Hello!

I'm going to make an audio spectrum analyzer using AVR microcontroller and LCD dot display. The input signal sources of the device are intended to be computer or mobile phone audio outputs, which have output signal few milivolts peak to peak depending on volume set. ADC of the microcontroller will be set to 5 V reference voltage. I'm planning to put an op-amp after input signal to set the offset and amplify signal to max 5 V level. The problem starts when I change the sound volume of my laptop or mobile phone. As the input signal level of spectrum analyzer changes, the output of the op-amp changes too so the circuit can not maintan constant output peak to peak voltage. I found out that AGC (Automatic Gain Control) circuit should be used. Maybe someone has an experience similar to this situation? And could you please help me with choosing the right solution for this input circuit?

 

Google comes up with several AGC circuits. Here's one, for example.