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About ground filtering

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kpec1988

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Hi can anyone tell is it necessory to put decoupling capacitors on ground line in high speed digital design?
 

That is the conventional way (putting decap between vdd and ground)to reduce the noise on the digital circuit. You had a question about the necessity of decoupling cap.

I would say that depends on:

1.the rise and fall time of you digital signals increases
2. The value supply and ground inductance.
3. How big is your digital circuit.
4. Does it communicate with a sensitive analog circuit.
5. What is THE nml and nmh of your inverters.

As an example, let say the vdd has 2 nh inductance and the rise time of your signal is 100 ps and let say the load of inverter is 50 fF and vdd is 2 volts.

The transient current through load capacitor is cl*dv/dt=0.05*2/100=1mA within 100ps. Therefore the voltage drop across inductor is L*di/dt=2*0.001/0.1=20mv. If you have 20 of these inverters working at the same time, the voltage drop will be ~0.4 which might reduce considerably the nml and nmh of your circuit.

therefore I advice you to use decap.
 

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