Encounted a little problem, sure its easy to solve, I have an audio signal (2.5v) which I wish to feed into the ADC port pins of a pic32mx, now on the face of it it seems I need to attenuate the signal then add a DC level; shifting the waveform above zero. I believe I need to do this to keep it within the cofines of the 0v to 3.3v input?
Hi John,
You are correct, you're goint to have to add a DC bias to the signal, and make sure that the peak to peak voltage of the signal does not exceed 3.3V. Neither of the two requirements should be much of a problem.
First scaling, I don't know your signal source but assuming it from a sensor of some sort then you could lower the gain of your analog front end to make sure the input to the ADC will not have a peak to peak voltage greater than 3.3V.
Changing the DC Bias is also very simple you can do it using a simple RC circuit with a twist. I've included a picture to show you, first we have a series capacitor which is used to block any DC in your ac signal. Now instead of referencing the resistor to ground you reference the resistor to the desired bias level (1.65 is 3.3/2). Using superposition it is easy to prove that input to the ADC is the sum of your AC signal plus the bias voltage.
hth,
Jorge Garcia
