I’m working on a board at the moment and am wondering something about bypass caps for one of the chips.
It’s an Analog AD6640, which is a 65 Msps A/D converter. The chip has six power pins per “plane” (analog VCC and digital VCC) and the 12 ground pins to go with them. For the most part, they are grouped (VCC,VCC,GND,GND,VCC,VCC,GND,GND…) on opposite sides of the chip. When placing bypass caps, would it be acceptable to place one cap per pair of power pins? If so, how would you change the cap size?
Typically you do want one bypass cap per VCC pin, as close to the pin as possible. The value you want depends on a number of factors (quality of power supply, number of elements being powered by the supply, distance to the pins, etc), but typically you want around .1 uF caps.
Looking at their example schematic, (pg 15 of the datasheet) they are using 6 .1uF bypass caps on the dvcc line, and three on the avcc line. Adding one more .1uF cap to the avcc line to have one per pin shouldn’t hurt anything.
Thanks. I’ll have to bang on that part of the design some more. Either that, or try my hand at soldering 0402 components. I’ve dealt with 0805 before, and am designing for 0603’s. I think I can make that jump easily enough, but not sure on smaller yet.
–edited–
I could only wish that the board layouts printed on page 16 weren’t so low resolution, and I wouldn’t have had this issue with reading it in the first place.
According to their reference design, there are 3x 0.1uF on the analog VCC, and 3x on the digital VCC. The others are for the 74LVQ00 and 74LCX574s (At least they are directly connected to them.)