ISOLATING ANALOG CIRCUITS FROM DIGITAL INTERFERENCES

摘要

Recent advances in analog integrated circuit techniques and continued improvement in VLSI technology are enabling more and more mixed-signal analog digital integrated circuits to be implemented on the same chip. One of the important recent topics in mixed signal circuit design, is the coupling of digital switching noise onto sensitive nodes in the analog circuits [1,3]. Traditionally, guard rings formed by substrate contacts have been used to prevent such crosstalk, as illustrated in Fig.1a. As the substrate is lightly doped, noise coupled from the digital circuit, such as that through the pn junction of the drain of an inverter, tends to travel to the analog part of the circuit along the surface, which can be easily intercepted by guard rings formed by p~+ diffusion. In modern technologies, however, the substrate usually starts with a heavily doped p~+ layer several hundred microns thick, followed by a thin p~- epi layer on which circuits are formed. This is illustrated in Fig.lb. While this kind of process is much less susceptible to latch-up, the guard ring method for isolating analog from digital circuits is no longer effective. As the substrate resistance between any two point in the p~+ layer is only a few ohms, the shortest resistive path between any two points near the surface of the epi-layer is no longer along the surface but rather first vertically down to the heavily doped p~+ layer, through it and then vertically back up to the second point on the surface. The vertical paths down to the p~+ layer also have low resistance because of the large average cross section of the path as compared to the vertical distance.