Noise Less Power Supply

CHAPTER 1

1.4 Noise less Power supply

All ICs need power supply to operate. The power is supplied to the ICs using power and ground planes and using thick traces connecting to the power and ground pins of the ICs. A number of events lead to fluctuation of voltage level on the power supply and ground nets. One of the reasons can be change in the load current because of the simultaneous switching of the output pins. The noise generated on the power supply may have frequencies from few kHz to several hundred MHz. A number of techniques are used to ensure that power supply rail is free from any noise or unwanted fluctuations. This involves use of power and ground plane, proper use and placement of decoupling capacitors. Low ac impedance between power and ground pins of all ICs at all frequencies ensures that the noise on ac rail is shorted to ground. When noise frequency is low, a big bulk capacitor between the power and ground plane removes the low frequency noise. At higher frequencies small decoupling capacitors with low parasitic inductance, placed near the ICs creates low impedance ac path between power and ground planes. At very high frequencies, power and ground planes placed adjacent to each other provide low impedance ac path.




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