Input/ Output Stages

Hi,

When we first wrote the article about pedalSHIELD we did not go very technical to keep it simple and not to scare the people with formulas, filters and gain calculations

But I see that learning how the circuit really works and gaining information to create your own designs is very interesting, so I will get some time to write something more technical and complete about the pedal hardware and why the things where designed like that.

In the meantime I can give you some directions:
1- The first op-amp of the input stage is basically an MXR Micro Amp, you can have a look to the analysis.
2- The second op-amp of the input stage is an inverting buffer with some caps (C5 and C4) to remove highs and lows.
3- Then we have two clamping diodes to protect the arduino DUE board from voltage peaks.
4- The 3rd and 4th op-amps are a differential and a summing amplifier respectively, t here is a GREAT book by Mancini explaining this topologies among others.
5- Then we have some bits and bolts, like a ±5 power supply with a TC1044 and some pots switches and leds connected to Arduino.

I hope this keeps you busy!!!

Looking at other projects getting guitar signal into an arduino, the signal in amplified, then a DC offset is required to move the centre voltage to 2.5V, allowing the Arduino to read the whole of the signal, right? Do the pedalShield not do this?

Yes, the input stage amplifies the guitar signal, gives some high/low pass filtering to remove unwanted frequencies and add an offset voltage.
The DC offset is needed, in the Arduino DUE the DACs work from 0 to 3V3, so the offset voltage is 1.65V(3V3/2).
This offset is created in the input stage by the voltage divider created by R19 and R20 and applied trough the biasing resistor R2.

I hope it makes all a bit more clear, some other boards (like Arduino UNO) use another biasing, to 2.5V because its ADCs work from 0 to 5V.

Hi,
The Arduino DUE ADCs input voltage is from 0 to 3.3V.
If you inject a signal lower than 0V or higher than 3.3V you could damage the ADCs, this is why the clamp diodes 1N5817 are placed.
The guitar signal needs a "virtual ground" of 1.65V (3.3V/2). The sound signal will "float" above and below 1.65V and the ADCs will read it without problems.