Low Voltage, Low Power, Inverter-Based Switched-Capacitor Delta-Sigma Modulator

An operational transconductance amplifier (OTA) is a major building block and consumes most of the power in switched-capacitor (SC) circuits, but it is difficult to design low-voltage OTAs in scaled CMOS technologies. Instead of using an OTA, this paper proposes an inverter-based SC circuit and its application to low-voltage, low-power delta-sigma (DeltaSigma) modulators. Detailed analysis and design optimizations are also provided. Three inverter-based DeltaSigma modulators are implemented for an implantable pacemaker, a CMOS image sensor, and an audio codec. The modulator-I for an implantable pacemaker achieves 65-dB peak-SNDR for 120-Hz bandwidth consuming 0.73 muW with 1.5 V supply. The modulator-II for a CMOS image sensor implemented with 320-channel parallel ADC architecture achieves 63-dB peak-SNDR for 8-kHz bandwidth consuming 5.6 muW for each channel with 1.2-V supply. The modulator-III for an audio codec achieves 81-dB peak-SNDR with 20-kHz bandwidth consuming 36 muW with 0.7-V supply. The prototype DeltaSigma modulators achieved high power efficiency maintaining sufficient performances for practical applications.

I. Introduction

The continuing feature size scaling in CMOS technology has enabled the digital system to decrease power consumption and lower costs while also increasing reliability. The supply voltages must be scaled along with transistor dimension to maintain the device's reliability. However, the threshold voltage is not scaled as aggressively as the supply voltage to avoid leakage current in transistors. Therefore, the design of low-voltage analog circuits in the scaled CMOS technology poses significant challenges. Especially, the design of an operational transconductance amplifier (OTA), a key analog building block, has been the main bottleneck in low-voltage analog circuits. Low voltage OTAs have been explored [1]–[3], but the supply voltages of the OTAs are restricted and have reached the limits of further scaling, because they are strictly limited by the input common-mode voltage.