Where's the analog ground pin on ATmega1281/2561?

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Regarding AVR devices that have an ADC, AVR042 (AVR Hardware Design Considerations) says, "The analog ground, AGND, is also separated from the digital ground – for the same reasons." Figure 122 on p.276 of the datasheet (12/05) implies that pin 63 is AGND, given the schematic and its placement between AVCC and AREF, but it's labeled "GND." Is this the AGND I should connect to my ground plane? Thanks.

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yes, I don't think any of the AVR's have an official AGND pin, they are always labelled as GND. But the one that is closest to the analog circuitry is certainly the one you want to use for AGND, sinice that part of the die will be most closely coupled to that ground connection. (also note that if you have a separate AGND it must to be coupled to GND through a single point connection)

Writing code is like having sex.... make one little mistake, and you're supporting it for life.

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Thanks for the confirmation. Any suggestions on minimum size for the analog ground plane? I was going to lay out a roughly CPU-sized rectangle under the CPU on the solder side of the board, but the size is just a guess.