[SI-LIST] Re: AC cap placement on Clocks
- From: wolfgang.maichen@xxxxxxxxxxxx
- To: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- Date: Thu, 22 Oct 2009 10:15:28 -0700
Hello Sam,
an ideal capacitor will be completely transparent and high-frequency
signal (down to the cutoff frequency - the capacitor C in combination with
the Thevenin-equivalent line impedance of 2*Zo form a high-pass filter
with a time constant of 2*Zo*C). So from that standpoint it does not
matter at all where you place it, as Lee already stated.
Of course a real capacitor always has some parasitic package and mounting
inductance, so if you go to very high frequencies or data rates
(multi-Gbit/sec) you will end up seeing reflections caused by that.
Although at your speeds (below a GHz) that won't be an issue unless you
reeally mess up the design (e.g. use a through-hole capacitor instead of a
good surface mount ceramic one). In that case it will be better in improve
your design rather than try to find a "sweet spot" for the placement which
will make your design very sensitive against any changes (e.g. line
length, other parasitics). Failing that, the best bet is probably to place
it very close (within ~1/4th of the shortest wavelength of interest, given
by the frequency 0.33/rise_time) to either your driver or your receiver.
The second consideration would be whether you put in the AC coupling
purely for signaling reasons (e.g. to avoid debiasing driver or receiver),
or whether it shall also act as protection. If e.g. it shall protect the
driver against short circuits (e.g. if driver and receiver reside on
different boards that get hot-plugged together), it may be better to place
the cpacitor on the driver side. But that willd depend on the specific
design.
Wolfgang
"Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
Sent by: si-list-bounce@xxxxxxxxxxxxx
10/22/2009 06:39 AM
To
"icer world" <icermail@xxxxxxxxx>, "Sam Pete" <cygnul@xxxxxxxxx>,
si-list@xxxxxxxxxxxxx
cc
Subject
[SI-LIST] Re: AC cap placement on Clocks
Form an SI point of view, it does not matter where along the path the
capacitors are placed.
> [Original Message]
> From: icer world <icermail@xxxxxxxxx>
> To: Sam Pete <cygnul@xxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
> Date: 10/21/2009 8:48:26 PM
> Subject: [SI-LIST] Re: AC cap placement on Clocks
>
> It's hard to say where the AC cap should be placed ,so you'd better do a
simulation if you have device models .
> I'm in doubt that why you use the AC coupling manner since the driver
and
the receiver are both LVDS level.
>
>
>
> ________________________________
> From: Sam Pete <cygnul@xxxxxxxxx>
> To: si-list@xxxxxxxxxxxxx
> Sent: Thu, October 22, 2009 7:22:17 AM
> Subject: [SI-LIST] AC cap placement on Clocks
>
> Hi All,
> I have a situation like this:
> LVDS Driver, ac cap, LVDS receiver (internal term) 156MHz clock.
> |>-----------------||----------------|>
> |>-----------------||----------------|>
>
>
> What is the optimum place to put coupling cap when the clock is
ac-coupled.
> Should it be close to driver or should it be close to receiver.
>
> From my understanding, the discontinuity should not be visible to the
electrical length of the signal. Having said that, ac cap should be as
close as possible to Driver.
>
> please share your thoughts.
>
> Thanks
> Sam
>
>
>
>
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