Re: [SI-LIST] : Signal Ground at the connector

ycchien (ycchien@alpha1.srrc.gov.tw)
Fri, 18 Dec 1998 09:23:52 +0800

Hello Vinu,
The EMI filter type I am trying to use is RC filter(CMD device) in the hope that it will provide some series termination in addition to filtering. The daughter card is inside the chassis.
What really bother me in applying the filter at the connector is the connection of filter ground and cable return ground connection. Many suggest that should both be connected to the chassis plane if there is one in the stack
because the signal ground is so noise that common-mode current will flow in the cable, which in turn will cause serious emission problem.

My question is if there is a chassis plane which is tightly capacitive-coupled to the signal ground, any pertubation on the signal ground will induce noise on the chassis plane(capacitive coupling), then what the difference does
it make to connect the filter and signal return ground to the chassis plane instead of to the signal ground?

Second, should there be no chassis plane in the stack for the cost reason and an external chassis plate or the chassis body itself is employed as "quiet ground", then how to make the connection between the filter/signal return
ground and chassis ground since they are located in the circuit board? and how to connect the signal ground to the chassis ground? I saw in Dr. Howard Johnson's book that discrete capacitor is not adequate to serve as the connection
between these two planes because of the parasitic inductance of the capactor.

Thanks for your time reading this

Eric Chien
Assistant Researcher
Synchrotron Radiation Research Center
Taiwan, R.O.C.
ycchien@alpha1.srrc.gov.tw

Vinu Arumugham wrote:

> What type of EMI filter are you planning to use? Is the daughter card inside or outside the chassis?
>
> Vinu
>
> ycchien wrote:
>
> > Hi,
> > I'm currently designing a circuit board with ribbon cable running
> > outward to another daughter board. The signals' tr/tf time riding on the
> > cable are approximately 2ns. To eliminate the RF energy enemating from
> > the signal line, EMI filter is considered to be inserted along the
> > signal line near the connector before it leaves the board.
> > After reading some articles on filtering the signal line, I'm still
> > confused at how to ground the filter. Most of the articles suggest
> > grounding the filter to the chassis ground not the signal ground and
> > then capacitively couple these two grounds. Several measures can be
> > adopted, they are
> >
> > A. Introduce a chassis plane to the layer stack, the filter ground is
> > connected through the via to the this chassis plane. But, where should
> > the signal return grounds of the cable be connected to? The chassis
> > plane or the signal ground plane?
> >
> > B. Moat the signal ground around the connector, leaving a wide piece of
> > copper to serve as bridge accross the moat. Reserve srew holes on the
> > ground plane of the connector part to provide a shorted path to the
> > external chassis plate, when low frequency isolation is not a concern.
> > However if low frequency isolation do matter, how to capacitively couple
> > this ground plate to external chassis plate?
> >
> > C. Moat the signal ground along the board circumference, with the
> > segmented ground plate along the circumference to serve as chassis
> > plane. The filter and signal return ground are connected to this plane.
> > The same question is encounterd as in item B.
> >
> > Any comments and advices are greatly appreciated?
> >
> > Eric Chien
> > Assistant Researcher
> > Synchrotron Radiation Research Center
> > Taiwan, R.O.C.
> > ycchien@alpha1.srrc.gov.tw
> >
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