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Analog Front End for ADCs

Question asked by JoshuaK on Mar 7, 2013
Latest reply on Mar 8, 2013 by ColemanR

Hi all,


I am working on a small personal project. It will be a small DSP based mixer that will mix together a few different audio sources. I have decided on the SigmaDSP chip I want to use, and I'm now trying to work out the analog front ends needed to make it all go. The problem is I'm not very good at analog.


Here is a small summary of my 8 inputs:


Input 1 and 2 - In Ear Left and Right - These will take the feed from an in ear wireless pack. It's designed to drive headphone type loads, and according to the specs will do 100mW into 32 Ohms.

Input 3 - Intercom - Feed from an intercom beltpack. According to specs on one of the beltpacks I've seen, it'll do 60mW into 150 Ohms.

Input 4 - Radio - Feed from a two-way radio. I haven't been able to find exact specs, but this is the signal that drives the speaker mic, so I'm guessing it's meant for 8-16 Ohms at a decent power level. I generally have to keep the volume control pretty low when feeding this signal into something else.

Input 5 and 6 - Ambient - The idea here will be to have a couple of small mics clipped to me that can be selectively added to my mix to let me hear the room around me, even with my full block in ear monitors in my ear canals. They will probably be electrets (for ease of use and size).

Input 7 and 8 - Microphone in - Mic in for my voice. Only one will be active at a time. In my prototype the main one will be a dynamic. The second input is for an electret that may never be used, but I've got the input channel so I want to be ready for it.


I had originally planned on using an AD1938 and an AD1974 as with the combo I'd cover the 8 in 6 out that I need. Over on the SigmaDSP forum I was pointed towards the ADAU1977 as a lower power solution, which looks quite nice in some respects. I had considered using two of them. I'm also open to using the AD1938 and one ADAU1977. I'm not sure how best to feed my signals to these chips, which is what is delaying my decision.


I like that according the datasheet, the ADAU1977 can be directly coupled to the source. I'm having a bit of trouble understanding exactly the best way to do this, and which inputs are the best to choose for it. The datasheet talks a bit about electret mics, but outputting levels that I haven't seen before for electrets.


I'm considering going with one ADAU1977 for inputs 1 through 4, as they will be the inputs with the highest voltage swing. Then I will need to build some sort of op-amp preamp for inputs 5 through 8, to feed those signals into the AD1938. I have looked at the MEMS electrets that Analog offers, but none of them is what I'm looking for at the moment.


So, is anyone able to help me out, or at least point me to some example schematics? Low power and low parts count are nice, which is why I'm trying to avoid just copying the two op-amp input circuit that's in the ADAU1446 eval board and then adding additional op-amps to act as a mic preamp.


All help is appreciated!


Thank you,