Variable Conversion Approach for Design Optimization of Low-Voltage Low-Pass Filters

This article describes a design optimization method based on variable conversion applied to low-voltage (LV) CMOS analog integrated circuits. For supply voltages in the range of 0.3 to 0.6 V, traditional design variables (e.g., transistor channel width W and gm/I-D) are not suitable for exploring the design space, leading to impractical design solutions. We show that it is more efficient to convert the design variables into current I-D and channel length L, since these parameters are directly related to circuit performance for LV applications. The proposed optimization algorithm explores the design space to obtain solutions that satisfy the design constraints while analyzing the effects of process, voltage, and temperature variations (PVT). We apply the proposed method for design optimization in terms of area and power consumption of a fourth-order LV active filter preceded by a programmable gain amplifier (PGA), in 130 nm CMOS technology. A fully differential 0.6 V operational amplifier (OTA) is designed as a building block and used in the filter in a bottom-up approach. Measurements of a prototype chip validate the proposed method and resulted in a 0.784 mW, 0.066 mm(2) active low-pass filter with a cutoff frequency of 2.5 MHz and attenuation that meet the requirements of the IEEE 802.15.4 technical standard for 2.4 GHz receivers.