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Protecting high impedance sections of PCB with metallic Sheets ratherthan metallic molds

P

PureSine

Jan 1, 1970
0
Hello,
In many sensitive devices/ instruments which have a sensitive high
impedance section like a high impedance amplifier(10^12 Ohm or even
higher) or other high impedance signals, I've seen they are protected by
a metallic mold(mostly copper) that sits on top of sensitive parts and
tracks and is usually screwed to the PCB. Such metallic molds which are
specific to the shape of PCB seems very expensive. I wonder those of you
that are familiar with such protection do you think it is necessary ?
They are for protection against EMI but for instruments that have no
signal/Clock faster than a few hundreds of MHz, the EMI wavelength
should be fairly smaller than 30 Cm, So effective it would be to instead
of designing a board specific metallic mold for each high impedance
section, Just cutting copper or aluminum sheets approximately to the
size of sensitive area and then mount them on PCB using 5mm metallic
Spacers that are grounded. Well there is a 5mm gap but this method is
much cheaper and simpler and to the extent of theory that I know it
should stop all the EMIs that their wave length is higher than a few Cm
and it is translated to about 10 GHz.
Even harmonics of a few hundred MHz clock are very weak at such frequency.

What do you think ?

Regards.
 
We make irregularly shaped covers out of sheet metal, bent into sorta
boxes, sometimes with mounting flanges that can be bolted hard against
the pc board. In colors!

http://www.highlandtechnology.com/DSS/V470DS.html
http://www.highlandtechnology.com/DSS/V450DS.html


For small stuff, Zero or equivalent deep-drawn aluminum covers work
great.

ftp://jjlarkin.lmi.net/Ferrite.JPG


Flat sheets on spacers work pretty well at moderate frequencies, if
you don't need things like air-current shielding too.

This is the stuff we (try to) use to keep 2.4GHz out of our analogs.

http://www.gore.com/en_xx/products/electronic/emi/snapshot/index.html

Once you get the hang of the SMT "balls" and the placement it works rather
well.
 
P

PureSine

Jan 1, 1970
0
Ok, Thanks everyone for useful inputs. I will use a flat aluminum sheet
connected via spacer to PCB for my next design.

Regards.
 
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