[SI-LIST] Re: 100nF AC coupling caps
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: Zheng Edison <edisonzheng@xxxxxxxx>
- Date: Thu, 8 Jan 2015 10:32:32 -0500
Impedance of a capacitor is
Z = 1/(2 x pi x f x C)
Capacitor feeds a 50 ohm transmission line
Capacitor and transmission line form a voltage divider
Vout = Vin x (50/(Zcap(f)+50)
charging time constant of a capacitor
t = 1/RC = 1/(50*C)
These equations along with the spectrum of your serial bit stream tell you
everything you need to know.
As eric bogatin says: "Do the math."
Scott McMorrow
Consultant - R&D
16 Stormy Brook Rd
Falmouth, ME 04105
(401) 284-1827 Business
http://www.teraspeed.com
On Thu, Jan 8, 2015 at 10:05 AM, Zheng Edison <edisonzheng@xxxxxxxx> wrote:
> Hi Scott,
> in most applications,the value of 0402 ac coupling cap is 100nf,my
> question is why 100nf?is 220nf ok?What is the difference?
> Edison
>
> --- Original Message ---
>
> From: "Scott McMorrow" <scott@xxxxxxxxxxxxx>
> Sent: January 8, 2015 8:39 PM
> To: "Carson Au" <carson.au@xxxxxxxxx>
> Cc: edisonzheng@xxxxxxxx, "Boris Bakshan" <bbakshan@xxxxxxxxx>,
> andrew@xxxxxxxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: 100nF AC coupling caps
>
> Carson,
> If you have a 2D simulator available, as a first approximation take your
> capacitor pads and join them together into one trace that is 20 mil wide
> and 60 mil long, and insert into your 50 ohm controlled impedance trace
> that is somewhere around 4 or 5 mil wide. Perform a simulation with your
> traces, connectors and packages in the circuit and see what happens.
>
> As for "free", engineering is a non-recurring expense, and as such is free
> with regards to manufacturing costs. Of course, it is not free in terms of
> capital expense if you do not have the appropriate tools at hand. That is
> why I gave a very simple rule of thumb, which is to relieve the plane under
> the capacitors ...with the caveat ... if it is possible to do so without
> disrupting other signals, and if there are two planes directly under the
> top layer. IF there is only one plane, then the problem just got much more
> difficult. If the 2nd plane is a power plane, just provide a ground fill in
> the region underneath the capacitors. These are really simple things to do
> for "free margin", without the time and expense of using a $100K full wave
> 3D field solver.
>
> best regards,
>
> Scott
>
>
>
>
>
>
>
> Scott McMorrow
> Consultant - R&D
> 16 Stormy Brook Rd
> Falmouth, ME 04105
> (401) 284-1827 Business
> http://www.teraspeed.com
>
> On Thu, Jan 8, 2015 at 6:58 AM, Carson Au <carson.au@xxxxxxxxx> wrote:
>
> > Hi Scott,
> >
> > I do not doubt that impedance compensation of the pad will improve
> channel
> > margins. However, compensating for this is not quite free margin - there
> is
> > considerable difficulty in implementing this on lower end EDA tools (let
> > alone working out the appropriate compensation using a 3D field solver
> > tool). The ground copper removal could also disturb the return path of
> > signals on the opposite side of this plane.
> >
> > I guess my question is - when is pad compensation critical? What does it
> > depend on?
> >
> > For example, a typical pad on an 0402 component could be 0.6mm in length.
> > Wavelength of the nyquist frequency of a 10Gb channel on a PCB would be
> > approximately 33.4mm (200ps/6ps/mm) (I acknowledge that higher frequency
> > content would exist, but this is just a quick estimation). Is the
> impedance
> > discontinuity of 0.6mm in comparison to the nyquist wavelength
> > insignificant?
> >
> > Regards,
> > Carson
> >
> > On Thu, Jan 8, 2015 at 10:38 PM, Scott McMorrow <scott@xxxxxxxxxxxxx>
> > wrote:
> >
> >> Carson
> >>
> >> I'm not sure what you are asking. We can certainly document that
> >> compensation of capacitor transitions improves performance in Serdes
> >> channels. That is not up for debate any more than tight impedance
> control
> >> is. Whether there is a specific design study that says a system failed
> due
> >> to just a cap ... who knows. Systems fail for all sorts of reasons, and
> >> most of those reasons are the interaction between reflections across
> >> multiple components. Optimization is free margin. Once done, the
> margin
> >> is always there and costs nothing to implement. Optimization of
> structures
> >> always provides better system performance.
> >>
> >> regards
> >>
> >> Scott
> >>
> >>
> >>
> >>
> >>
> >>
> >> Scott McMorrow
> >> Consultant - R&D
> >> 16 Stormy Brook Rd
> >> Falmouth, ME 04105
> >> (401) 284-1827 Business
> >> http://www.teraspeed.com
> >>
> >> On Thu, Jan 8, 2015 at 6:30 AM, Carson Au <carson.au@xxxxxxxxx> wrote:
> >>
> >>> Are there studies/papers on the practical effect of not compensating
> for
> >>> the pad impedance in SERDES channel design? Has there been any
> documented
> >>> channel failures due to not compensating for these pad impedances?
> >>>
> >>> On Thu, Jan 8, 2015 at 9:53 PM, Zheng Edison <edisonzheng@xxxxxxxx>
> >>> wrote:
> >>>
> >>>> Hi,experts
> >>>> What is the difference between using 100nf caps and using 200nf
> caps(or
> >>>> some other values) in serdes links?thanks.
> >>>> HW
> >>>> edison
> >>>>
> >>>> --- Original Message ---
> >>>>
> >>>> From: "Scott McMorrow" <scott@xxxxxxxxxxxxx>
> >>>> Sent: January 8, 2015 6:20 AM
> >>>> To: bbakshan@xxxxxxxxx
> >>>> Cc: andrew@xxxxxxxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
> >>>> Subject: [SI-LIST] Re: 100nF AC coupling caps
> >>>>
> >>>> 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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> >>
> >
>
>
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