[SI-LIST] Re: 100nF AC coupling caps

  • From: Boris Bakshan <bbakshan@xxxxxxxxx>
  • To: andrew@xxxxxxxxxxxxxxxxxx
  • Date: Wed, 7 Jan 2015 23:53:55 +0200

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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