On Sunday 10 October 2004 22:06, Garst R. Reese wrote: > The connection examples for these two supply voltages all show them > externally tied together. Can they actually be separate supplies with > different voltages? I assumed they could be, but got worried when I > could not find an example.
If you are actually using the analog facilities on the chip, performance will be poor if these are merely tied. My experience is that careful filtering of AVCC is necessary, both for the comparator and for the ADC12. Poor decoupling of DVCC is a wonderful way to get exotic, erratic, and non-repeatable bugs, as a look through the archives of this mailing list would show. If you don't need the analogue stuff, tie the supplies together and dcouple them as one. The AVCC and DVCC voltages should be nominally equal, though a difference of a few tens of millivolts seems not to matter. > >Murata, AVX, and Kemet all show 0805 10uF 6V 10% Ceramic capacitors. > >Would these require the extra 0.1uF? Yes. I've tried it. The large ceramics have enough intrinsic inductance to resonate at a surprisingly low frequency, and the 0.1uF takes care of high-frequency decoupling. No free ride here, I'm afraid. The 0.1uF must be as close to the chip as you can get it, but a centimeter or so of total lead length on the 10uF is OK. This can be used to advantage to "isolate" the two capacitors, so that you don't need separate traces to the chip pins, and it eases layout too. -- Rick Jenkins <[email protected]> Hartman Technica http://www.hartmantech.com Phone +1 (403) 230-1987 voice & fax 221 35 Avenue. N.E., Calgary, Alberta, Canada T2E 2K5
