I. INTRODUCTION
Variable delay elements are often used to manipulate the rising or falling edges of the clock or other pulses in integrated circuits (ICs). They are widely used in digital systems and are essential parts for self-timed operation in high speed VLSI and phase modulation such as delay locked loops (DLL's) or phase locked loops (PLL's) [1]–[4]. In switched-capacitor circuits, they are used to generate non-overlapping clock. And the delay element is one of the crucial components and its precision directly affects the overall performance of all the above-mentioned applications.