, not a Mems one.
The audio opamp has low noise. The first opamp is a preamp with a gain of 22, the second opamp is a Sallen-Key active 2nd-order lowpass filter with a gain of 1.6 and the power amp is fed from a volume control and has a gain of 20. Today I would use an OPA2134 audio dual opamp that has less noise and a single 9V battery.

Your 1st-order passive lowpass filter does not do much so it passes the hiss.
 

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tvtech

Well-Known Member
Most Helpful Member
Audioguru is yet another Legend that hangs out at ETO. There are many here. Sometimes legends get full of crap .... that is their right...ETO and it's moderators understand that. In fact, the Moderation here is superb. We here at ETO understand life. Most of here are are specialists in our chosen fields. As far as I am concerned, I trust ETO and it's advice 100% of the time.

I've been here around 9 years... never regretted a moment of it.

We're a class act. Team ETO. I don't post much Technical stuff cause noone really uses CRT anymore. So what I do is try bring smiles to Technical guys faces.

Once a tech always a tech.

That's me anyway. Naughty...comes the territory.

:) :) :)
 

unclejed613

Well-Known Member
Most Helpful Member
something else you might want to try is using a small speaker as the microphone, and running it into a differential amplifier. speakers normally don't make good microphones because they only pick up low frequencies well. in your case that would be an advantage. the low impedance of the speaker will also help reduce electrical noise pickup, and the differential amplifier will only amplify difference between the speaker terminals, and reject most everything else.
119439

the speaker being used as a microphone would be connected across the V1 and V2 terminals. R1 and R2 should be equal, R3 and R4 equal, and the ratio of R3/R1 sets the gain.
 
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