I have in my project 1 analog input (camera microphone of my computer) and I am reading its value. But I need to convert this value to the dB scale, how can I do it?
I check the fórmula: 20*log(Vo/vi), but i dont know what to put in "vi".
You can help me?
dB scale is relative scale, voltage in decibels shows ratio between input voltage (Vi) and output voltage (Vo). So, you need to know, with which input voltage you're comparing your measured voltage, so you can calculate voltage ratios in dB.
In addition to what Sam Rudneff said, there are some "absolute" references that are often used in audio applications. If you're looking at microphone sensitivity, you might use dBV, which uses 1VRMS as a reference. Otherwise you may want to use a power reference liker dBu or dBm. dBu is a popular measurement in audio applications, which refers to the VRMS needed to output 1mW into a 600Ω resistor [√(1mW×600Ω) ≈ 0.775VRMS]. dBm is essentially the same, except you need to know the load impedence you are interested in. Do some googling, there are a lot of sites that talk about these things.