Possibility of using Audio codecs for measuring DC stuff

[Wimberleytech]

"Silicom" Labs  Too funny.

[Berni]

Running the ADC from a stable supply voltage wont solve everything. The internal reference at least won't be affected by supply variation but it is still affected by temperature and its possible that it slowly walks around on its own even.

As for audio ADCs handling DC i have put DC in to some of the higher end chips and they take it just fine, but they did have a configuration pin for turning off the internal high pass filter.

The only way to really know is to take a a few ADC chips and test it with the sort of input your application might provide it. Most audio ADC chips are easy to run where you only need to give it a clock and it spits out data so you could just bodge wire one on a SMD breakout board and hook it to a scope with I2S serial decode to display the data. One good measurement is worth a 1000 expert opinions.

[coppice]

As for audio ADCs handling DC i have put DC in to some of the higher end chips and they take it just fine, but they did have a configuration pin for turning off the internal high pass filter.

The problem with audio ADCs and DACs is not whether they can handle a DC signal. Almost all of them can. Its the huge DC offset you typically get. If this were rock solid you could probably calibrate away most of it, but it isn't always that stable.

[b_force]

Wasn't it that the topology used in audio ADCs is not working for DC signals, cause they keep 'hunting' or something?

[coppice]

Wasn't it that the topology used in audio ADCs is not working for DC signals, cause they keep 'hunting' or something?

Pretty much anything with a feedback loop has some amount of hunting going on. However, a sigma-delta converter starts out with a very high 1, 2, or whatever bit sampling rate, and decimates down to a much lower 16, 24, or whatever bit sampling rate. The great majority of the noise due to hunting, which is like a rapid minor oscillation in the pre-decimated domain, is way beyond the Shannon rate of the converter, and gets filtered off during decimation.

[Kleinstein]

Some DAC / ADCs might show rather large idle tones at DC. Some ADCs (don't know about DACs) do remove DC and thus won't work with DC at all.  Many audio codecs also need a constant flow of data, even if constant values are wanted. Usually the gain and offset drift is not that good, but often still not very bad.

As audio usually is at least 2 channels one could use one ADC channel to monitor an external reference and thus way tie the data to an external reference even if the ADCs/DACs use an internal one only.

For the ADCs there are some alternative, relatively cheap ADCs (e.g. MCP39xx series) with reasonably DC specs.

[b_force]

Some DAC / ADCs might show rather large idle tones at DC. Some ADCs (don't know about DACs) do remove DC and thus won't work with DC at all.

This was actually what I was referring to.

I personally don't really get the issue, there are plenty of affordable 12 and 16 bits ADCs for DC signals.
Even 24 bit it not that much more.

[Kleinstein]

The ADC part is easy and also affordable. The MCP3911 is affordable (e.g. $2-3  range) and in many respects similar to the Audio ADC, but with good DC specs too.

The more difficult part is the DAC - essentially all cheap 16 Bit DACs are audio. At least I have not found a SD 24 Bit DAC with good DC specs and a low price. I am sure some people would like to have some, even if the price is not that low.

So it is a very legitimate question if there are some of the audio DAC  (or complete codecs) that do have good enough specs to make them attractive for DC use.

[coppice]

The ADC part is easy and also affordable. The MCP3911 is affordable (e.g. $2-3  range) and in many respects similar to the Audio ADC, but with good DC specs too.

The more difficult part is the DAC - essentially all cheap 16 Bit DACs are audio. At least I have not found a SD 24 Bit DAC with good DC specs and a low price. I am sure some people would like to have some, even if the price is not that low.

So it is a very legitimate question if there are some of the audio DAC  (or complete codecs) that do have good enough specs to make them attractive for DC use.

Put an audio DAC in the mode where it can output DC. Feed its output back to a spare ADC input, measure the error in the resulting output signal, and correct the value fed to the DAC. Monitor for drift, and update as needed.