Magic Capacitors

Capacitors play an important role in providing the low impedance path between power supply and ground and thereby it reduces the noise. At high frequency the effective series inductance of the capacitor becomes the bottleneck in providing low impedance. The series inductance of the capacitor lowers the resonance frequency and makes the capacitor ineffective at high frequency. The inductance of the traces connecting the capacitor to the ground must not be ignored. It is the sum of the mounting inductance and the capacitor series inductance that must be taken into account to find the overall inductance. The resonance frequency must be calculated based upon the overall system in mind.

There are a class of capacitors called IDC capacitors or X2Y capacitors that have markedly reduced ESL as compared to the normal capacitors. The inductance is lowered by keeping a number of capacitors in parallel or in an array to reduce the effective series inductance. However, these capacitors are expensive. Their prices are about 5 to 10 times that of the normal capacitors.

Experts disagree on the use of these array capacitors. Some engineers favors using two inexpensive 0603 capacitors in place of using one IDC array capacitor. “The cost of these units is at least 10 times that of an 0603 standard ceramic chip capacitor. It is usually a better choice to use two 0603 capacitors instead of one array to get the same effective ESL at a much lower cost. This is due to the fact that the mounting inductance will dominate over the low inductance of the array package.” – Lee write in Right the First Time, A Practical Handbook on High Speed PCB and System Design, Page 132.

Effectively it comes down to pricing in addition to the engineering. Expect the cost of the IDC or the X2Y capacitor to fall in future. If these capacitors are only marginally expensive than the normal ceramic capacity do not hesitate to use them selectively.


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