Cheap Audio ADC with many channels

[bryce1]

Hi guys,

I need to have 32 channels to capture audio. My problem is that most "real" audio ADCs are to high spec (16bit upwards, 44khz) while I only need 12bit and 22khz.

Do you guys have any recommendations on how to do this in the most cost effective way?

My assumption is that I would probably need to substitute the audio ADC for some differential Sigma-Delta ADC, but I am not sure.

Regards
bryce

[Karel]

Be aware that most Sigma-Delta ADCs, not intended to use in audio applications, suffer from "idle tones".

Most sigma-delta converters exhibit some spikes in the noise floor, called idle tones.
In general, these spikes have low energy, not enough to substantially affect the S/N of the converter.
Despite that, however, many applications cannot tolerate spikes in the frequency spectrum that extend
much beyond the white noise floor. In audio applications, the human ear, for example, does an
excellent job of detecting tones in the absence of large input signals even though the tones are well
below the integrated (0-20-kHz) noise of the system.

http://www.analog.com/library/analogDialogue/Anniversary/15.html

[jitter]

But that does not exclude audio AD conversion per definition.
E.g. the Cirrus Logic CS5368 is an 8 channel SD-ADC aimed at audio applications. No doubt if you google a bit more, you can find SD audio ADCs with more channels and lower sample rates.

[Kleinstein]

Audio ADCs are usually quite cheap, because of high volume production. So you get 2 or 4 channel version in the $1-$4 range. They usually come as 16 Bit (or more) and something like 48 kHz, but many can run slower as well.
Most of them are sigma delta, and as quality is not that important, I would not care about idle tones - this might be a thing of high end audio speculations. A nice thing about sigma delta converters is, that they simplify (nearly eliminate) the anti-aliasing filter a lot - this alone is an important point if you want low cost. Besides audio there are some cheap ADCs made for mains power measurement (e.g. MCP39xx).

If 12 Bits is good enough, one could in principle use µC internal converters (e.g. ARM M3 based). But the need for AA filtering might be the problem. So even with sampling at 40 kHz (to easy on AA) one could get something like 16 channels from a single chip.

With the 32 Channels, an important question is, if the all channels are needed active at the same. Also the interfacing circuit might be important. More channels in one chip might help here to keep the interface simpler.

[bryce1]

In theory, I only need 8 channels to be active at any given time. I have not figured out a way though to "choose" the channels automatically.
More resolution is nice, but I plan on using full speed USB, so the 12bit constraint actually helps to get the data rate down to something that allows me to deliver all 32 channels over USB. More than 12bit does not do me any good.

[Kleinstein]

For choosing the active channel one can use a MUX, on the low cost end this could be something like 74HC4051.  Some ADCs already include a MUX to allow for more channels, but only one active at a time or with interleaving, reducing the sampling rate.

With only 8 active channels at a time, one could get away with the ADC inside an µC like STM32F3xx. I don't know if they have some with 32 possible analog inputs, but it is possible. For 8 active channels it possible to sample at something like 160 kHz and thus do oversampling to easy on the AA filters.