I'm playing around with sigma-delat ADC and DAC for audio. It's amazing how good this works without any active components. Just Rs and Cs.
The question now is: Can I hook a speaker direct to the output pins of an FPGA?
The idea is to omit any passive components as the speaker acts as low-pass filter and to use two pins, feeding one with the inverted sigma-delta stream (or PDM), to build a bridge amplifier (class D?). With 3V3 output swing this should result (on an 8 Ohm speaker) in an output power of:
Peff = Ueff^2/R = (Up/sqrt(2))^2/R = = (3.3V/2*sqrt(2)/sqrt(2))^2/8Ohm = 340mW
First question: Is the multiplication with sqrt(2) correct? The idea is that it is a class D amplifier with rectangular output resulting in a sine wave that is sqrt(2) times higher than Up of the square wave.
Second question: Is the inductive load from the speaker an issue for the output drivers?
Martin