[SI-LIST] Re: PCI Express AC Coupling

It is a theoretical question. I've seen plenty of systems with double caps, and 
like Steve, never have seen problems related to a charge buildup on the 
floating node. It doesn't seem enough of an issue that "best" design practices 
should include a drain resistor when it isn’t possible to guarantee the node 
won't float. Thanks to all responders.

-----Original Message-----
From: steve weir [mailto:weirsi@xxxxxxxxxx] 
Sent: Thursday, March 11, 2010 2:39 PM
To: Bowden, Ivor
Cc: si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: PCI Express AC Coupling

All capacitors have some leakage.  If you have a really clean PCB and the 
static builds up faster than the leakage can drain it off you might have an 
issue.  Although I have never heard of a failure actually traced to this 
mechanism, a 10K - 1M drain or shunt resistor should take care of any such 
concerns. The biggest impact of such a solution is the additional parasitics.  
In that respect, I think the drain resistor is easier to implement 
transparently than the shunt is.

Steve.

 
Bowden, Ivor wrote:
> I wonder about having 2 series capacitors in a signal line. Could the 
> floating node be subject to a static voltage buildup due to no DC path to 
> ground? Enough to damage the capacitor (ESD event for example)?
>
> -Ivor
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx 
> [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Michael Rose
> Sent: Wednesday, March 10, 2010 2:47 PM
> To: Chris Johnson; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: PCI Express AC Coupling
>
> Chris, I thought PCIe required AC coupling on both ends(?). Generally, I like 
> to place the coupling caps on the RX end since the channel attenuates the 
> slew rate making the impedance discontinuity a bit less severe. In some 
> cases, AC coupling on both ends has the advantage of tolerating a short 
> circuit through the interconnect without damaging the driver.
>
> Mike
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx 
> [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Chris Johnson
> Sent: Wednesday, March 10, 2010 4:37 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] PCI Express AC Coupling
>
> Could someone tell me if any of the TX (from motherboard to card), RX (from 
> card to motherboard), and REFCLK (from motherboard to card) PCI Express 
> differential pairs are guaranteed to be AC coupled on the motherboard?
>
> Also, is there significant signal integrity downside to AC coupling on the 
> plug in card, when the signals are already AC coupled on the motherboard, 
> assuming that the AC coupling is right at the I/Os to the chip on the plug in 
> card?
>
> Thanks,
>
> Chris
>
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