Linearity Improvement of VCO-Based ADC via Complementary Bias Voltage Control for IoT Devices

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Abstract

<p>Ring-VCO (Voltage controlled oscillator)-based ADCs are suitable for the data acquisition in embedded sensors which are at the core of AI enabled IoT (Internet of Things) devices. Fundamentally, the ring-VCO can generate digital code by counting frequency from its voltage-controlled oscillation. However, the ring-VCO has some issues related to non-linearity and power dissipation due to the voltage-to-frequency (V-to-F) tuning characteristics. Particularly, for low power operation with low voltage supply, the linearity further degrades. This paper presents a complementary bias voltage control approach to attain a linear V-to-F characteristics with low-power dissipation. The novel voltage-to-current (V-to-I) conversion provides the linear bias current source and sink matching for current-starved inverter-based delay elements. Furthermore, the proposed circuit can be extended to optimize nonlinearity error by selecting an optimal transistor size. Simulation results with a 0.5V power supply circuit designed in TSMC 180nm CMOS technology shows that maximum nonlinearity error is below 0.24% for 4-stage and below 0.49% for 8-stage ring-VCO.</p>

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