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I am using this stethoscope circuit digital stethoscope I want to give TL072 butterworth filter output to Arduino Due ADC which input is max 3.3 but the supply of the mentioned circuit is split +9-0-(-9) volts. How can I interface this?

I tried to shift reference voltages with two back to back diodes but it did not give output

My question is what is the best way to interface this circuit to arduino due ADC so that we can sample data there Thanks

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If you can modify your circuit, you can remove R15 and then take the output from C6 and add a simple DC bias circuit as described. C6 will already "isolate" the circuits. You have to make sure not to set R11 too high.

schematic

simulate this circuit – Schematic created using CircuitLab

If you can't modify the stethoscope circuit, then you can simply add another capacitor like C6 from the output, essentially like the circuit in this answer.

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  • \$\begingroup\$ I was thinking to interface from U1b pin 7? I want thinking to omit LM358 as it is just a power amplifier is it necessary to use lm358 stage? or can I use same bias circuit on Pin 7 of TL072? \$\endgroup\$ – Abdul Rehman Dec 20 '15 at 21:10
  • \$\begingroup\$ You can use the same bias circuit from U1b, but you may have to reduce the gain of the circuit. You can do that by modifying R4 or R8, although R8 should change some aspects of the filter. \$\endgroup\$ – pipe Dec 20 '15 at 21:15
  • \$\begingroup\$ c6 in original circuit is 1000uF but you mentioned 1uF? is 1uF enough for this? \$\endgroup\$ – Abdul Rehman Dec 21 '15 at 11:23
  • \$\begingroup\$ I used 100uf Capacitor and it worked perfectly fine. I have now fully arduino compatible functional stethoscope. I want to do some dsp operations on sampled data is there any recommended library for arduino due? thanks \$\endgroup\$ – Abdul Rehman Dec 21 '15 at 12:42
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    \$\begingroup\$ I did not mention 1 µF, I just copied the value from C6 which is 1000 µF but CircuitLab rewrote it as 1 mF which is the same. As you have already noticed, the value is not critical at all, and it depends on the lowest frequencies you want to capture, so you should be fine. I know nothing about Arduino so you have to start a new question with that I'm afraid! \$\endgroup\$ – pipe Dec 21 '15 at 15:23

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