Time Domain Comparator Based SAR-ADC

Team Members

1. M. Taufiqul Huda (Leader)
2. Nahwan Faza Assaify
3. Yohanes Stefanus
4. Ahmad Jabar Ilmi
5. Raditya Eka Putra
6. Astria Nur Irfansyah, S.T., M.Eng., Ph.D. (Mentor)

Introduction

The ADC was proposed for photoplethysmogram signal acquisition that works on a low-frequency domain. A photoplethysmogram signal or PPG signal is a biomedical signal obtained from a change in light intensity caused by a fingertip blood volume change [1]. PPG signal can be detected using red and infrared LED spectrum and recorded using photodiode then converted A/D using ADC. SAR ADC matches the specification of the signal that will be recorded (PPG signal) that works in low frequency (0 Hz to 25 Hz, average bandwidth 0.25 Hz to 10 Hz) [2]. Thus, the challenge is how to design low-power ADC for PPG recording purposes that have specific applications for wearable devices (smartwatches, oximeters, etc).

The innovation with this design is to replace the comparator (the old one still using Dynamic Latch Comparator) with time-based comparison processing. The comparator that will be used is EPC (Edge Pursuit Comparator) which works with a time delay caused by the comparator input voltage difference based on inverter logic delay block (containing 2 PMOS and 2 NMOS) [3].

The design contains several block modules:

1. Op-Amp [ref: https://tinytapeout.com/runs/tt06/490/ & https://github.com/RTimothyEdwards/sky130_ef_ip__opamp ]
2. DAC [ref : https://tinytapeout.com/runs/tt06/229/ ]
3. Logic Level Shifter [ref: https://github.com/RTimothyEdwards/sky130_ef_ip__analog_switches ]
4. SAR Logic
5. Sample Hold Circuit (S/H Circuit)
6. Edge Pursuit Comparator Circuit
7. Clock Generator [ref: https://github.com/RTimothyEdwards/sky130_ef_ip__rc_osc_500k ]

The purpose of this design is to create a time-based SAR ADC design for PPG signal recording. The specifications will be broken down below.

Specifications

Pin Layout

Reference

[1] C. Orphanidou, Signal Quality Assessment in Physiological Monitoring. 2018. [Online]. Available: https://doi.org/10.1007/978-3-319-68415-4_2%0Ahttp://link.springer.com/10.1007/978-3-319-68415-4

[2] Z. Chen, H. Qin, W. Ge, S. Li, and Y. Liang, “Research on a Non-Invasive Hemoglobin Measurement System Based on Four-Wavelength Photoplethysmography,” Electron., vol. 12, no. 6, pp. 1–12, 2023, doi: 10.3390/electronics12061346.

[3] D. Kumaradasan, S. K. Kar, and S. Sarkar, “An 8-bit 100 kS/s Low Power SAR ADC with Modified EPC for Bio-Medical Applications,” Conf. Proc. - 2023 IEEE Silchar Subsect. Conf. SILCON 2023, pp. 1–6, 2023, doi: 10.1109/SILCON59133.2023.10404914.

[4] M. Shim et al., “Edge-Pursuit Comparator: An Energy-Scalable Oscillator Collapse-Based Comparator with Application in a 74.1 dB SNDR and 20 kS/s 15 b SAR ADC,” IEEE J. Solid-State Circuits, vol. 52, no. 4, pp. 1077–1090, 2017, doi: 10.1109/JSSC.2016.2631299.

Refer to README for this sample project documentation.