introduction
Heart disease is one of the major diseases that seriously threaten human health and life. Statistics show that about 60% of heart disease patients die at home. If these patients can get timely rescue and care, it is entirely possible to avoid death. Due to the great chance and suddenness of heart attacks, extending ECG monitoring from hospital beds, hospitals to communities, and families to implement remote monitoring, whether it is from the perspective of reducing the financial burden of patients, or from the perspective of enhancing hospital service capabilities All have important practical significance.
2 Hardware design of ECG monitoring terminal
The hardware circuit of the ECG monitoring terminal is designed from the perspective of small size, low power consumption and easy operation. Figure 1 is the hardware block diagram of the entire monitoring terminal, which is mainly composed of 5 parts: conditioning circuit, ECG data acquisition module, ARM7 microprocessor module, network port module, and power module. The monitoring terminal completes the collection and preprocessing of ECG signals, and sends it to the server of the monitoring center in real time through the network port, thereby realizing remote real-time monitoring.
2. 1 signal conditioning circuit design
The human electrocardiogram signal is a low-frequency weak electrical signal with an amplitude of about 10μV ~ 5 mV and a frequency range of 0.05 ~ 100 Hz. It is necessary to enlarge thousands of times (that is, to reach the order of V) to facilitate observation and A / D conversion, and the specific gain needs to be determined in conjunction with the measurement parameters of the A / D conversion module. The ECG signal obtained through the ECG lead is first differentially amplified by the AD620 instrumentation amplifier. Since the primary battery may be formed between the body surface fluid and the electrode, resulting in a fixed potential difference between the electrodes, the differential amplification gain of the first stage cannot be Too high, otherwise it is easy to saturate. Therefore, the gain of the first stage is selected as 20; in order to make the signal meet the A / D conversion requirements, the signal needs to be amplified to the level of V, so the secondary amplification gain is set to 30, and the ordinary four op amp LM324 is used here. Since the DC and baseline drift interference is often mixed in the ECG signal, a high-pass filter should be added between the first stage and the secondary amplifier circuit to effectively avoid the baseline drift of the ECG signal, and the corresponding high-frequency interference signal can pass through the amplifier The input circuit and a second-order low-pass filter with a cut-off frequency of 100 Hz are used for filtering. In addition, the 50 Hz power frequency interference is filtered by the notch filter. After the above signal processing, the ECG signal is connected to the P0.27 pin (AIN0) of the microprocessor LPC2210 to complete the A / D conversion by its internal A / D conversion module. According to the sampling theorem, the sampling frequency should be guaranteed to be higher than 2 times, so the sampling frequency is set to 500 Hz in the A / D conversion. Figure 2 shows the signal conditioning circuit.
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