Analog-to-digital converter using CML based Ring oscillator with tuning range 1–7.2 GHz in 90 nm CMOS

Voltage controlled oscillator(VCO) based analog-to-digital converter(ADC) utilizes the superior time resolution and digital processing power of time domain signal processing. Voltage controlled oscillator based analog-to-digital converters also have inherent first order noise shaping property and very high accuracy which can be achieved by reducing both area and power consumption. This paper analyses the performance of VCO implemented by current mode logic(CML) based fully differential Ring oscillator for high speed and applications that require high noise immunity. Analytical model of limitation on maximum achievable oscillation frequency of Ring oscillator by CMOS inverter is proposed showing why it cannot be used in high speed applications. Designed VCO has linear tuning range from 7.2 GHz to 1 GHz. Using control voltage for the above VCO a 6-bit analog-to-digital converter is designed in 90nm CMOS.