Voltage-to-frequency converter with high sensitivity using all-MOS voltage window comparator

Abstract

A high-sensitivity voltage-to-frequency converter (VFC) using an all-MOS voltage window comparator is presented in this work. The circuit is composed of one voltage-to-current converter, one charge and discharge circuit, and one all-MOS voltage window comparator. The input voltage is converted into a current which in turn triggers the charge and discharge circuit, where a built-in capacitor is driven. The voltage window comparator monitors the variated voltage on the capacitor and generate an oscillated output of which the vibration frequency is linearly dependent to the input voltage. In this way, the worst-case linear range of the output frequency of the proposed VFC is 0–55.40 MHz verified by simulations given a 0–0.9 V input range. The physical measurement of the proposed VFC shows a 0–52.95 MHz output frequency given a 0–0.9 V input range. The error in linearity is better than 8.5% while the power dissipation is merely 0.218 mW.

Introduction

Lots of work have been done to develop the methods for designing sensors which convey information, the values of passive or active elements, into the oscillation parameters, e.g., the magnitude, phase, frequency, or duty cycle of the vibration. The values to be conveyed might be functions of some other factors, e.g., mechanical pressure, magnetic field, or temperature. The variation of the oscillation must faithfully track the corresponding change in these factors. Hence, bandwidth, sensitivity, and linearity are the most important measures to judge the quality of these circuits. Converting the physically estimated values into oscillations is much preferred because the oscillations are more noise immune. Thus, the correctness of the information can be ensured. A traditional VCO (voltage-controlled oscillator) cannot fulfill such a demand, because the linearity and sensitivity of such a conversion are required.

This paper presents a high-bandwidth linear interfacing circuit which converts the sensed voltage into frequency. The frequency output is very much noise resistant compared to other types of outputs, e.g., the magnitude of current or voltage. It meets the requirement for integration with other IP (intellectual property), e.g., uP or communication MAC (media access circuitry). In contrast to the bipolar or BiCMOS implementations [1], [2] the proposed design is realized in TSMC (Taiwan Semiconductor Manufacturing Company) $0.25\mathrm{\mu }\mathrm{m}$ 1P5M CMOS technology. It possesses the advantages of low power, small area and high bandwidth. Although there were several prior CMOS-based converters, they either required an extra OSC [3], or additional timing control signals and many switched capacitors [4], which became overhead in those designs. By contrast, our proposed voltage-to-frequency converter (VFC) is composed of one voltage-to-current converter ($V$-to-$I$), and one charge and discharge circuit (CDC) which is driven by the voltage window comparator (VWC). The physical measurement of the proposed VFC on silicon shows a 0–52.95 MHz output frequency given a 0–0.9 V input range.