Signal Integrity TUTORIAL

Power Supply Noise

Consider the following circuit where V1 represents the voltage source and L1 represents the inductance of the path delivering the voltage to the load Z.

There is a voltage drop of L*dI/dt where dI/dt represents the rate of change of he current by the load. The higher the rate of change the more is the voltage drop of across the inductor. This creates a power supply noise.

We can also think of this phenomenon in terms of the frequency of the voltahe flowing through the inductor L. The impedance of the inductor is given by 2*pi*f*L. The higher the frequency the more is the drop accross the inductor.Beyond a certain frequency the voltage drop accross the indutor becomes intolerable. If dV is the tolerable voltage and dI is the current then, the frequency beyond which the ripple will become intolerable is given by

2*pi*f*L = dV / dI

To solve this problem we add a capacitor as shown in the following diagram.

When the frequency of the AC current flowing through the wiring goes beyond the frequecy given by previous equation, the bypass capacitor comes into action. The impedance of the Capacitor is given by

Xc = 1/2*pi*f*C

Higher the frequncy, smaller is Xc. Therefore at higher frequencies, the capacitor acts as short for the ac signals. This effectively shorts the ac signal across the capacitor. The DC signal or the power supply voltage continue to flow through the inductor.

The value of this Capacitor will depend upon the amount of the change of the current and the power supply wiring or PCB inductance value.

From the PCB designer perspective you should undertand the value of this Bypass Capacitor. The inductor in the equation signifies that its value should be kept at small possible by keeping the traces thick and small. The length of the power supply wiring should be small and as thick as possible. There is a method to calculate the desired value of the capacitor which is beyond the scope of this tutorial which is written from PCB designer point of view. You can find it in advanced tutorial section ( TBD).

The Bypass capacitor is not perfect. Each Bypass capacitor has series inductor. The series inductance of the capacitor makes it ineffective beyond a certain frequency. Also, the traces going from this big Bypass capacitor has inductance. The solution to this problem is to add small capacitors right at the individual ICs and very close to it. The smaller capacitors have smaller lead inductance. These capacitors should be place as close to the supply pins of the ICs as possible with a wide short trace.

This completes the broad guidelines of the Bypass and local decoupling capacitors. You may like to read more on advanced topics in the Advanced tutorial section ( TBD).