I know this is an old thread, but I have a suggestion. You could try using a JFET-input op amp... I would recommend a specific one, but it's not from Analog Devices! Set it up in a non-inverting buffer configuration with the input signal from the guitar going through an AC-coupling capacitor to the non-inverting input. You'll want a pull-down to ground on the non-inverting input of the op-amp to set the impedance - 1 Megaohm should be good. That will give you a nice impedance converter and you can feed the output of the op amp directly to the voltage-to-current conversion resistor on the ADAU1701's ADC input.
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