Level-Crossing Detection based Low-Power Sigma-Delta ADC for Sensor Applications

A Current Controlled Oscillator (ICO), integrates its information-bearing current-input-signal iin using a capacitor, C. The voltage VC across C, namely the integral, is reset or folded when it reaches a level VREF. If iin (dq/dt) is constant, then VC is a saw-tooth waveform. Since the frequency of the oscillator is defined as the rate of change of phase, the phase wraps itself after every 2π radians or its corresponding voltage VREF. With this property we propose an open-loop continuous-time Sigma-Delta ADC combining level-crossing detection and folding techniques using an ICO. We also propose a technique to increase the effective resolution of the ADC by inserting additional voltage-levels for detection between 0 and VREF. The complete ADC is designed in TSMC 40nm CMOS technology. The simulation results show an ENOB of 12.2-bits consuming 1.5μW of power from a 1V supply corresponding to a Schreier-FOM of 167.2dB and a Walden-FOM of 157fJ per conversion cycle.

[1]  T. R. Viswanathan,et al.  Operational current to frequency converter , 2013, 2013 IEEE 56th International Midwest Symposium on Circuits and Systems (MWSCAS).

[2]  Pieter Rombouts,et al.  Why and How VCO-based ADCs can improve instrumentation applications , 2018, 2018 25th IEEE International Conference on Electronics, Circuits and Systems (ICECS).

[3]  Mark Vesterbacka,et al.  Ring Counters as Phase Accumulator in VCO-Based ADCs , 2018, 2018 25th IEEE International Conference on Electronics, Circuits and Systems (ICECS).

[4]  Henrik V. Sorensen,et al.  A level-crossing sampling scheme for A/D conversion , 1996 .

[5]  Michael H. Perrott,et al.  A 78 dB SNDR 87 mW 20 MHz Bandwidth Continuous-Time � ADC With VCO-Based Integrator and Quantizer Implemented in 0 . 13 � m CMOS , 2009 .

[6]  Nan Sun,et al.  Analog signal processing in deep submicron CMOS technologies using inverters , 2014, 2014 IEEE 57th International Midwest Symposium on Circuits and Systems (MWSCAS).

[7]  Marko Kosunen,et al.  A VCO-based ADC with Relaxation Oscillator for Biomedical Applications , 2018, 2018 25th IEEE International Conference on Electronics, Circuits and Systems (ICECS).

[9]  M.H. Perrott,et al.  A 78 dB SNDR 87 mW 20 MHz Bandwidth Continuous-Time $\Delta\Sigma$ ADC With VCO-Based Integrator and Quantizer Implemented in 0.13 $\mu$m CMOS , 2009, IEEE Journal of Solid-State Circuits.

[10]  Pavan Kumar Hanumolu,et al.  A VCO-based current-to-digital converter for sensor applications , 2014, Proceedings of the IEEE 2014 Custom Integrated Circuits Conference.

[11]  Maliang Liu,et al.  A 10-kS/s 625-Hz-Bandwidth 65-dB SNDR Second-Order Noise-Shaping SAR ADC for Biomedical Sensor Applications , 2020, IEEE Sensors Journal.