The Design Of Multi-channel Active Flexible Array Electrodes And Signal Conditioning Circuit

Physiological condition of the human body can be understood in time by the use of EMG, ECG, EEG and other electrophysiological measurements, which contribute to accurate diagnosis and treatment for many diseases. Electromyography (EMG) is usually used for examining the nerve conduction velocity to assist the clinical diagnosis of neuromuscular disorders. Due to its noninvasive and painless feature, surface EMG (sEMG) has been widely used in sport rehabilitation, the human-machine interface and human body engineering and so on. In a variety of sEMG-related clinical and scientific applications, it is of great demand to develop surface EMG acquisition with multi-channel, low-noise, high anti-interference features, for the purpose to obtain signals with more detailed temporal and spatial information. Therefore, the design of high-density surface electrode array has become a hotspot in development of sEMG acquisition devices in recent years, where the acquisition of multi-channel weak signals with high-performance becomes a key issue to be addressed.This paper focuses on the design of a multi-channel sEMG signal acquisition device. A flexible electrode array based on polyimide flexible materials was designed with active components, and the corresponding multi-channel signal conditioning circuit and A/D converter circuit were also designed for high performance acquisition of sEMG signals. Not limited to the multi-channel sEMG acquisition, the designed system can also be employed for collecting other electrophysiological signals such as ECG and EEG. In addition, a portable ECG monitoring module was also developed, including biplor leads for ECG measurement, digital filtering, and the data display interface for real-time use in PC.The achievements of this work include but not limited to the following aspects:(1) The flexible electrode array using active components and the corresponding conditioning circuit were designed. The flexible electrode array was implemented in two different ways by considering multiple aspects of the design such as differential structure of the electrode, center-to-center electrode spacing, and noise suppression and so on.For addressing the issue derived from large skin-electrode contact impedance that it is easy to introduce high-frequency noise, two types of active electrode, namely single-gain and multi-gain electrode, were examined. By considering the demand of electrode spacing and the desired number of channels, the single-gain electrode was finally selected to carry out the design of multi-channel electrode array. Meanwhile, in order to maintain a good contact between each electrode and uneven skin surface, the polyimide was used as a basic material of printed circuit board to enhance its flexibility. A number of different flexible active electrode arrays with up to32single-differential channels were designed on the basis of high signal quality. The corresponding multi-channel signal conditioning circuit was designed according to the range of sEMG signal amplitude and frequency bandwidth, mainly including a first-stage amplification and filtering circuit based on an instrumental amplifier, a multiple feedback filtering amplifier circuit based on a general purpose amplifier and multi-channel A/D converter, consequently yielding approximately a signal amplication gain of1500, and a bandpass width of20～00Hz per channel.(2) The performance of the designed flexible electrode array based on active components was examined by conducting two different experiments. In the first experiment, the designed flexible active electrode array was compared with a previously developed passive flexible electrode array, and spring needle electrode array, in terms of SNR, signal power spectrum and root mean square of the baseline, respectively. Experimental results showed that the designed active electrode array was able to reduce interference to original sEMG signals from the power line and impedance of electrode-skin contact, and that the signal quality was improved significantly with the flexible electrode array applied to uneven skin surface. The second experiment was to compare the designed active electrode array with a commercialized system,64-channel passive flexible electrode array designed by TMSi Company. It was also found from the experimental results that the designed active electrode array can collect sEMG signals with higher total amplification gain, whereas the baseline noise from the64-channel passive flexible electrode array by TMSi was much lower. The reason might be that additional double-sided adhesive and conductive gel was applied for the use of the electrode array from TMSi.(3) A portable ECG module was built and designed. According to the demand of portablilty in personal healthcare, some special designs were applied to the ECG module. The original ECG signals were detected by bipolar leads. The circuit of the module consisted of the signal amplification and filtering circuit, an A/D converter and a data transmission circuit. A simple and effective Levkov filtering algorithm was adopted for real-time signal filtering and processing, due to its low computational cost and ease to be transplanted in MCU. Finally, a small size, portable ECG monitoring module was developed with functions of real-time display and data storage on the PC.