Power Supply Bypass Capacitors-Myths and Realities

Power supply bypass capacitors are critical to the operation of high-frequency analog and digital circuits, yet are seldom considered in the depth they deserve. This paper investigates their use and the resulting power integrity effects as seen from an RFIC bondpad looking out to a host PCB. We show how designs can be either successful or unsuccessful depending on the capacitor values selected and the frequencies involved. In particular, the common practice of placing two or three graduated-size capacitors in an attempt to create a broadband AC ground may actually be counterproductive and potentially damaging to circuit operation. The associated problem of placing partial bypassing on-chip with larger bypass capacitors off-chip is also studied and guidance for avoiding hidden dangers is given.