Investigation of voltage-controlled oscillator circuits using organic thin-film transistors (OTFT) for use in VCO-based analog-to-digital converters

• VCO circuits using p-type OTFTs to realize a VCO-based ADC.
• Investigation of various tuneable delay cells.
• Equation to determine the ADC resolution from VCO output frequency.
• Effect of jitter and harmonics.
• Equation to determine the tuning voltage range of a VCO-based ADC.

A VCO-based ADC is a time-based ADC architecture that is highly digital with regard to its composition. In this paper, we analyze the performance of an organic voltage-controlled oscillator (VCO) employing different delay elements and investigate their suitability for use in a VCO-based analog-to-digital converter (ADC). An equation to calculate the theoretical limit of the resolution of the VCO-based ADC from the voltage versus frequency characteristics of the VCO was formulated. Using this equation we analysed various VCO architectures to realize the VCO-based ADC. We also investigated the impact of jitter and 1/f noise on the performance of the ADC. We have employed a ring oscillator based VCO in our design. The investigated single-ended delay elements were analyzed with respect to the linearity in their voltage versus frequency characteristics. This measure of linearity governs the resolution of the VCO and the VCO being the critical part of a VCO-based ADC, determines the maximum possible resolution of the whole VCO-based ADC. The resolution of all the investigated delay cells were calculated. Based on these results it was found out that the diode-load inverter delay cell is the most promising option to realize the VCO-based ADC. For such a VCO-based ADC using diode-load inverter delay stages, the measured results show that a maximum possible resolution of 5.8 bits can be achieved. In addition to the diode-load inverter based VCO, we also measured the VCO circuit using cut-off load inverter delay stages. The OTFTs use poly-3-hexylthiophene (P3HT) as the P-type semiconductor. Furthermore, the circuits were fabricated in a clean-room process that is compatible with printing processes for mass production.