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I am designing a circuit to amplify audio from microphone as well as removing dc offset from biasing the microphone. I searched for circuits available online for my personal project. I came across circuit from Sparkfun. I understood everything except the capacitor and resistor connected to non-inverting terminal of op-amp. Please explain the use of the capacitor C2 in the above schematic and why it is connected with the resistors R2 and R3.

Also, I am going to convert the analog signal after amplification to digital form using ADC. I want suggestion on bit resolution for audio applications that will provide acceptable voice quality during recreation of the voice.

Here is the schematic from the Sparkfun web site shared under the Creative Commons license.

enter image description here

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closed as off-topic by Marcus Müller, Dmitry Grigoryev, Voltage Spike, ThreePhaseEel, Daniel Grillo Jan 16 '17 at 19:59

This question appears to be off-topic. The users who voted to close gave this specific reason:

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  • \$\begingroup\$ Without knowing the power supply voltages, suggesting an ADC is guesswork. Apart from that, shopping questions are off-topic because relevant answers become out of date. I think you could do some research on google about bit resolution for voice systems. Not enough effort shown in this question to solve it hence VTC. \$\endgroup\$ – Andy aka Sep 3 '16 at 11:16
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    \$\begingroup\$ I'm voting to close this question as off-topic because of several factors listed above. \$\endgroup\$ – Andy aka Sep 3 '16 at 11:16
  • \$\begingroup\$ Go find a tutorial on op-amps \$\endgroup\$ – Voltage Spike Jan 16 '17 at 17:05
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Resistor R2 and R3 form a voltage divider to provide a virtual ground for the op-amp at half the VCC voltage. (Note the equal values resistors).

Capacitor C2 operates as a filter capacitor to keep the virtual ground node at a quiet level even if there are noise spikes and small disturbances on the VCC supply rail.

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  • \$\begingroup\$ Thanks for your answer. I initially thought that op-amp's output will be able to swing in both directions like an AC. But i was wrong. I didn't see that pin 2 in op-amp is connected to gnd. Due to this, it wouldn't be able to go in negative direction while capturing and amplifying input from microphone. However, R2 and R3 sets virtual ground at half the VCC voltage that helps ap-amp to swing in both direction with reference to new Vcc/2. Please reply back to validate it. \$\endgroup\$ – abhiarora Sep 3 '16 at 11:51
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R2 and R3 bias the opamp input (and thus its output, since C1 blocks any DC component on the input) to half the supply rail.

All resistors generate noise, which you don't really need on a microphone amplifier, so C2 simply decouples that input to ground, reducing that noise. However, R4 generates more than enough noise that C2 probably doesn't matter. (just checked the datasheet : the opamp itself is noisier still : 30nV/rtHz versus 13nV/rtHz for a 10K resistor)

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