About Figure-of-Merit for Continuous-Time Analog Filters

Closed-loop filters are widely used in several mixed-signal systems. They are often preferred to the open-loop g(m)-C counterpart for the intrinsic in-band linearity provided by the large in-band loop-gain. Unfortunately, all feedback analog filters suffer from poor linearity when the input tones frequency is in close proximity to the closed-loop poles frequency, where loop-gain reduces. This concept is not included in the Figure-of-Merit (F.o.M.) widely used in the past to compare different filters designs, in different technology processes. In this scenario, the aim of this paper is to provide a more complete F.o.M. that includes the linearity performance frequency dependence. In order to validate these considerations, this work presents and models the most relevant harmonic distortion metrics (IIP3 and IM3) as a function of the loop-gain frequency behavior. The reference schematic is a classical CMOS 0.18 mu m 1st-order filter based on a fully-differential Operational Amplifier (Opamp). Finally the whole analog filters State-of-the-Art is compared using the classical F.o.M. and the hereby proposed F.o.M.