How to select bypass CAPsUltraCAD Design, Inc. ESR and Bypass Capacitor Self Resonant Behavior How to Select Bypass Caps Douglas G. Brooks, MS/PhD Introduction It’s no news that the band pass requirements for power systems on PCB’s are increasing and that power supply impedance requirements are getting tighter. Bypass capacitor fabrication and assembly techniques are improving and pushing higher the normal self-resonant frequencies we have to deal with. ESR’s (equivalent series resistance) are decreasing, pushing further down the floor of the power supply impedance curve. All this has created increased debate as to how to take advantage of this higher self-resonant requency and lower ESR. One argument is that lower ESR is thoroughly beneficial. Another is that, while lower ESR lowers the impedance at the minimum p……