Protecting high impedance sections of PCB with metallic Sheets ratherthan metallic molds
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From: PureSine on 19 Apr 2010 02:02 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. |