Design of Low Cost ECG Monitoring System Based on 51 Single Chip Microcomputer

1 Introduction

Virtual medical instruments make full use of the computer's rich hardware and software resources, and only a small number of dedicated software and hardware modules can be added to realize all the functions of traditional instruments and functions that cannot be realized by traditional instruments, and shorten the development cycle. The system consists of two parts: a data acquisition device with C8051F320 microcontroller as the core and an analysis and processing system with PC as the platform. In the design, the data acquisition device is considered to be small in size, low in power consumption, and fast in operation. Therefore, all components in the SMT package are used. The PC monitoring terminal receives data through the USB interface and has a high transmission rate. The graphic programming language LabVIEW is used to write functions such as display, storage, analysis and processing. The system can monitor and provide parameters such as cardiac cycle and heart rate in real time, and can also store and play back data to provide a basis for cardiovascular disease diagnosis. The software development of the system and the integration test between the hardware and the host computer software show that the system runs stably and reliably and achieves the expected results.

2 system hardware design

The system consists of two parts: C8051F320 data acquisition module and PC, as shown in Figure 1.

The data acquisition module is mainly composed of an ECG acquisition circuit and a DAQ interface card based on the C8051F320 microcontroller, as shown in Figure 2.

Figure 2 data acquisition module

The module collects the pre-processed ECG signal through the C8051F320 on-chip A/D converter, and then transmits it to the PC display via the USB bus. The PC part is mainly software design, including writing the data acquisition graphical user interface through the C8051F320 single chip on-chip USB host API function and LabVIEW software; realizing the process of receiving, displaying and processing the data collection module to send the collected data through the USB interface. Both the LabVIEW application and the C8051F320 application use the API and drivers of the Silicon Laboratories USB Xpress Development Kit to read and write to the underlying USB device.

The ECG signal is a weak signal, and the amplitude of the surface ECG signal ranges from 1 to 10 mV. There is a strong interference in measuring ECG signals, including measuring the DC polarization voltage generated by the chemical half-cell formed between the electrode and the human body, and the 50 Hz power frequency interference in the form of common mode voltage. Exercise of the human body, baseline drift caused by breathing, and myoelectrical interference caused by muscle contraction. The use of telemetry HOLTER three-lead wire and disposable ECG electrodes to contact the human body can reduce the myoelectric interference caused by exercise and breathing. The front-end amplifier uses an AD620 amplifier with a very high common-mode rejection ratio (CMRR) for a magnification of approximately 50 times; a 0.05-100 Hz bandpass filter and a 50 Hz notch circuit are used to suppress the signal. Baseline drift, high frequency noise and power frequency interference. In order to make full use of the accuracy of A/D conversion, the signal is amplified to about 70% of the reference voltage of the A/D conversion circuit before conversion. Considering that the DC component is added to the signal, it is necessary to increase the level before the A/D conversion circuit. Adjust the circuit. The difference of the individual ECG amplitude requires the design of the program-controlled amplifier circuit in the circuit, and in order to facilitate the calibration of the ECG signal and take into account the deviation between the actual device amplification factor and the theoretical value, a manually adjustable amplification circuit is arranged in front of the program-controlled amplification circuit (1) ~10 times).

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