[SI-LIST] Re: chassis ground connections
- From: daniel.bauer16@xxxxxx
- To: olaney@xxxxxxxx
- Date: Wed, 11 Feb 2009 09:07:31 +0100
>These two techniques are
>most applicable where there is no isolation from the power line or there
>are other reasons for floating PCB ground with respect to earth ground
that means, if the power and ground is isolated by an dc-dc converter it makes
not really much sence to connect both grounds via an huge resistor.
>A direct metallic connection is the best from the standpoint of EMI control.
and the use of capacitors will provide more ESD EMC protection (suggested from
many application notes for ethernet connections). I will try to use capacitors
with very low impedance (< 100mOhm) at all connectors... (together with an
option for zero ohm resistors).
*Von:* <olaney@xxxxxxxx>
*Gesendet:* 10.02.09 21:59:16
*An:* daniel.bauer16@xxxxxx
*CC:* si-list@xxxxxxxxxxxxx
*Betreff:* [SI-LIST] Re: chassis ground connections
Different grounding methods reflect different considerations. A
capacitor gives an RF ground (to a first approximation) and if properly
sized is effectively an open circuit at power frequencies and DC. A high
value resistor creates a path for static charge and low level leakage
currents so that the PCB isn't floating in a DC/LF sense with respect to
the outside world, but is still a high impedance (to a first
approximation) for all frequencies of interest. These two techniques are
most applicable where there is no isolation from the power line or there
are other reasons for floating PCB ground with respect to earth ground.
Sometimes you will see a resistor and capacitor used in parallel. A
direct metallic connection is the best from the standpoint of EMI
control. Assuming that the power supply is line isolated (the normal
case for anything with user accessible I/O connectors), a mounting screw
that contacts ground on the PCB and goes into a metal mounting post on
the chassis is sufficient from a safety viewpoint. Placement next to I/O
connectors maximizes the RF benefit, as do grounded connector shells
mounted tightly against the chassis wall, shielding, and lots of other
EMC stuff that goes well beyond the immediate question.
Orin Laney
On Tue, 10 Feb 2009 20:57:16 +0100 Daniel Bauer <daniel.bauer16@xxxxxx>
writes:
> Hi,
>
> could someone of you tell me the best way to connect the chassis
> ground to the supply ground of the PCB? I`ve read a few books and
> pdfs discussing this subject, but the methods are very different.
>
> The PCB contains two connectors to other PCBs, one ethernet
> connector and the power supply connector. Many manufactorer like Ti,
> SMCS have their application notes describing the way to connect the
> chassis ground for the ethernet connection. They suggest using an
> capacitor (2kV, approximately 1nF) and no direct connection between
> the supply - and the chassis ground. Micron also suggest to use one
> or two capacitors to connect them between the two grounds near by
> the transformer - for better esd protection.
>
> Johnson Howard wrote in one of his articles about this subject that
> each connector should have one direct connection (no use of
> capacitors) to the chassis ground in order to avoid antennas. It`s
> not possible to use both suggestion - direct connection / connection
> via a capacitor.
>
> Another point is that each capacitor has its impedance; but the
> impedance of the supply ground plane / chassis ground plane is very
> low. Could you give me some more information about the maximal
> impedance of the capacitor (when using an capacitor to connect both
> grounds on each connector and the power supply connector)? Is it
> maybe better to use an Pi filter (with one ferrite bead and one
> inductor)?
>
> Last week, I saw a pcb board using an isolated ground (for an rs485
> connector) with one big resistor (approximately 10MOhm) which
> connects both ground (isolated ground and the normal supply ground).
> Is this a common way to avoid big ground differences between these
> two grounds?
>
> best regards
> Daniel
>
>
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