Improving the stability of on-chip automatic tuning loops for continuous-time filters with an analog adaptive controller

Continuous-time filters (CTF) with automatic tuning loops are nonlinear feedback systems with potential instability. Therefore, the appropriate linear dynamic modeling of the tunable filter should be obtained to assure stability in case an improved design of the loop controllers is to be carried out. With this aim, starting from a general and systematic analysis in order to obtain an equivalent small-signal linearised incremental model, from which transfer functions between output variables and control voltages are derived, the subsequent design of compensated loops with enhanced stability and dynamic performance is proposed. In particular, this modeling allows proposing both a non-adaptive and adaptive controllers with improved stability, together with their implementation. As a demonstrative example of application, the modeling of a particular band-pass CNIOS CTF is presented in this paper. Experimental results are shown for a CMOS 0.8-mu m technology.