[SI-LIST] Re: 100nF AC coupling caps
- From: Hermann Ruckerbauer <Hermann.Ruckerbauer@xxxxxxxxxxxxx>
- To: scott@xxxxxxxxxxxxx, bbakshan@xxxxxxxxx
- Date: Thu, 08 Jan 2015 08:32:53 +0100
HI Scott,
is your concept using 0402 up to 16Gb only valid for "designed"
implementations (so optimizing the structure for a 0402 capacitor), or
also for a best guess implementation (just cutout the GND underneath the
pads) ?
or if one just uses some kind of best guess (without detailed analysis
of the structure) one should start 0201 earlier ?
Guess this "best guess" approacht is still used in most of the designs
out there..
Hermann
EKH - EyeKnowHow
Hermann Ruckerbauer
www.EyeKnowHow.de
Hermann.Ruckerbauer@xxxxxxxxxxxxx
Itzlinger Strasse 21a
94469 Deggendorf
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Am 07.01.2015 um 23:19 schrieb Scott McMorrow:
> All,
> For an 0402 capacitor, use pretty much any MLCC and you will be fine. In
> the configuration that a DC blocking capacitor is used, the important
> criteria is distance from the PCB to the lowest plate in the capacitor. An
> 0402 DC blocking capacitor in a differential configuration can be designed
> to be mounted on a PCB and have a return loss of better than -35 dB out to
> 12.5 GHz. Such a capacitor has essentially flat response. I just designed
> such a transition last week, and modeled the pcb, traces, pads,planes, full
> body of the capacitor and all of it's plates. Having done this a few times
> in the past, I can tell you that an 0201 capacitor can be designed to have
> about twice the bandwidth, and would definitely work up to 56 Gbps NRZ.
>
> For guardband against Murphy, I will run 0402 capacitors up to 16 Gbps, and
> switch to 0201 capacitor for 25+ Gbps designs.
>
> As a first approximation, in a board with two planes directly underneath
> the capacitors, make those two planes ground in the vicinity of the
> capacitors, stitch them with vias, and place a hole in the plane adjacent
> to the capacitor that extends under both pads. This will provide some
> amount of compensation, and get you to around -15 dB return loss out to 10
> GHz.
>
> best regards,
>
> Scott
>
>
>
>
>
>
>
> Scott McMorrow
> Consultant - R&D
> 16 Stormy Brook Rd
> Falmouth, ME 04105
> (401) 284-1827 Business
> http://www.teraspeed.com
>
> On Wed, Jan 7, 2015 at 4:53 PM, Boris Bakshan <bbakshan@xxxxxxxxx> wrote:
>
>> Hi Andrew,
>> For high rate signaling it won't matter that much if you choose 0201 caps
>> over 0402.
>> For 0201, the ESL of just the capacitor alone (excluding the artifacts
>> associated with the mounting on a PCB) is in the order of 400pH.
>> For shape of 0402, it is ~550pH.
>> Remember that the impedance of the bulk capacitance and the series
>> resistance (ESR) are negligible when it comes to high speed signaling.
>> Furthermore, you will not benefit from reducing the package but instead
>> what you should be doing is reducing the parasitic-shunt capacitance of the
>> 0402 structure (shape a void area in the plane underneath) and target the
>> effective impedance to match your transmission line.
>>
>> Hope it helped..
>> Boris Bakshan.
>>
>>
>> On Wed, Jan 7, 2015 at 8:32 PM, Andrew Holme <andrew@xxxxxxxxxxxxxxxxxx>
>> wrote:
>>
>>> Can anyone recommend by manufacturer's part number an 0402 size 100nF AC
>>> coupling capacitor for use in a DisplayPort Main Link at 5.4 Gbps? Many
>>> 0402 size 100nF caps have self-resonant frequencies in the tens of MHz
>>> range. We would consider 0201 devices but shy away from 01005. The VESA
>>> spec says min 70 nF to max 265 nF is the allowable range of values for AC
>>> coupling caps in the main link. I presume such large values are required
>>> because of DC wander?
>>>
>>> TIA
>>> Andrew.
>>>
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