A Sub-0.1% THD Sinusoidal-Signal Generator for Impedance Measurement Using a Delta-Sigma-Modulated Look-Up Table

Abstract

Many frequency-based electronic measurement applications require a sinusoidal stimulus with high linearity, such for accurate impedance measurement systems used in chemistry, biology, and physical sensing. In this brief, we present a highly linear sinusoidal-signal generator (SSG) on chip for use in such impedance measurement systems. Building upon prior digital SSG approaches that use a digital multi-stage noise-shaping (MASH) modulator to achieve high linearity while using a small digital-to-analog converter (DAC), we introduce delta-sigma modulation directly to the look-up table, achieving similarly high linearity without requiring a MASH modulator in the signal chain, which adds both design complexity and in-band errors. A prototype integrated circuit was fabricated using 180nm CMOS and measured while generating a 20 kHz, 140 mVpp sine wave. It achieves 0.088% THD through the $20^{th}$ harmonic and 63.5 dB SFDR. The approach is highly digital in nature and therefore amenable to implementation using automated synthesis and place and route, which makes it a promising approach for future scalability in advanced CMOS process nodes.