From: Krishnakumar (firstname.lastname@example.org)
Date: Mon Mar 12 2001 - 08:12:47 PST
i am having this problem, can anybody help?
we have a new system with a new type of backplane. this backplane is a
virtual back plane - it is made up of strips - one strip for each card.
power distribution is through cables. we distribute -48V, -48V RETURN, and
Digital GND through cables. all cards get frame ground through the screws.
our cards have 48V to 5/3.3V DC to DC converters. we have soft start
circuitry on the cards, and have good (LC) filters on -48V. i believe we
have good decoupling on 5V and 3.3V. we have stacked pins for power pins.
cards get Frame GND first, then Digital GND, then -48V return and then -48V.
now this is the problem. i get data errors on one card when i plug in
i made the following observations:
1. the error seems to reduce if i apply power to the board using a banana
jack rather than plugging in the board.
2. adding inductors on the Digital Ground lead does not help. (we already
have inductors on -48V and -48V return)
3. scope readings show big ringing on almost all signals - N48V Return,
Digital GND, 3.3V, 5V. there is no ringing on N48V as such(surprise to me).
but i donot believe in the scope readings. scope shows ringing on Digital
GND with respect to Digital ground itself. Looks like it is measuring noise
on some big ground loop. (i am using a scope with the Ground pin removed
from the power cord.) the amplitude of ringing, for ex., on 3.3V, is 7V to
0V, dying down in about 100-500us, the ringing frequency is 50MHz. should my
first attempt be to kill this ringing?
can anybody suggest some thing to help me out?
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