I'm laying out a 2-layer board (~5" x 5") for an ATmega2561 design and I'd like to use a ground plane for the ADC. I can't devote a whole layer to it so I was wondering if there's a minimum useful size. I was thinking of a square roughly the size of the CPU package, on the solder side just beneath the CPU, offset slightly to underly the ADC pins (I know about the single connection to digital ground). Is this adequate? Thanks.