Printed Circuit Board Power Bus Decoupling Questions

1. What is the best strategy for connecting decoupling capacitors to a printed circuit board?

The answer to this question depends on the type of board you are trying to decouple and what you are trying to accomplish by adding decoupling capacitors. For the moment we will assume that your goal is to supply all of the current needed for the proper operation of the active component while minimizing noise on the power bus. With this in mind, there are three possible decoupling strategies depending on the type of power bus you have.

PCB without power planes

PCB with closely spaced (<.3 mm) power and ground planes

PCB with power and ground planes that are not closely spaced

In all cases, the following guidelines apply:

• It is the capacitor's impedance that determines how much current it can supply for a given change in its voltage. The lower the impedance, the better.
• At low frequencies, the capacitor's impedance is determined by its capacitance value. More capacitance is better.
• At high frequencies, the capacitor's impedance is determined by its connection inductance plus its internal inductance. Generally speaking, smaller capacitor packages tend to have lower internal inductance and lower connection inductance. The smaller the package, the better.
• Most printed circuit board designs have one or more large-valued bulk capacitors to reduce the impedance of the power bus at low frequencies and several low-inductance capacitors to reduce the impedance of the power bus at high frequencies.
• It is particularly important to minimize the connection inductance of capacitors that are intended to reduce power bus noise at high frequencies.

References:

Power Bus Decoupling on Multilayer Printed Circuit Boards (pdf - 821 kB)
T. H. Hubing, J. L. Drewniak, T. P. Van Doren, and D. Hockanson
IEEE Transactions on Electromagnetic Compatibility
vol. EMC-37, no. 2, May 1995, pp. 155-166.

An Experimental Investigation of 4-Layer Printed Circuit Board Decoupling (pdf - 363 kB)
T. H. Hubing, T. P. Van Doren, F. Sha, J. L. Drewniak, and M. Wilhelm
Proceedings of the 1995 IEEE International Symposium on Electromagnetic Compatibility
Atlanta, GA, August 1995, pp. 308-312.

Simulation and Measurement for Decoupling on Multilayer PCB DC Power Buses (pdf - 478 kB)
H. Shi, F. Sha, J. L. Drewniak, T. H. Hubing, T. P. Van Doren, and F. Yuan
Proceedings of the 1996 IEEE International Symposium on Electromagnetic Compatibility
Santa Clara, CA, August 1996, pp. 430-435.

An Experimental Procedure for Characterizing Interconnects to the DC Power Bus on a Multilayer PCB (pdf - 242 kB)
H. Shi, F. Sha, J. L. Drewniak, T. P. Van Doren, and T. H. Hubing
IEEE Transactions on Electromagnetic Compatibility
vol. EMC-39, no. 4, November 1997, pp. 279-285.

Designing Power Bus Decoupling for CMOS Devices (pdf - 366 kB)
S. Radu, R. E. DuBroff, J. L. Drewniak, T. H. Hubing, and T. P. Van Doren
Proceedings of the 1998 IEEE International Symposium on Electromagnetic Compatibility
Denver, CO, August, 1998, pp. 375-380.

Experimental Evaluation of Power Bus Decoupling on a 4-layer Printed Circuit Board (pdf - 365 kB)
J. Chen, M. Xu, T. Hubing, J. Drewniak, T. Van Doren, and R. DuBroff
Proc. of the 2000 IEEE International Symposium on Electromagnetic Compatibility
Washington D.C., August 2000, pp. 335-338.

Quantifying Decoupling Capacitor Location (pdf - 902 kB)
J. Fan, J. Knighten, A. Orlandi, N. Smith and J. Drewniak
Proc. of the 2000 IEEE International Symposium on Electromagnetic Compatibility
Washington D.C., August 2000, pp. 757-762.

"Quantifying SMT Decoupling Capacitor Placement in DC Power-Bus Design for Multilayer PCBs," (pdf - 406 kB)
J. Fan, J. Drewniak, J. Knighten, N. Smith, A. Orlandi, T. Van Doren, T. Hubing and R. DuBroff
IEEE Transactions on Electromagnetic Compatibility
vol. EMC-43, no. 4, November 2001, pp. 588-599.

"Reducing Power Bus Impedance at Resonance with Lossy Components," (pdf - 370 kB)
T. Zeeff and T. Hubing
IEEE Transactions on Advanced Packaging
vol. 25, no. 2, May 2002, pp. 307-310.

Decoupling Strategies for Printed Circuit Boards without Power Planes (pdf - 240 kB)
H. W. Shim, T. Zeeff and T. Hubing
Proc. of the 2002 IEEE International Symposium on Electromagnetic Compatibility
Minneapolis, MN, August 2002, pp. 258-261.