[SI-LIST] Re: Analog/Digital Gnd
- From: "Bart Bouma" <bart.bouma@xxxxxxxxx>
- To: weirsp@xxxxxxxxxx
- Date: Thu, 13 Mar 2003 08:47:46 +0100
Steve and Jack,
did you guys ever try to use X2Y caps? They have excellent CM suppression
due to two (2) balanced caps with extreme low inductance.
Low inductance because of opposing currents inside the component. They do
a good job at EMI-filtering of connectors, upto several GHz's.
Two (2) connector pins can be filtered with one (1) X2Y part.
Bart
www.yageo.com
Jack,
In a small product like a Walkman, you can think about an AC short near
the connectors. But in something large like a 19" chassis, that will
almost always be inadequate. Every aperture, including the slots that
form
at the card edges, ejectors, ventilation, etc. provides an antenna for the
CM between the signal and frame. I have seen data where the BeCu fingers
used to short adjacent cards become ineffective around 1GHz. So to get
rid
of the radiation, you need to get rid of the CM by shorting the HF
potentials in many locations.
Regards,
Steve.
At 10:36 PM 3/11/2003 -0800, Jack Stone wrote:
>Steve,
>
>Are'nt there special suppressors other than 180/220pf caps
>that can be used in this case? Usually placed as close as
>possible to the suspect connector?
>
>jack ~
>
>--- steve weir <weirsp@xxxxxxxxxx> wrote:
> >
> > Hariharan,
> >
> > Frame ground is a different beast. Depending on the approach taken to
> > powering the device, frame ground will often, ( as in PC's and most
> telecom
> > equipment ) be bonded to the signal ground in multiple locations in
order
> > to minimize EMI radiation. Exceptions to this rule occur when frame
and
> > signal ground are DC isolated as in some telecom gear due to safety
> > considerations. However, in these cases, the manufacturer will
usually
> > still do their best to achieve tight AC coupling for EMI suppression.
> >
> > Regards,
> >
> >
> > Steve.
> > At 11:16 AM 3/12/2003 +0530, hariharan wrote:
> > >Does Frame Ground (FGND) in a product follows the same rules for
> AGND/DGND?
> > >
> > >Hariharan
> > >
> > >-----Original Message-----
> > >From: si-list-bounce@xxxxxxxxxxxxx
> > >[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of steve weir
> > >Sent: Wednesday, March 12, 2003 4:03 AM
> > >To: shane.san.miguel@xxxxxxxxx; si-list@xxxxxxxxxxxxx
> > >Subject: [SI-LIST] Re: Analog/Digital Gnd
> > >
> > >
> > >
> > >Shane,
> > >
> > >PLL's usually make up a special case. The corollary to: "Follow the
> > >money." is: "Follow the current."
> > >The supply for a PLL can be effectively isolated because significant
high
> > >speed currents do not need to flow between the PLL and the digital
world
> > >outside of the chip. By isolating the connection on the outside, the
PLL
> > >rides on the digital ground potential inside the chip where both
grounds
> > >are most definitely connected, and fast digital currents do not
traverse
> > >the PLL ground node inducing noise.
> > >
> > >Now, contrast that to the VDAC problem just mentioned. There, high
speed
> > >currents definitely flow between the analog and digital. By
inserting
> > >impedance between the two "grounds", significant common mode voltages
> > >develop showing up as the bounce reported. That's not a good thing.
> > >
> > >The key here is to control where currents flow, and keep currents
from
> > >taking unwanted paths and disturbing victims we need to protect.
Often,
> > >this is a matter of placement and routing. In other cases, moating
can
> > >prevent closure of unwanted secondary current loops.
> > >
> > >Regards,
> > >
> > >
> > >Steve.
> > >At 01:30 PM 3/11/2003 -0800, San Miguel, Shane wrote:
> > >
> > > >I am for sure not the expert on this but in my experience, you want
=
> > > >analog isolated to take advantage of extra filtering. Usually I
run =
> > > >into a seperate analog gnd when there is a PLL involved. You have
to =
> > > >keep the digital supplies away from this so there is nothing to
> confuse =
> > > >the PLL.=20
> > > >
> > > >Shane San Miguel=20
> > > >
> > > >
> > > >-----Original Message-----
> > > >From: Parthasarathy Sampath [mailto:parthsv@xxxxxxxxx]
> > > >Sent: Tuesday, March 11, 2003 12:44 PM
> > > >To: si-list@xxxxxxxxxxxxx
> > > >Subject: [SI-LIST] Analog/Digital Gnd
> > > >
> > > >
> > > >
> > > >Hi All,
> > > > Why is that Analog ground and Digital ground are
> > > >kept seperate during Layout? I have even noticed in
> > > >some cases, seperate power rails are provided for
> > > >analog and digital block even though they require same
> > > >VDD.
> > > >
> > > >Thanks in Advance,
> > > >Partha!
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