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Problems with internal temperature sensor - MCP3561

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Dave_PT:

--- Quote from: uer166 on July 14, 2021, 11:01:41 pm ---Filtering power is fine, what I meant is, splitting the ground planes that are not galvanically isolated is a not fine. This has been discussed ad-nauseam here in other threads, but basically the datasheet here is plain incorrect, you will verifiably get worse performance if you split the ground planes per their recommendation. Maybe it was something an intern wrote that kept being copy/pasted, maybe they just don't bother to fix it or care, but in such a mixed signal system as yours you'd be much better off keeping a solid ground plane, separating the analog/digital signals, ensuring all traces have a reference plane underneath, and filtering AVCC if you wish at the point of load.

--- End quote ---

It's not worth opening the discussion here too much, but separate plans are recommended by several manufacturers and have their supporters.
It's like having a star shaped ground.
One example I've worked with recently was TI's ADS1299. A frontend for EEG measurement and the recommendation is also to have separate ground, even following the EVB design.

I'm not completely disagreeing with you, I understand what you're saying too, but it makes some sense to me the use of separate grounds.

MasterT:

--- Quote from: Dave_PT on July 15, 2021, 07:27:53 am ---
--- Quote from: MasterT on July 14, 2021, 10:41:32 pm ---Probably, you are referring to MCP6V91 voltage follower, but there is no capacitive load. I see C29/C30 and it's make difference. What capacitance over there, on Vref, Vdd & Vaa?

--- End quote ---
Vref = 10nF, after the voltage follower and next to the MCP3561 pin. The noise level is ~3mVpp.

--- End quote ---
You can't put a cap at the OPA output, read DS mcp6002 - paragraph 4.3 Capacitive Loads
This is always a problem,  I know only a few specifically designed OPA that may handle 10uF w/o series resistance.
Setting series resistance with common OPA creates an error. Probably, offset may be calibrated out for constant sampling rate

Dave_PT:

--- Quote from: MasterT on July 15, 2021, 01:15:54 pm ---You can't put a cap at the OPA output, read DS mcp6002 - paragraph 4.3 Capacitive Loads
This is always a problem,  I know only a few specifically designed OPA that may handle 10uF w/o series resistance.
Setting series resistance with common OPA creates an error. Probably, offset may be calibrated out for constant sampling rate

--- End quote ---

Yes sure.
I was going to put an RC of 100R + 10uF, similar to what Microchip has in the voltage ref app note.

Dave_PT:

--- Quote from: Dave_PT on July 15, 2021, 02:17:18 pm ---Yes sure.
I was going to put an RC of 100R + 10uF, similar to what Microchip has in the voltage ref app note.

--- End quote ---

At first glance it's hard to tell apart, but I would say the end result is slightly worse.
Same test condition, result in a noise ADC count of 644.

floobydust:
There is the MCP3x6x Rev. B silicon errata with Data-Ready work around, looks like you're supposed to use the newer MCP3561R  :palm:

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