FPGA Central

Hi all

Does anyone know whether the ac coupling caps for PCIExpress should be
located at the gold fingers or at the bridge chip. I have a layout in
front of me and putting the caps next to the gold fingers would make
the discontinuity just a little bit bigger, but putting them next to
the bga adds another seperate discontinuity. The only advice that I've
found says that they should go at the bridge but doesn't say why.

Any pointers appreciated.

Colin

I don't see why it would matter where the caps go. At the frequencies
of
the link they look pretty much like wires anyway. This is not the same
as
a series termination resistor.

The important thing is to use small parts that don't present a large
bump in the Zo of the transmission lines. I'm assuming this is
the discontinuity you speak of. If this gets large, then it makes
sense to keep it close to the driving end of the runs.

colin wrote:
> Hi all
>
> Does anyone know whether the ac coupling caps for PCIExpress should be
> located at the gold fingers or at the bridge chip. I have a layout in
> front of me and putting the caps next to the gold fingers would make
> the discontinuity just a little bit bigger, but putting them next to
> the bga adds another seperate discontinuity. The only advice that I've
> found says that they should go at the bridge but doesn't say why.
>
> Any pointers appreciated.
>
> Colin

Thats what i have found out, as for the caps the spec i have seen says that
they should be no bigger than 0603 and between 75nF to 200nF, if that is of
any help.

REGARDS IAN.

Home of the Low cost development board: RAGGEDSTONE1.
WEB: www.enterpoint.co.uk

"Gabor" <[email protected]> wrote in message
news:[email protected] ups.com...
>I don't see why it would matter where the caps go. At the frequencies
> of
> the link they look pretty much like wires anyway. This is not the same
> as
> a series termination resistor.
>
> The important thing is to use small parts that don't present a large
> bump in the Zo of the transmission lines. I'm assuming this is
> the discontinuity you speak of. If this gets large, then it makes
> sense to keep it close to the driving end of the runs.
>
> colin wrote:
>> Hi all
>>
>> Does anyone know whether the ac coupling caps for PCIExpress should be
>> located at the gold fingers or at the bridge chip. I have a layout in
>> front of me and putting the caps next to the gold fingers would make
>> the discontinuity just a little bit bigger, but putting them next to
>> the bga adds another seperate discontinuity. The only advice that I've
>> found says that they should go at the bridge but doesn't say why.
>>
>> Any pointers appreciated.
>>
>> Colin
>