Hi - I'm currently working on designing a board around the Acam TDC-GPX time to digital converter chip. I've run into something which I found to be incredibly odd. They reccomend 47uf caps on almost every single vdd line. 12 in all. See page 54 (second to last page) of the datasheet for the drawing I'm referring to:
They reccomend use of Taiyo-Yuden LMK325BJ476MM, which is a 1210 47uf cap available at digikey at the price of $34.10/10 caps. Thus for a single board I would be using about $40 in just decoupling caps!!
Typically, when I design a board, I put a .1uf 0603 ceramic on every single supply pin of every IC, and then a single 10uf tantalum/ic on the more sensitive ICs. My understanding was that this was more or less the standard way to deal with decoupling.
So anyways - I'm starting to lay out this board - and these 12 1210s are really getting in my way. They are making my life very difficult. I'm trying to keep this as just a 2 layer board - but I'm not sure if I can do it with all these damn 1210s all over the place.
My question is this: are such huge capacitors necessary? Are they even worth it? The sales engineer for the distributor we purchased these chips through didn't seem to have a good idea one way or the other. My personal understanding was that it was better to have small but very low esr caps on the supply pins, as opposed to really large capacity, higher esr caps like these 1210s.
Thanks for any comments or suggestions,
-Mike