Cheap Audio ADC with many channels - Page 1
Forum section: electronics / beginners

author: bryce1
author_rank: Regular Contributor
timestamp: July 16, 2016, 05:15:44 am
content: 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.Regardsbryce

author: Karel
author_rank: Super Contributor
timestamp: July 16, 2016, 06:21:15 am
content: Be aware that most Sigma-Delta ADCs, not intended to use in audio applications, suffer from "idle tones".Quotehttp://www.analog.com/library/analogDialogue/Anniversary/15.html

author: jitter
author_rank: Frequent Contributor
timestamp: July 16, 2016, 07:04:12 am
content: But that does not exclude audio AD conversion per definition.E.g. theCirrus Logic CS5368is 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.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 16, 2016, 07:46:04 am
content: 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.

author: bryce1
author_rank: Regular Contributor
timestamp: July 16, 2016, 12:26:06 pm
content: 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.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 16, 2016, 02:09:54 pm
content: 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.