How would you personally design a circuit for EMC ?
can you design it for combinde ESD/EMC ?

Hallo Mahmood,

Have a careful look at Michael's last reply. He just has done that!

ESD events have highest priority. It's not wise to filter first and to limit secondly, but opposite way. Many devices are not capabable of handling extreme overvoltages. Others loose their functionality. E.g., many chokes and coils cannot limit extremely high overvoltages, because core goes into saturation and permeability reduces to 1, means inductivity drastically decreases. But that's not general valid! Understanding chokes and coils often equals to find the way in jungle. It's very hard to do any reliable calculations...

So, it's always wise to first capture ESD energy by the help of a transzorb. They are expensive but really brillant. If you can add some capacitance, do it! It's the cheapest way to deal with ESD, as I showed in a former reply. Unfortunatley, capacitance can only shunt very short overvoltages, because after a certain time they are charged, of course.

If high capacitances are not allowed, either transzorbs with low junction capacitance can be used, or a circuit like in Michael's last reply: A fast Schottky diode shunts any overvoltage to a transzorb, but at nominal voltage only the very small junction capacitance of the Schottky diode is present.

After destroying energy of ESD is removed, measures for increasing EMC (Electro-Magnetic Compatibility), means some HF-filtering should follow. Michael has done it by the help of common mode filtering: If two devices have to be connected, and they are separated by some distance, between both devices noise or high frequency interference can be built-up. This noise is called common mode noise. Left unfiltered, noise currents will flow through 0V routing of both devices and signal quality will be eroded. So, a common mode choke is used to present a HIGH IMPEDANT path for common mode noise current, which will run over 0V routing inside both devices. A decoupling capacitance alone is not helpful, because common mode noise is indeed grounded but common mode current is not limited! So, LC common mode filter makes both: Capacitance grounds, common mode choke limits current.
By the way, it's always necessary, not only to decrease voltage of interference, but also the currents, which will be caused to flow.
For the same reason it's wise to shunt the ESD currents through transzorb NOT to analog or digital ground, but to chassis ground!!!

The best way to handle ESD and EMC is:

1. Shunt noise on cable screen via capacitance to chassis ground. If possoble connect cable shield directly to chassis ground.

2. Shunt ESD via transzorbs to chassis ground.

Alone with these two measures, a really big portion of ESD and high frequncy noise is already shunted to chassis ground and is securely prohibited to penetrate chassis barrier.

3. Optionally use common mode filtering for further increasing EMC.

Common mode filtering is a big advantage when dealing with high speed digital borads, because even with massive ground plane ground potential shows some high frequency common mode noise between any two points. A cable directly connected to PCB would act like an antenna and violate all EMC rules!!! It's very wise then to use common mode filtering. But nevertheless, an as much as noise-free ground connection to PCB is essential, also. This leads to PCB design, where cable is connected to a point with 'no' high frequency activity, far away from micros and oscillators!!

Have a look into your latest PC. Analyze it's ground routing...

Feel free to contact me for further assistance.

Kai