FreeLists / si-list / [SI-LIST] Re: Noise on BGA core voltage rail

[ANAND KURIAKOSE <Anand.Kuriakose@xxxxxxxxxx>]

        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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