Protecting an ADC from negative voltage: Is it better use diodes OR to offset voltage?

Question

I'm working on a simple guitar amplifier and reader with Arduino. The Arduino will use this input to find the amplitude and frequency of the input signal.

I'm getting confused about how the ADC inputs should be protected from negative voltages. So far, there's I've found two solutions: Use a diode to stop negative voltages OR offset the voltage. Using a diode seems like it would be easier to calculate the frequency since the negative cycle is just 0V, but I'm afraid that the signal will get too distorted or weak. But I'm just not sure which is the correct one for this situation. Which is the better solution? And why?

This is my circuit using a diode on the output. Vout is across R3:

This is my circuit using offset voltage. Vout is the BJT collector voltage:

Answer 1

1) In that circuit, set the collector resistor == Rbase / beta.

This set the DC Vout to VDD/2.

That suggests 1,000 Ohm in the collector.

2) your circuit will have a gain of 0.5 * ( 5v / 0.026 ) = 0.5 * 5 * 39 = 100x

Thus more than 0.05v input will be clipping (in the rails);

3) for moderate (10%) distortion, keep the input voltage at 4 milliVolts or less

4) the ADC needs to grab a bunch of charge during each sample; you can evaluate the impact of these impulse_functions, by adding 10nF from collector to Ground.

Answer 2

Your output voltage (across R3 or at the collector of the transistor) can not be negative. You are powering the transistor with a 5V supply...there is no way to get a negative voltage at the collector of the transistor in this circuit.

Assuming that you have properly biased the transistor, and that you are using an Arduino operating at 5V, then no offset or diode is needed to prevent negative voltages.