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I would like to connect an electric guitar to ADC (or MCU internal ADC input) in order to process signal digitally. I'm not sure right now which MCU or ADC I'm going to use, nevertheless I believe that connecting a guitar to such input without amplification isn't going to work well.

What is 'typical' voltage range on such input in MCU? I guess that my signal should oscillate between GND and Vcc, being centered at 0.5*Vcc (to achieve maximum resolution).

I found one circuit that looks promising (the one with 'line input'): http://duino4projects.com/wp-content/uploads/2013/05/Audio-Input-to-Arduino-Schematic.jpg

Please, let me know if I understand how it works correctly. There's an op amp in non-inverting configuration with gain equal to 10K/1K + 1. With the 47pF capactitor it's also a high pass filter. 100k resistors between GND and 5V provide 2,5V DC offset. I'm not sure about the role of both 10uF conds. What is their role if the circuit makes sense?

Is this circuit what I need in order to connect my guitar to MCU/ADC? What are other options?

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    \$\begingroup\$ ne5532 is not specified for 5V supply. Consider a different op-amp. \$\endgroup\$ – markrages Oct 8 '15 at 21:18
  • \$\begingroup\$ @markrages ti.com/lit/ds/symlink/ne5532.pdf 5V is its recommended minimum. But of course I'm not in any way attached to this model/ \$\endgroup\$ – Rames Oct 8 '15 at 21:39
  • \$\begingroup\$ No 5V is not the minimum operating range of a NE5532 it is 10V. Read the flipping data sheet irrespective of what your ultimate intentions are. Also, what output voltage level can a guitar produce - without this information, the optimum circuit cannot be established. I know what it is because I play guitar and have meddled but, because you were so cocksure about the NE5532 I'm going to leave it for you to establish. \$\endgroup\$ – Andy aka Oct 8 '15 at 22:28
  • \$\begingroup\$ @Andyaka now I get my mistake, simple, stupid, beginner way thinking that 5V indeed means that I can put 5V on Vcc+ without considering what is on Vcc- and forgetting that voltage level is always relative. Thank you for forcing me to read it again! As far as guitar output voltage is concerned I believe that it depends on pickups and guitarist playing style, so your data doesn't have to be relevant in my case. I think I'm going to replace one of resistors with pot in order to control gain manually, however then probably it would be possible to select the right capacitor for desired frequency. \$\endgroup\$ – Rames Oct 9 '15 at 6:25
  • \$\begingroup\$ According to Wikipedia: "A hard strum on all 6 guitar strings can produce a larger output voltage swing, typically peak voltages of +/− 5 volts for single coil pickups and +/− 10 volt peaks on dual coil pickups". SoI'd consider a step-down transformer before the ADC. \$\endgroup\$ – Alan Campbell Oct 9 '15 at 13:36
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The 47pF actually forms a low pass filter, as it's wrapped round the op-amp. 10k//47pF = 0.5us = 300kHz (in round numbers). The gain rolls down from 11x above 300kHz. You could increase the value of the cap somewhat to reduce the bandwidth.

The 10uF caps are both high pass filters. The input cap into the parallel 100k resistors goes down 1 below 1Hz. It's mainly shifting the ground-based input up to 2.5v for the amplifier. The second cap decoupling the 1k resistor goes down to 16Hz. This is not so much a 'signal processing' filter, as much as a way to give the amplifier unity gain at DC, so that the output sits at 2.5v on average. You may want to reduce the values of these caps to increase the highpass filter corner, to pass guitar frequencies, while attenuating lower mains frequencies.

It's always a good idea if you are making a 'signal conditioning' amplifier/filter to restrict the range of wanted frequencies as much as you can, to avoid things like mains interference or radio interference to get into the input.

A gain of 10 sounds a reasonable place to start. It's easy enough to change if you find your input too low or too high.

[edit] markrages comment about the 5532, indeed a bizarre choice for this position. It is a very old skool, high rail, poor output swing close to rail, amp. You would be much better off with 1/4 of an LM324, and you could have 4 input channels, or better still a CMOS rail to rail type, there are plenty of those speced for 5v rail. [/edit]

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