Measurements and Analysis of PLL Jitter Caused by Digital Switching Noise

When integrating analog and digital circuits onto a mixed-mode chip, power supply noise coupling is a major limitation on the performance of the analog circuitry. Several techniques exist for reducing the noise coupling, or which one of the cheapest is separating the power supply distribution networks for the analog and digital circuits. Noise coupling from a digital noise-generating circuit through the power supply/substrate into an analog phase-locked loop (PLL) is analyzed for three different power supply schemes. The main mechanisms for noise coupling are identified by comparing different PLLs and varying their bandwidths. It is found that the main cause of jitter strongly depends on the power supply configuration of the PLL. Measurements were done on mixed-mode designs in a standard 0.25-µm digital CMOS process with a low-resistivity substrate. The same circuits were also implemented with triple-well processing for comparisons.