Novel structure and design of a DC-coupled sensor interface IC for biopotential acquisitions

Abstract

Wearable biomedical devices are gaining importance in personalized healthcare due to their important role in improving life quality and prevention-oriented telemedicine system. The performance of such biomedical sensor systems is very often determined by that of the interface integrated circuit (IC) since it is located at the front-end of the system. However, biopotential signals feature low amplitudes and frequencies, leading to low signal to noise ratio (SNR), which poses great challenges to the design of high performance interface IC with low noise and high linearity. In addition, to enhance user comfort and convenience, dry-electrodes, rather than traditional wet-electrodes, circuits with small system size, and more functionality are preferred for wearable applications. This work presents a novel fully integrated interface IC for bopotential signal acquisitions which aim to address the above design requirements. It possesses the following distinctive features: 1) To reject the direct current (DC) offset and lower the input noise, a novel preamplifier topology with forward offset suppression and noise modulation is proposed. The proposed preamplifier is implemented based on a new multi-input differential difference amplifier (DDA). Compared with previous state-of-the-art designs, the proposed approach is able to achieve DC offset suppression and a noise efficiency factor (NEF) of 2.83, which can be used for DC-coupled biopotential recording systems with dry-electrodes input. 2) To achieve small chip area and high linearity, a new continuous-time filter architecture is proposed by adopting the current-steering integrator and the second-order feedback structure. Tunable high-pass corner frequency as low as 0.05 Hz and around -76 dB total harmonic distortion (THD) are obtained. This design outperforms the traditional low cutoff frequency filters with pseudo-resistors with lower THD and smaller number of capacitors per pole. 3) To cater for wirelessly powered biomedical sensor systems with high energy efficiency and supply interference suppression, a new power converter system consisting of a full-active rectifier with leakage current rejection and a low dropout (LDO) regulator with power supply rejection ratio (PSRR) boosting scheme is proposed. The rectifier is able to achieve an average voltage conversion efficiency (VCE) of 93.4% at 2 kΩ load with a 3 MHz 4.5 V input and the LDO regulator is able to achieve a dropout voltage as low as 25 mV and a PSRR of -62 dB. The proposed interface IC consists of an analog conditioning channel with tunable gain and bandwidth, a power converter block, a clock generator and a serial peripheral interface (SPI). The interface IC was fabricated in a 0.18 μm standard Complementary Metal Oxide Semiconductor (CMOS) process and occupies only 0.72 mm2 silicon area. Based on the customized interface IC chip and a flexible dry-electrode, a flexible and miniaturized Electrocardiography (ECG) -patch prototype for wireless ECG signal recording has been implemented. In-vivo tests showed that the flexible ECG-patch is able to record ECG signals with dry-electrodes from human chest or fingers. The proposed circuit design techniques for the interface IC and biopotential acquisition systems provide a promising and attractive solution for wearable biomedical sensor applications.