We selected the 1N5187
because they are cheap, easy to find and they were used in similar applications
with good results.
These four 1N5187 diodes are low-voltage-drop Schottky clamping diodes
. The clampling diodes will prevent the input guitar signal to be more negative than –0.24V and more positive than 3.54V.
The Arduino Due ADCs can work with signals from -0.3 to 3.6, but overcoming these limits can damage the ADCs.
The series 4K7 resistors R9 and R10 will limit the current though the diodes, lowering the diodes VF to a good value to protect the ADCs. At the same time these resistors values are low enough to keep a good source impedance for the ADC (remember that the Atmel ATSAM3X A/D datasheet
page 8 mentions that the source impedance must be lower than 10K)