??? 11/21/08 13:46 Read: times |
#160175 - Microchip people seems careless Responding to: ???'s previous message |
Kai Klaas said:
You could even cut down the width of ground connection by moving the right edge of gap (directly next to the most right ferrite) slightly under the ferrites to the left.
Hopefully you mean to reduce the split area till the powersupply choke, as changed above. Kai Klaas said:
I say this, because pin 30 and pin 31, the oscillator pins, are rather close to this ground connection and the unavoidable spreading of corresponding ground return currents could add noise to the analog ground plane. Especially, if Ap plans to use only a double sided board, which cannot be recommended here at all, by the way.
Ap said: Maybe I may also answer this question? Ap, the pi-filter is a part that has to work up to and beyond 1GHz. Every millimeter of signal traces added or routed improperly between the involved parts will make the filter suffer. The reason, why the pi-filter caps must be so close to the ferrites, is, that the ground return currents between the two ground pins of involved caps form a loop, which must be as small as ever possble. Yes, you can also say, the ground pins of pi-filter caps must be as close to another as ever possible. So, when you plan to move the cap on analog side farer away from the ground connection by inserting jumpers, then you again risk to contaminate the analog ground.
Michael you made the diagram great, can we use a Jumper from the VDD filter caps till we reach the ferrite bead where we are making AVDD, just to add some more continuity to the Ground plane. Kai Its my honour Kai, Yes again as you point out the Oscillator pins orientation,which shows Microchip's carelessness with their MCUs ADC section.If you have a look at the Atmegas you wont find this , the Oscillator is on the extreme left side of the die. Even the Silabs people provide a NC pins around the oscillator. Regards/ Ap |