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EMI, ground plan and analogic ground plan

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Dec 19, 2005
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emi ground


I'm designing a battery recharger and I have some question about EMI effects. Should I put a ground layer under the inductor?
The switch is a PNP transistor. It has a dissipator connected in the collector. If I put the inductor near of the dissipator, could it work like a antenna (and it interferes on command circuit)?
I will use a two layer board and the circuit has a ground and an analogic ground. Should I make a ground plan and a analogic ground plan? or only a ground plan?

Does anybody have suggestions?


short earth and digital ground

This is only a battery charger applcation, so you are not after nV or µV precision ..
As you have two-layer PCB I would suggest that you make a common “ground plane” at the bottom layer.
It will have the least possible resistance and inductance of any ground configuration ..

emi vs grounding

My experience says that it is better to have a solid and single ground plane common to analog and digital parts.

Usually producers of A/D and D/A converters suggest to have a separate analog and digital ground plane. This usually optimize the performance of the mixed signal device.

However in real life situations the splitting of the planes is not so easy to define and you have to accept compromises and often it is simpler and more effective to use a single ground layer
emi grounding with inductor

IanP: this is only the charger, but if He want to produce it for the market, then must pass all EMC test procedure, the requirements is to same for the simple charger and for the VCR.

Do You have a dc/dc converter ? And .. will be better if you can give the schem, or part of the schem.

About ground and polygon.
1. You must have separate Analog and Digital ground, if you have the ADC you must connect two ground under the ADC in only one point. If you use CPU ADC then mae a connection on the Aground pin.
2. If you want to pass EMC test, then you must the reduce to minimum radiation from CPU, on the other hand you should made your processor resistive to external emission. Use for CPU crystal clear ground polygon, connect only in one point to the digital ground polygon. Put the resistor in to oscilation circuit. Use short track to the crystal. Use the crystal with possible low frequency. Put the capacitor near the CPU.
3. If you have a DC/DC converter, this is a very depend from converter, this will be a DC/Dc or a AC/DC, what be a input source? Minimize the capacitance between power MOSFET and ground, neutral or Earth. This i to same and for inductors. Use short and wide track. Put input filters.

If you give me a bit more information i will can give you a more specific suggestion.
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