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The classic problem-Mixed Circuit isolation

Question asked by diogenes on Jul 30, 2012
Latest reply on Feb 26, 2018 by Pra@5456

Hi there, I am working with the AD74111 mono audio codec as part of a voice recognition system. I want to power the codec and the supporting microphone preamp from a 5 V power feed off the processor that uses the codec output. I drive the codec with separate LDO regulators on the digital and analog sides of the codec to minimize noise and take the 5 V power out to the microphone circuit. What I find is that when the codec is operating, it causes a large noise signal on the 5 V supply. Note that the system ground is unaffected by the codec. I tried to isolate the LDO regulator by placing several ferrite chips with bypass capacitors on the input side of the regulator to filter the input 5 V and bypass the noise back to the regulator but this doesn't seem to work. My question: Given that the codec is driving noise back through the regulator onto the 5 V source line, what is the best way to isolate the codec/regulator from the microphone source? Any suggestions or guidance?