An All-Digital Delay-Locked Loop Using an In-Time Phase Maintenance Scheme for Low-Jitter Gigahertz Operations

Traditional all-digital delay-locked loops (ADDLLs) have a long control loop, and false skew compensation may occur due to late code adjustment. To avoid this problem, the ADDLLs either simply sacrifice the maximum operating frequency or adopt a lower code adjustment rate to achieve a higher maximum operating frequency. However, lowering the code adjustment rate not only increases the number of the locking cycles but also results in large output jitter because the clock skew induced by run-time variations cannot be compensated in time. This paper presents a 55 nm 1.0 V 0.1-to-2.5 GHz ADDLL, which is constructed on a previously proposed half-delay-line skew-compensation circuit with several new circuit design techniques developed to achieve low jitter, small area, low power, fast lock-in, and high PVT- variation tolerance across a large operating frequency range. The key design feature is a ping-pong phase maintenance scheme that allows the code adjustment to be performed in time in each clock cycle, even for gigahertz operations. The measurement results show that the ADDLL achieves a peak-to-peak (p-p) jitter of 3 ps with 1.96 mW power consumption and 8 lock-in cycles when operated at 2.5 GHz.