Sub-μW Tow-Thomas based biquad filter with improved gain for biomedical applications Ricardo P ovoa a, b, * , Richa Arya a , Ant onio Canelas a ,F abio Passos a , Ricardo Martins a , Nuno Lourenço a , Nuno Horta a a Instituto de Telecomunicaç~ oes, Instituto Superior T ecnico - Universidade de Lisboa, Lisboa, Portugal b Escola Superior N autica Infante D. Henrique, Paço de Arcos, Portugal ARTICLE INFO Keywords: gm-C OTA Biquad Tow-Thomas Biomedical Energy-efficiency ABSTRACT This paper presents an innovative topology of a gm-C Operational Transconductance Amplifier (OTA), with improved gain and energy-efficiency and its corresponding implementation inside a second order Tow-Thomas based filter configuration, for biomedical and healthcare applications. The proposed OTA architecture takes advantage of a current division technique, as well as the usage of a pair of cross-coupled voltage-combiners in replacement of the static current source that traditionally bias the differential pair. The circuitry proposed in this paper is described at analytical level, fully-designed at sizing level and validated at simulation level compounded by Monte Carlo results, using a standard 130 nm technology node. Both the OTA and the biquad filter architecture are compared, in terms of performance indexes, with state-of-the-art bibliography, where the potential of both is demonstrated. The designed filter operates at weak inversion and sub-threshold, being supplied by a 0.9 V source, achieving a cut-off frequency of 15 Hz, a gain of 7 dB, hence improving the input-referred noise and consuming nearly 0.55 μW. 1. Introduction Outstanding developments in wearable/implantable devices, which record both physical and psychological signals online, have brought revolutionary improvements in a variety of biomedical applications such as health monitoring for early disease detection, neural prostheses and also brain stimulation techniques and therapies [1,2]. The development of wearable devices has grown so much during the last two decades that lead to a new name: Wearable Technology. The sensors used in this field have unique design and development constraints: it is crucial to take into account the portability, the size and weight, the longevity, the ergo- nomics and the power consumption, in parallel with the energy-efficiency [3,4]. The devices must have small form factor and active area, and low power consumption, enabling comfortable, unob- trusive and chronic monitoring, hence, being suitable for everyday use. Particularly for implanted devices, the power consumption of these electronics must be low enough to avoid excess heat flux tissue harmful damage. Ultra-low power consumption is of paramount importance to enable miniature battery size and prolong the operational lifetime. Simply to give an example, a 1.4 V Zinc-air button battery with a capacity of 620 mAh is, in general, able to provide continuous power if uninter- ruptedly used for three months [5]. Nowadays biomedical monitoring systems include frontend low-noise amplifiers (LNAs), filter circuitry and variable or programmable gain amplification stages (VGA/PGA), further followed by multiplexed analog-to-digital converters (ADCs) and radio-frequency circuitry that send/receive raw data, as briefly shown in Fig. 1 [4–7]. Sensing amplifiers usually dominate the power and the noise of the recording frontend, hence, significant research activity has been focused on designing this important block. The integration of a package of low frequency interface with the whole sensor in the same chip is made possible with the development of micro electromechanical systems and, lately, carbon nanotube sensors as well. The sensors that measure phys- ical quantities: flow rate, temperature and pressure, show slow variation and require low pass filtering with upper band frequency limit of a few hertz to reduce environmental noise. Furthermore, the voltage signals are low, usually with a few mV of amplitude, as detailed in Table 1 [5,6]. Hence, in terms of filtering inside biomedical and healthcare applica- tions, a low cut-off frequency is often desired, varying with the field of applications, yet always in the order of units of Hz [6–10]. A low-pass * Corresponding author. Instituto de Telecomunicaç~ oes, Avenida Rovisco Pais, 1, Torre Norte, Piso 10, 1049-001, Lisboa, Portugal. E-mail address: rpovoa@lx.it.pt (R. P ovoa). Contents lists available at ScienceDirect Microelectronics Journal journal homepage: www.elsevier.com/locate/mejo https://doi.org/10.1016/j.mejo.2019.104675 Received 21 May 2019; Received in revised form 24 November 2019; Accepted 28 November 2019 Available online 2 December 2019 0026-2692/© 2019 Elsevier Ltd. All rights reserved. Microelectronics Journal 95 (2020) 104675