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Very high resolution circuit
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nctnico:
I'd use a ground plane. Partial grounds can have currents running where you can't predict them. If the digital supply is filtered properly then it won't end up in the analog section (remember current flows in circles so if it can't flow in the supply, then it can't flow in the ground). A good trick is to use series resistors in the digital lines towards the ADC. Usually high resolutions ADCs have differential inputs. Connect the - from the input as close to the 0V of the voltage you are measuring so you basically do end up with a differential input.
iMo:
Interesting, especially the Figures 10,11,12..

https://www.maximintegrated.com/en/app-notes/index.mvp/id/5450
OM222O:
input protection doesn't matter here as I'm careful with using the device. I have SMD resistors from 100 ohms to 10K as well as 4.7u tantalums so I can experiment with the filter value (or if one is needed at all). the analog section is also fairly small so I'm guessing a ground plane would be good enough in this case?

Someone mentioned it's not a good idea to keep analog and digital sections on different sides of the PCB ... The manufacturers suggest that you keep them separate ... besides it acts as a natural star ground and I don't need to worry about digital return paths anymore. Also the final version would be a 4 layer board so there will plenty of decoupling provided between the signals. Currently I'm using a 47u tantalum right on the input section + lots of 100n local decoupling caps so I'm not really worried about supply noise either. I'm not sure why there is such a huge disagreement on different methods here but this thread has confused me even more XD I think I'll just test things for myself and find out.
DaJMasta:
Well typically with analog/digital planes, you split the pour with a small gap and then run the two under the chip, just keeping the input side on the analog plane and the output side on the digital - so not really two separate full planes, but one plane area split into analog and digital sections.


You may say you're not worried about power supply noise, but if you want 20 significant bits, you definitely will be  ;D  For a quantification, a million or so divisions (20 bits worth) at 0-5V full scale is just 5uV per division... and a decent linear lab power supply like the Rigol DP832 is specified for 350uV RMS and 2mVpp... meaning the peak to peak noise if coming from the power supply could lower your performance to about 13 significant bits.  Of course there's going to be some power supply rejection inherent to the ADC and some benefit gained from the reference voltage and low sample rate and using averaging.... but having an extra 30+dB worth of power supply noise rejection onboard could go a long way to actually getting your usable resolution close to the spec of the ADC.

My advice would be even if you don't want a common mode choke on the input, extra bulk capacitance, and local regulation for the analog section (if not both), add in the footprints, have a couple jumpers to bypass them, and build the boards like that.  Then if your first design without the extra power filtering stuff doesn't hit your performance target, you can add in the LDO or other filtering parts to see if they offer better performance.  I suspect they'll be needed, but doing the experiment yourself has its own value too.
Kleinstein:
For the signal filtering I would not use tantalum caps, as they have quite some possible leakage and dielectric absorption. So settling might take long. The better choice are film caps or NP0/COG ceramic capacitors. Unless the signal source is known low impedance, there is not much use in a large cap at the filter. It would be more about maybe having a two stage filter, to better suppress the really high frequencies from the RF range (e.g. GSM, WLAN).

A tantalum cap might be OK for reference filtering / buffering.

For a 4 layer board, one can have the analog and digital part directly on different sides, just have one closed layer of ground in between.

For the ground it is also important where the ground is connected to the outside world.

Power supply noise can become important, as the PSRR may no be that good. A common mode choke might be a good idea  - this could be for the input (and corresponding ground), but also for the supply and control.

At the low frequency 20 bit resolution is not as difficult as it sounds. If possible chose an aperture time (possibly manual averaging) that is a multiple of a power line cycle to suppress mains hum an related.
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