An Integrated Method of ECG Data Acquisition
By Yin Peng, R&D Department
Electrocardiogram is one of the most important kinds of bioelectric information of human body and electrocardiosignal is a kind of periodic electrophysiological signal which is transmitted from body tissues to body surface and then generate potential difference on the body surface. In clinical practices, these potential differences can usually be measured and described in curves by various equipment and devices to form electrocardiogram. The method for measuring body surface electrocardiosignal is usually affixing electrodes to specific positions on the body surface to acquire the weak electrocardiosignals and then send the signals to ECG preprocessing circuits by ECG lead cables for processing. After this, the signals will be transformed into digital signals by A/D digitalization and sent to MCU for recognition of ECG characteristics and processing. Last, the processed ECG signals will be sent to the upper computer for displaying.
The traditional ECG acquisition system or device has used discrete components to acquire electrocardiosignals. These components include front-end protection circuit, electrotome suppression circuit, front-end impedance matching circuit, lead loss recognition circuit, main amplifying circuit, pacemaker detection circuit, high-pass filter circuit, the secondary amplifying circuit, low-pass filter circuit, the right leg drive circuit, A/D conversion circuit and MCU circuit. Because mass discrete components are used, such as integrated circuits, resistors and capacitors, it will lead to the fact that the device has a big size, high power consumption, complex design and low flexibility. What’s more, since there are many differences between the integrated circuits, resistors and capacitors, the lead parameters and indexes of ECG such as common-mode rejection ratio and input impedance are different. If the difference between the impedance and capacitor is too big, the filter characteristics of the front-end filter circuits will change, which will lead to inconsistency in the channel bandwidths of ECG.
2. Detailed design scheme
This thesis is going to put forward an integrated method for acquiring ECG to address the current problems existed in ECG acquisition. In this method, the integrated ASIC chip takes the place of traditional discrete circuits and puts the front-end impedance matching circuit, lead loss recognition circuit, main amplifying circuit, pacemaker detection circuit, the secondary amplifying circuit, the right leg drive circuit and A/D conversion circuit for ECG acquisition on one ASIC chip, thus diminishing the size, power consumption and cost. The 24 bits high-resolution A/D converter integrated can reserve the details of electrocardiosignals to the greatest degree. And since electronic components are reduced, consistency of all the lead’s ECG indexes is raised. What’s more, the high-pass filter circuit and low-pass filter circuit in the discrete circuits are realized by digital filters, which is more stable, makes the design more flexible and helps avoid the inconsistency of the characteristics of filters due to the difference of resistors and capacitors, which will lead to distortion of electrocardiosignals. This method will also to a great extent improve the precision and signal-to-noise ratio of the ECG acquisition system and can be realized by hardware circuits and control of software.
2.1 Circuits of hardware
The circuits of hardware are composed of front-end protection circuit, electrotome suppression circuit, ASIC chip circuit, main control circuit, power source circuit and interface circuit. The electrocardiosignals are transmitted to the front-end protection circuit and electrotome suspension circuit through patient’s lead wires, then after RC filtering in the electrotome suspension circuit it will go through the internal processing on ASIC chip. This includes the amplification of electrocardiosignals, lead loss detection, pacemaker pulse detection and A/D digital processing. After ASIC processing, it will be transmitted to the main control chip DSP through SPI interface for filtering and recognition of its characteristics. Afterwards, the processed and recognized electrocardiosignals and the result of processing will be sent to the upper computer for displaying. The functional block diagram is shown below:
ASICThe ASIC chip circuit has an integrated ASIC chip which put the front-end impedance matching circuit, lead loss recognition circuit, main amplifying circuit, pacemaker detection circuit, the secondary amplifying circuit, the right leg drive circuit and 24-bits A/D conversion circuit on one ASIC chip. The ASIC chip is controlled by the main control DSP through SPI interface. After the electrocardiosignals are processed and A/D digitalized, ASIC chip will transit them to the main control DSP for data processing. The integration block diagram of ASIC is as follows:
2.2 Software control
The software parts mainly consist of system control software, filter software and ECG characteristics recognition software. The flow diagram of the system control software is shown on the right: First, power on to initialize the peripheral resources of the main control chip which will initialize ASIC internal registers by SIP bus. (The internal registers include ECG lead register, pacemaker detection register and lead loss control register.) After initialization, wait for the ASIC chip to return to shake-hands data, then time to receive the data packets by ASIC chip, including ECG data, pacemaker state data and lead loss state data. If there is lead loss data, the main control chip will send the lead loss marks and if there is pacemaker state data, the electrocardiosignals will be get rid of pacemaker signals, after which filtering of electrocardiosignals and recognition of their characteristics will be conducted. When all of these are done, the processed data will be sent to the upper computer for displaying.
The ECG acquisition device described in this thesis adopts an integrated ASIC chip to replace the traditional components, the result of which is diminished size and power consumption of the electrocardiosignal acquisition system as well as reduced system design cost. This method has also made possible miniaturization of electrocardiosignal acquisition devices and made them portable. At the same time, since this integration method for acquiring electrocardiosignals has considerably reduced the use of electronic components, consistency, reliability and stability of the leads’ ECG parameters and indexes have been greatly improved.