{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,4,16]],"date-time":"2026-04-16T08:24:21Z","timestamp":1776327861467,"version":"3.50.1"},"reference-count":38,"publisher":"MDPI AG","issue":"19","license":[{"start":{"date-parts":[[2024,9,24]],"date-time":"2024-09-24T00:00:00Z","timestamp":1727136000000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>Neural recording systems play a crucial role in comprehending the intricacies of the brain and advancing treatments for neurological disorders. Within these systems, the analog-to-digital converter (ADC) serves as a fundamental component, converting the electrical signals from the brain into digital data that can be further processed and analyzed by computing units. This research introduces a novel nonlinear ADC designed specifically for spike sorting in biomedical applications. Employing MOSFET varactors and voltage-controlled oscillators (VCOs), this ADC exploits the nonlinear capacitance properties of MOSFET varactors, achieving a parabolic quantization function that digitizes the noise with low resolution and the spikes with high resolution, effectively suppressing the background noise present in biomedical signals. This research aims to develop a reconfigurable, nonlinear voltage-controlled oscillator (VCO)-based ADC, specifically designed for implantable neural recording systems used in neuroprosthetics and brain\u2013machine interfaces. The proposed design enhances the signal-to-noise ratio and reduces power consumption, making it more efficient for real-time neural data processing. By improving the performance and energy efficiency of these devices, the research contributes to the development of more reliable medical technologies for monitoring and treating neurological disorders. The quantization step of the ADC spans from 44.8 mV in the low-amplitude range to 1.4 mV in the high-amplitude range. The circuit was designed and simulated utilizing a 180 nm CMOS process; however, no physical prototype has been fabricated at this stage. Post-layout simulations confirm the expected performance. Occupying a silicon area is 0.09 mm2. Operating at a sampling frequency of 16 kS\/s and a supply voltage of 1 volt, this ADC consumes 62.4 \u00b5W.<\/jats:p>","DOI":"10.3390\/s24196161","type":"journal-article","created":{"date-parts":[[2024,9,24]],"date-time":"2024-09-24T08:56:06Z","timestamp":1727168166000},"page":"6161","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":4,"title":["A Reconfigurable, Nonlinear, Low-Power, VCO-Based ADC for Neural Recording Applications"],"prefix":"10.3390","volume":"24","author":[{"given":"Reza","family":"Shokri","sequence":"first","affiliation":[{"name":"Biomedical Integrated Systems Lab, University of Tehran, Tehran 1439957131, Iran"},{"name":"Dipartimento di Ingegneria Navale, Elettrica, Elettronica e delle Telecomunicazioni (DITEN), University of Genoa, 16145 Genoa, Italy"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Yarallah","family":"Koolivand","sequence":"additional","affiliation":[{"name":"Department of Electronics, Khajeh Nasir Toosi University of Technology, Tehran 1631714191, Iran"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Omid","family":"Shoaei","sequence":"additional","affiliation":[{"name":"Biomedical Integrated Systems Lab, University of Tehran, Tehran 1439957131, Iran"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-2145-1869","authenticated-orcid":false,"given":"Daniele D.","family":"Caviglia","sequence":"additional","affiliation":[{"name":"Dipartimento di Ingegneria Navale, Elettrica, Elettronica e delle Telecomunicazioni (DITEN), University of Genoa, 16145 Genoa, Italy"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-6938-9806","authenticated-orcid":false,"given":"Orazio","family":"Aiello","sequence":"additional","affiliation":[{"name":"Dipartimento di Ingegneria Navale, Elettrica, Elettronica e delle Telecomunicazioni (DITEN), University of Genoa, 16145 Genoa, Italy"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2024,9,24]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"881","DOI":"10.1016\/j.mcna.2017.04.006","article-title":"Complementary and Integrative Medicine for Neurologic Conditions","volume":"101","author":"Wells","year":"2017","journal-title":"Med. Clin. N. Am."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"74","DOI":"10.1109\/MEMB.2006.1705750","article-title":"Stypulkowski. Electrical stimulation as therapy for neurological disorders","volume":"25","author":"Testerman","year":"2006","journal-title":"IEEE Eng. Med. Biol. Mag."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"173","DOI":"10.1109\/JSSC.2016.2624989","article-title":"A \u00b150-mV Linear-Input-Range VCO-Based Neural-Recording Front-End With Digital Nonlinearity Correction","volume":"52","author":"Jiang","year":"2017","journal-title":"IEEE J. Solid-State Circuits"},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"877","DOI":"10.1109\/TBCAS.2021.3112756","article-title":"Closed-Loop Neural Prostheses With On-Chip Intelligence: A Review and a Low-Latency Machine Learning Model for Brain State Detection","volume":"15","author":"Zhu","year":"2021","journal-title":"IEEE Trans. Biomed. Circuits Syst."},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"1083","DOI":"10.1007\/s00702-017-1751-6","article-title":"Systems for deep brain stimulation: Review of technical features","volume":"124","author":"Amon","year":"2017","journal-title":"J. Neural Transm."},{"key":"ref_6","doi-asserted-by":"crossref","unstructured":"Shokri, R., Koolivand, Y., Shoaei, O., Aiello, O., and Caviglia, D.D. (2023, January 4\u20137). Multipolar Stimulator for DBS Application with Concurrent Imbalance Compensation. Proceedings of the 2023 30th IEEE International Conference on Electronics, Circuits and Systems (ICECS), Istanbul, Turkiye.","DOI":"10.1109\/ICECS58634.2023.10382867"},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"2170","DOI":"10.1212\/01.WNL.0000145603.48221.B5","article-title":"Suicide after successful deep brain stimulation for movement disorders","volume":"63","author":"Burkhard","year":"2004","journal-title":"Neurology"},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"3314","DOI":"10.1109\/JSSC.2018.2867293","article-title":"A Fully Integrated 16-Channel Closed-Loop Neural-Prosthetic CMOS SoC With Wireless Power and Bidirectional Data Telemetry for Real-Time Efficient Human Epileptic Seizure Control","volume":"53","author":"Cheng","year":"2018","journal-title":"IEEE J. Solid-State Circuits"},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"1756286419838096","DOI":"10.1177\/1756286419838096","article-title":"An update on best practice of deep brain stimulation in Parkinson\u2019s disease","volume":"12","author":"Hartmann","year":"2019","journal-title":"Ther. Adv. Neurol. Disord."},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"24","DOI":"10.1109\/MDAT.2015.2504367","article-title":"Multichannel Wireless Neural Recording AFE Architectures: Analysis, Modeling, and Tradeoffs","volume":"33","author":"Tong","year":"2016","journal-title":"IEEE Des. Test"},{"key":"ref_11","doi-asserted-by":"crossref","unstructured":"Sharma, M., Gardner, A.T., Strathman, H.J., Warren, D.J., Silver, J., and Walker, R.M. (2018). Acquisition of Neural Action Potentials Using Rapid Multiplexing Directly at the Electrodes. Micromachines, 9.","DOI":"10.3390\/mi9100477"},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"1565","DOI":"10.1109\/JSSC.2020.3041289","article-title":"Opamp-Less Sub-\u03bcW\/Channel \u0394-Modulated Neural-ADC With Super-G\u03a9 Input Impedance","volume":"56","author":"Pazhouhandeh","year":"2021","journal-title":"IEEE J. Solid-State Circuits"},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"371","DOI":"10.1109\/TBCAS.2013.2270178","article-title":"Nonlinear Signal-Specific ADC for Efficient Neural Recording in Brain-Machine Interfaces","volume":"8","author":"Judy","year":"2014","journal-title":"IEEE Trans. Biomed. Circuits Syst."},{"key":"ref_14","doi-asserted-by":"crossref","unstructured":"Badami, K., Ramos, J.-C.P., Lauwereins, S., and Verhelst, M. (2018, January 11\u201315). Mixed-signal programmable non-linear interface for resource-efficient multi-sensor analytics. Proceedings of the 2018 IEEE International Solid-State Circuits Conference\u2014(ISSCC), San Francisco, CA, USA.","DOI":"10.1109\/ISSCC.2018.8310325"},{"key":"ref_15","doi-asserted-by":"crossref","unstructured":"Danial, L., Sharma, K., Dwivedi, S., and Kvatinsky, S. (2019, January 17\u201319). Logarithmic Neural Network Data Converters using Memristors for Biomedical Applications. Proceedings of the 2019 IEEE Biomedical Circuits and Systems Conference (BioCAS), Nara, Japan.","DOI":"10.1109\/BIOCAS.2019.8919068"},{"key":"ref_16","first-page":"1073","article-title":"A Widely Reconfigurable Piecewise-Linear ADC for Information-Aware Quantization","volume":"68","author":"Sengupta","year":"2021","journal-title":"IEEE Trans. Circuits Syst. II Express Briefs"},{"key":"ref_17","doi-asserted-by":"crossref","unstructured":"Sirimasakul, S., and Thanachayanont, A. (2022, January 24\u201327). A Logarithmic Level-Crossing ADC with Fixed Comparison Window. Proceedings of the 2022 19th International Conference on Electrical Engineering\/Electronics, Computer, Telecommunications and Information Technology (ECTI-CON), Prachuap Khiri Khan, Thailand.","DOI":"10.1109\/ECTI-CON54298.2022.9795458"},{"key":"ref_18","first-page":"15","article-title":"A Low-Power Logarithmic CMOS Digital-to-Analog Converter for Neural Signal Recording","volume":"69","author":"Jomehei","year":"2022","journal-title":"IEEE Trans. Circuits Syst. II Express Briefs"},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"3140","DOI":"10.1109\/JSSC.2018.2868328","article-title":"A Fully Configurable Non-Linear Mixed-Signal Interface for Multi-Sensor Analytics","volume":"53","author":"Badami","year":"2018","journal-title":"IEEE J. Solid-State Circuits"},{"key":"ref_20","doi-asserted-by":"crossref","unstructured":"Sundarasaradula, Y., Constandinou, T.G., and Thanachayanont, A. (2016, January 11\u201314). A 6-bit, two-step, successive approximation logarithmic ADC for biomedical applications. Proceedings of the 2016 IEEE International Conference on Electronics, Circuits and Systems (ICECS), Monte Carlo, Monaco.","DOI":"10.1109\/ICECS.2016.7841123"},{"key":"ref_21","doi-asserted-by":"crossref","unstructured":"Shokri, R., Koolivand, Y., Shoaei, O., Aiello, O., and Caviglia, D. (2023, January 25\u201326). A Nonlinear, Low-Power, VCO-Based ADC for Neural Recording Applications. Proceedings of the 2023 5th Iranian International Conference on Microelectronics (IICM), Tehran, Iran.","DOI":"10.1109\/IICM60532.2023.10443199"},{"key":"ref_22","unstructured":"(2024, February 26). Available online: https:\/\/www.niktek.ir\/index.php\/products\/experimental\/ndl."},{"key":"ref_23","unstructured":"Razavi, B. (2017). Design of Analog CMOS Integrated Circuits, Mcgraw-Hill Education."},{"key":"ref_24","doi-asserted-by":"crossref","unstructured":"Tong, X., and Wang, J. (2017, January 19\u201321). A 1 V 10 bit 25 kS\/s VCO-based ADC for implantable neural recording. Proceedings of the 2017 IEEE Biomedical Circuits and Systems Conference (BioCAS), Turin, Italy.","DOI":"10.1109\/BIOCAS.2017.8325226"},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"1283","DOI":"10.1109\/TBCAS.2021.3133531","article-title":"A 174.7-dB FoM, 2nd-Order VCO-Based ExG-to-Digital Front-End Using a Multi-Phase Gated-Inverted-Ring Oscillator Quantizer","volume":"15","author":"Pochet","year":"2021","journal-title":"IEEE Trans. Biomed. Circuits Syst."},{"key":"ref_26","doi-asserted-by":"crossref","first-page":"1684","DOI":"10.1109\/JSSC.2021.3114006","article-title":"A Deep-Subthreshold Variation-Aware 0.2-V Open-Loop VCO-Based ADC","volume":"57","author":"Nguyen","year":"2022","journal-title":"IEEE J. Solid-State Circuits"},{"key":"ref_27","doi-asserted-by":"crossref","unstructured":"Rubino, R., Crovetti, P.S., and Aiello, O. (2019, January 11\u201314). Design of Relaxation Digital-to-Analog Converters for Internet of Things Applications in 40nm CMOS. Proceedings of the 2019 IEEE Asia Pacific Conference on Circuits and Systems (APCCAS), Bangkok, Thailand.","DOI":"10.1109\/APCCAS47518.2019.8953168"},{"key":"ref_28","unstructured":"Stanchieri, G.D.P., Aiello, O., and De Marcellis, A. (2024, January 19\u201322). A 0.4 V 180 nm CMOS Sub-\u03bcW Ultra-Compact and Low-Effort Design PWM-Based ADC. Proceedings of the 2024 IEEE International Symposium on Circuits and Systems (ISCAS), Singapore."},{"key":"ref_29","doi-asserted-by":"crossref","unstructured":"Della Sala, R., Spinogatti, V., Bocciarelli, C., Centurelli, F., and Trifiletti, A. (2023). A 0.15-to-0.5 V Body-Driven Dynamic Comparator with Rail-to-Rail ICMR. J. Low Power Electron. Appl., 13.","DOI":"10.3390\/jlpea13020035"},{"key":"ref_30","doi-asserted-by":"crossref","first-page":"4642","DOI":"10.1109\/ACCESS.2023.3349244","article-title":"Rail to Rail ICMR and High Performance ULV Standard-Cell-Based Comparator for Biomedical and IoT Applications","volume":"12","author":"Centurelli","year":"2024","journal-title":"IEEE Access"},{"key":"ref_31","first-page":"1841","article-title":"A VCO-ADC Linearized by a Capacitive Frequency-to-Current Converter","volume":"70","author":"Garvi","year":"2023","journal-title":"IEEE Trans. Circuits Syst. II Express Briefs"},{"key":"ref_32","doi-asserted-by":"crossref","unstructured":"Yeon, P., Bakir, M.S., and Ghovanloo, M. (2018, January 8\u201311). Towards a 1.1 mm2 free-floating wireless implantable neural recording SoC. Proceedings of the 2018 IEEE Custom Integrated Circuits Conference (CICC), San Diego, CA, USA.","DOI":"10.1109\/CICC.2018.8357048"},{"key":"ref_33","doi-asserted-by":"crossref","first-page":"623","DOI":"10.1109\/JSSC.2019.2892426","article-title":"A Noise-Shaped VCO-Based Nonuniform Sampling ADC With Phase-Domain Level Crossing","volume":"54","author":"Wu","year":"2019","journal-title":"IEEE J. Solid-State Circuits"},{"key":"ref_34","doi-asserted-by":"crossref","first-page":"666","DOI":"10.1109\/JSSC.2019.2959479","article-title":"A 0.025-mm2 0.8-V 78.5-dB SNDR VCO-Based Sensor Readout Circuit in a Hybrid PLL-\u2206\u2211M Structure","volume":"55","author":"Zhao","year":"2020","journal-title":"IEEE J. Solid-State Circuits"},{"key":"ref_35","doi-asserted-by":"crossref","first-page":"121","DOI":"10.1109\/LSSC.2021.3092020","article-title":"A 94.2-dB SNDR 142.6-\u03bcW VCO-Based Audio ADC With a Split-ADC Differential Pulse Code Modulation Architecture","volume":"4","author":"Huang","year":"2021","journal-title":"IEEE Solid-State Circuits Lett."},{"key":"ref_36","unstructured":"Hu, C.M. (2010). Modern Semiconductor Devices for Integrated Circuits, Prentice Hall."},{"key":"ref_37","doi-asserted-by":"crossref","unstructured":"Li, M.-X., Jiang, C.-Y., Pan, Y.-Y., Chen, H.-H., and Hsu, Y.-W. (2019, January 3\u20136). Using Inversion-mode MOS Varactors and 3-port Inductor in 0.18-\u00b5m CMOS Voltage Controlled Oscillator. Proceedings of the 2019 International Symposium on Intelligent Signal Processing and Communication Systems (ISPACS), Taipei, Taiwan.","DOI":"10.1109\/ISPACS48206.2019.8986327"},{"key":"ref_38","doi-asserted-by":"crossref","first-page":"2263","DOI":"10.1109\/TED.2020.2989726","article-title":"Highly Tunable High-Q Inversion-Mode MOS Varactor in the 1\u2013325-GHz Band","volume":"67","author":"Saadi","year":"2020","journal-title":"IEEE Trans. Electron Devices"}],"container-title":["Sensors"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/1424-8220\/24\/19\/6161\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,10]],"date-time":"2025-10-10T16:01:04Z","timestamp":1760112064000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/1424-8220\/24\/19\/6161"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2024,9,24]]},"references-count":38,"journal-issue":{"issue":"19","published-online":{"date-parts":[[2024,10]]}},"alternative-id":["s24196161"],"URL":"https:\/\/doi.org\/10.3390\/s24196161","relation":{},"ISSN":["1424-8220"],"issn-type":[{"value":"1424-8220","type":"electronic"}],"subject":[],"published":{"date-parts":[[2024,9,24]]}}}