In a message dated 11/24/2002 7:44:59 PM Pacific Standard Time,
bdewitt@xxxxxxxxxxxxx writes:
> Maybe someone can explain it, but I've never understood adding _any_
> additional impedance to a reference plane (call it "ground") connection.
> It
> seems I should always be aiming for the common impedance here for reference
> purposes. Creating an isolated power to reference current loop does seem
> to
> show advantage when both high speed digital and low level analog live on
> the
> same board. Am I missing something?
Right on target, Brent. Decoupled moats with power input PI-filters and
ground "bridges" (under the outputs) to the islands created are an effective
noise isolation mechanism for clocks, but increasing the impedance on the
ground connection (in my experience) has always increased the SI and EMC
problems. On the contrary, I have successfully reduced radiated emisions,
reduced jitter performance, and cleaned up other SI problems on over 200
PCBs for ~80 different clients over the past decade by using continuous
ground planes (along with several other of my tricks of the trade).
Mike
Michael L. Conn
Owner/Principal Consultant
Mikon Consulting
Cell: (408)821-9843
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