[SI-LIST] Re: Noise on BGA core voltage rail
- From: ANAND KURIAKOSE <Anand.Kuriakose@xxxxxxxxxx>
- To: "Pfeifer, Alan" <apfeifer@xxxxxxxx>
- Date: Thu, 8 Aug 2002 08:51:30 -0700
Alan,
No, the bulk caps are electrolytic.
Anand.
From: "Pfeifer, Alan" <apfeifer@xxxxxxxx> on 08/08/2002 11:09 PM
To: ANAND KURIAKOSE/Apex Data Inc/01@Apex Data Inc,
si-list@xxxxxxxxxxxxx@SMTP@Exchange
cc: =20
Subject: RE: [SI-LIST] Noise on BGA core voltage rail
=A0=20
Anand,=20
Are the buld capacitors ceramic?=A0 If so, it might help to remove
these.=20
Alan=20
-----Original Message-----=20
From: Anand.Kuriakose@xxxxxxxxxx=20
To: si-list@xxxxxxxxxxxxx=20
Sent: 8/8/02 9:36 AM=20
Subject: [SI-LIST] Noise on BGA core voltage rail=20
Hi,=20
Heres a situation where i have a BGA (chipset with interface to=20
processor, DDR=20
memory and other high speed proprietry buses) with sufficient
decoupling=20
sprinkled around the BGA. The decoupling on the core voltage rail
(2.5V,=20
which=20
is also the I/O voltage for DDR interface)=A0 basically consists of 2
high=20
value=20
bulk capacitors, six 1uf caps, ten each of 0.1uF and 0.01uF caps. I
am=20
noticing=20
noise around 150mv of noise during activity on the DDR (using
software=20
utilities) and roughly 80-100 mV=A0 during almost no activity across
the=20
chip.=20
This amplitude is significantly more than the noise noticed at other
high speed=20
chips on the board. Using the FFT function on the DSO, i figured out
that the=20
frequencies where it peaks are 200Mhz and integral multiples of
200Mhz.=20
I tried a couple of things:=20
1>=A0=A0 Since i am seeing peaks at 200Mhz and its integral multiples, =
i
thought=20
that there could be insufficient high frequency decoupling and hence
i=20
replace=20
the=A0 0.1uF caps with 1000pF caps.=20
2>=A0=A0 Secondly, fearing that there could be some resonance =
happening
due=20
the=20
different values of caps used, i replaced all the 0.1 uF caps with=20
0.01uF caps=20
(in=A0 addition to existing 0.01uF caps).=20
Both the above strategies failed to reduce the noise. There was no=20
change in the=20
amplitude of the noise and also the frequencies where peaks were=20
noticed.=20
Can somebody out there throw some light on what is lacking in the=20
strategies=20
mentioned above and how to reduce this noise satisfactorily.=20
Thanx in advance.=20
Anand.=20
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