I am trying to design an op-amp input stage to capture a guitar through an ADC, specifically the ADC on an Arduino Due (3.3V, 12 bit). I am aware there are numerous ways to configure and bias the op-amps. I have designed a simple circuit based on a inverting amp followed by an active LP filter. The system will be running off a 9V battery and the power will be provided to the op-amps and uC (regulated) from this.

The aim being:


Bias the signal around Vdd/2 (mid of ADC input range)
Amplify the signal to use best of the ADC's range
Lowpass filter the input signal at Nyquist rate (fs/2) to avoid aliasing


However, I have been  unable to find much information on what the benefits or drawbacks of different configurations and methods of biasing are, specifically for this kind of signal. I have not yet measured the voltage output of my guitar so the specifics of the required gain properties are not known yet! Aslo, will I require a high input impedance for a guitar?

Any advice or comments would be greatly appreciated.







simulate this circuit – Schematic created using CircuitLab

It is generally a good idea to include a gate resistor to avoid ringing. Ringing (parasitic oscillation) is caused by the gate capacitance in series with the connecting wire's inductance and can cause the transistor to dissipate excessive power because it doesn't turn on quickly enough and hence the current through drain/source in combination with the somewhat high'ish drain-source impedance will heat the device up. A low ohm resistor will solve (dampen) the ringing.

As @PhilFrost mentiones, a high value resistor to ground is a good idea to avoid capacitive coupling driving the transistor when it is otherwise not connected.







simulate this circuit – Schematic created using CircuitLab

At all times keep the wiring between logic output, transistor gate, transistor source and ground as short as possible. This will ensure fast turn on/off.
