A Charge-Sharing-Based Two-Phase Charging Scheme for Zero-Crossing-Based Integrator Circuits

Dong-Jick Min,Jae Hoon Shim

doi:10.3390/electronics8070821

Abstract

As an effort to improve the energy efficiency of switched-capacitor circuits, zero-crossing- based integrators (ZCBI) that consist of zero-crossing detectors and charging circuits have been proposed. To break the trade-off between accuracy and speed, ZCBI typically employs a two-phase charging scheme that relies on an additional threshold for zero-crossing detection. This paper proposes a simpler realization method of the two-phase charging scheme by means of charge sharing. To demonstrate feasibility of the proposed method, we designed and fabricated a second-order delta-sigma modulator in 180-nm complementary metal–oxide–semiconductor (CMOS) technology. The measurement results show that the modulator exhibits a peak signal-to-noise-and-distortion ratio (SNDR) of 46.3 dB over the bandwidth of 156 kHz with the power consumption of 684 µW. We also designed the same modulator in 65-nm CMOS technology and simulation results imply that the proposed circuit is able to achieve a much better energy efficiency in advanced technology.

Highlights

In conventional switched-capacitor integrators, the negative feedback around an operational transconductance amplifer (OTA) with a high DC gain forces the two input terminals to have the same potential, controlling the charge transfer from the sampling capacitor to the feedback capacitor
The measurement results show that the modulator exhibits a peak signal-to-noise-and-distortion ratio (SNDR) of 46.3 dB over the bandwidth of 156 kHz with the power consumption of 684 μW
zero-crossing-based integrator (ZCBI) is an attractive alternative to the conventional power-hungry switched-capacitor integrator because the ZCD can be implemented using a simple inverter circuit and auto-zeroing techniques

Summary

Introduction

In conventional switched-capacitor integrators, the negative feedback around an operational transconductance amplifer (OTA) with a high DC gain forces the two input terminals to have the same potential, controlling the charge transfer from the sampling capacitor to the feedback capacitor. If the current source charges the load with the constant current I, the overshoot voltage at the output of the integrator is given by VOVS =. Depending on the direction of charging during the fine phase, the two-phase charging scheme can be bidirectional or unidirectional (Figure 2). If the charging scheme is bidirectional, the coarse phase ends when the ZCD detects zero-crossing through the threshold VCM. The unidirectional two-phase charging scheme, on the other hand, introduces an additional threshold Vth that is lower than VCM to decide when to make a transition from the coarse phase to the fine phase. This paper presents, in more detail, the design of a second-order ∆Σ analog-to-digital convertor (ADC) that employs the proposed two-phase charging scheme and discusses its effectiveness with measurement results.

Proposed ZCBI Circuit

Circuit Implementation and Measurement Results

Conclusions