Analog Filter Design Using Ring

Abstract

Integrators are key building blocks in many analog signal processing circuits and systems. The DC gain of conven- tional opamp-RC or - integrators is severely limited by the gain of operational transconductance ampli fier (OTA) used to im- plement them. Process scaling reduces transistor output resistance, which further exacerbates this iss ue. We propose applying ring os- cillator integrators (ROIs) in the design of high order analog filters. ROIs implemented with simple CMOS inverters achieve in finite DC gain at low supply voltages indep endent of transistor non-ide- alities and imperfections such as finite output impedance. Conse- quently, ROIs scale more effectively into newer processes. A proto- type fourth order filter designed using the ROIs was fabricated in 90 nm CMOS and occupies an area of 0.29 mm . Operating with a0 . 5 5Vs u p p l y ,t h efilter consumes 2.9 mW power and achieves bandwidth of 7 MHz, SNR of 61.4 dB, SFDR of 67.6 dB and THD of 60.1 dB. The measured IM3 obtained by feeding two tones at 1 MHz and 2 MHz is 63.4 dB.