IEEE TRANSACTIONS ON BIOMEDICAL CIRCUITS AND SYSTEMS, VOL. 3, NO. 1, FEBRUARY 2009 53 Systematic Design and Modeling of a OTA-C Filter for Portable ECG Detection Shuenn-Yuh Lee, Member, IEEE, and Chih-Jen Cheng, Student Member, IEEE Abstract—This study presents a systematic design of the fully differential operational transconductance amplifier-C (OTA-C) filter for a heart activities detection apparatus. Since the linearity and noise of the filter is dependent on the building cell, a precise behavioral model for the real OTA circuit is created. To reduce the influence of coefficient sensitivity and maintain an undistorted biosignal, a fifth-order ladder-type lowpass Butterworth is em- ployed. Based on this topology, a chip fabricated in a 0.18- m CMOS process is simulated and measured to validate the system estimation. Since the battery life and the integration with the low-voltage digital processor are the most critical requirement for the portable diagnosis device, the OTA-based circuit is operated in the subthreshold region to save power under the supply voltage of 1 V. Measurement results show that this low-voltage and low-power filter possesses the HD3 of 48.9 dB, dynamic range (DR) of 50 dB, and power consumption of 453 nW. Therefore, the OTA-C filter can be adopted to eliminate the out-of-band interference of the electrocardiogram (ECG) whose signal bandwidth is located within 250 Hz. Index Terms—Continuous-time filter, ECG, low power, nonide- ality, nonlinearity, portable circuits, SIMULINK, subthreshold. I. INTRODUCTION W ITH the help of the integrated circuit technology, medical diagnostic instruments can be compacted to portable devices for the purpose of homecare to diagnose heart disease [1]. This can be done by creating an apparatus which simultaneously records electronic signals from the surface electrodes on the human body. These assistant technologies are not only used to diagnose patients, but also provide them access to handy and convenient medical instruments. Hence, the power-saving requirements of portable and durable equip- ment give circuit designers the impetus to reduce the current it consumes and extend the battery lifetime [2]. In addition to the issue of power consumption, the analog front-end circuits as the interface between physical signals and digital processor must operate under a low-supply voltage for the purpose of Manuscript received March 27, 2008; revised May 21, 2008 and July 31, 2008. First published January 06, 2009; current version published January 28, 2009. This work was supported by the Chip Implementation Center (CIC) and the National Science Council, Taiwan, under Grants NSC 96-2628-E-194-015- MY3, NSC 96-2220-E-194-008, and NSC 96-2220-E-194-012. This paper was recommended by Associate Editor K. Chakrabarty. The authors are with the Electrical Engineering Department, National Chung Cheng University, Min-Hsiung Chia-Yi, 62107 Taiwan (e-mail: sylee@vlsi.ee. ccu.edu.tw; 92bobs@vlsi.ee.ccu.edu.tw). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TBCAS.2008.2007423 being integrated into the low-voltage system-on-a-chip (SoC) system [3], [4]. To preprocess the cardiac signal (also called ECG measured on the surface of the body), referring to Fig. 1, a preamplifier with low input-referred noise will be utilized to perform the 10–100 times amplification of the weak ECG signal (amplitude range of 100 V–4 mV) [5]. Behind the amplifier, an ultra-low power filter with low cut-off frequency is adopted to decrease the out-of-band noise (typically above the frequency of 250 Hz [5]). However, for low frequency biomedical applications, real- izing lowpass filter circuits with large time constant under an acceptable capacitor’s value (typically 10 pF for the realizable area in the circuit implementation) isn’t an easy task. For instance, the ECG application 250 requires a resistance greater than 100 . Switched-capacitor (SC) is a popular integrated circuit technique used in implementing this long-term ECG monitor system [6], [7]. Due to the leakage problem of the advanced process, the sample-hold circuits of the switch-based topologies are not suitable for applications requiring large time constant (of the order of millisecond or more). In order to overcome the leakage problem, the leakage-reducing mechanism is usually required in the switched-capacitor circuits [8]. Thus, there have been the proposal to use low-power continuous-time well-suited OTA-based filters composed of the open-loop OTA-C integrators [9]–[13] in which the devices are operated in the subthreshold region to realize a very low transcon- ductance ( , typically of the order of a few nanoamperes per volt). In OTA-based circuits, the OTA will dominate the performance of the filter circuit, and the ratio of the capacitor to the small transconductance determines the time constant of OTA-C integrators. When performing a systematic OTA-based filter design, two considerations must be taken into account: first, we determine the topology and the specification of the filter appropriate for the biomedical signal acquisition; second, with a chosen topology, we determine whether a given building cell used to construct this filter fulfill the requirement of such signal processing. The decision in the first consideration depends on the characteristics of the bio-signals, such as the bandwidth as well as the toler- ance to distortion, and the second consideration must be esti- mated by a detailed analytical model of nonideal factors caused by the OTA. In the continuous-time filter design, total harmonic distortion (THD) and signal-to-noise ratio (SNR) are often used to inspect the performance of the whole filter circuit. The main impact on THD is the inherent linearity of OTA’s transconduc- tance because it induces the harmonic distortion (HD) to the filter system. In the low-frequency application, the flicker and thermal noise of OTA, especially the former, will be a severe 1932-4545/$25.00 © 2008 IEEE