[SI-LIST] : Awkward analog power filtering

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From: Chris Bobek (cbobek@cadence.com)
Date: Thu Aug 17 2000 - 16:23:39 PDT


Hi All,

We are using a mixed-signal framer BGA for 4 ports of T-1.
The vendor (who shall remain nameless) provided a reference
design that shows a very unusual scheme for filtering power to
their device.

The device has about 20 digital power pins, which are to be
directly connected to the power plane and decoupled using
standard 0.1uF caps. The device also has about 20 analog
power pins. These are divided into:

- transmit power for each of the four ports
- receive power for each of the four ports
- misc power for each of the four ports
- generic analog power

The ref design shows an RC network for each group/port
combination above, so that's 16 RC power nets! Each RC net is
a 4.7ohm resistor, 47uF cap, and 0.1uF cap. This becomes an
issue because the package is a BGA. That means I'll have to
snake these power traces to each of the pins. I think I can
make the traces pretty wide, but it won't be as nice as a
plane.

I proposed a "better" solution to them which is to have a
single analog power plane under the BGA. Then, we'll have a
few of the 47uF caps and a bunch of the 0.1's. The analog
plane would be tied to the digital plane through a ~1ohm
resistor (that can handle the current). This scheme is a lot
more popular and I think it achieves a better power supply
system (less L between pin and caps) and a lot less board
space. They didn't "ok" this solution because they only
tested their device per the recommendation. They can't defend
the recommendation, so that's why I thought I'd ask everyone
on the list if they've seen something this elaborate before
(especially for something ~ 1.5MHz).

Thanks,

Chris

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