With the clinical application of electrocardiography technology and the development of electronic technology, electrocardiogram, as a mature and widely used technology in biomedical measurement, has gradually become a routine clinical examination method, and is diagnosed and monitored in heart disease. The effects of pharmacodynamic analysis play an important role. At present, most commonly used ECG detection circuits are powered by dual power supplies. This solution requires many power supply devices and a large area of ​​layout and wiring, which will increase the cost of the product. The design given in this paper uses a single power supply to solve the above problems and reduce the cost of the product. At the same time, the design also uses a simple and practical algorithm in the ARM core-based embedded system, which can quickly and accurately locate the QRS complex wave (ie, the computing person) Heart rate). Designed for the majority of home users, the design is small and requires only a personal computer to connect to it, allowing real-time operation and observation of ECG signals. The basic architecture of the ECG signal acquisition system is shown in Figure 1. The human body's ECG signal is sent from the body to the system via electrodes and dedicated lead wires. Through the filtering and amplification adjustment circuit, the weak ECG signal is amplified to the appropriate amplitude and is within the A/D conversion range. System control and data processing is performed by Analog Devices' MCU ADuC7020 based on the ARM7 TDMI core. This chip has a wealth of on-chip peripheral circuits, processing speed of up to 40MIPS, A / D conversion speed of up to 1MSPS, with a high cost performance. Finally, the result is sent to the computer by the ADuC7020 through the UART port, and the result is visually displayed or stored by the computer through the interface written by LabVIEW. Figure 2 is a basic hardware circuit diagram. Figure 1: The basic architecture of an ECG signal acquisition system. The amplitude of the ECG signal collected from the human body or the ECG signal generator is between 0.05 and 5 mV (generally 2 mV) and the frequency is between 0.05 Hz and 75 Hz. The ECG signal is subjected to several stages of circuits such as buffering, matching resistor network, voltage amplification and filtering. The ECG signal first passes through a two-stage RC low-pass filter circuit and enters the buffer stage. Before the signal enters the system, the high-frequency component needs to be removed. Therefore, a low-pass filter with a cutoff frequency of 300 Hz is designed to ensure that the weak ECG signal of 0.05-75 Hz is not attenuated. The buffer stage consists of a voltage follower that increases the input impedance of the entire amplifier circuit and reduces the output impedance. In order to ensure the consistency of the differential signal, the amplifier integrated on the chip should be selected. The matching resistor network usually uses the Wilson electric center network, which obtains the Wilson electric center end as a reference point for the entire ECG system through a specific resistor network. The front stage of the filter amplifier circuit uses a negative feedback differential amplifier circuit. Here, AD8221, an instrumentation amplifier with ADI's adjustable gain and high common-mode rejection ratio, is used as the preamplifier. The magnification is set to 8 times and is calculated by the formula G=49.4kΩ/RG+1, where RG is the resistance value between the two RG pins of the AD8221. Figure 2: Basic hardware circuit. This resistor should use high-precision, low-temperature drift Metal film resistors to ensure low noise performance of the AD8221. The REF pin of the AD8221 is not grounded, but instead forms a negative feedback loop with a low-pass filter to effectively filter out the DC component, so that the voltage at U1 is always clamped at 1.25V. Because it is a single power supply, and the voltage of the ECG signal of different leads is positive and negative, it is necessary to provide a suitable clamping voltage. The voltage input range of the ADuC7020 in AD conversion mode is 0-2.5V, and the intermediate value of 1.25V is selected as the clamp voltage. The single-ended signal output of the AD8221 is very small, and there is a lot of interference mixed with it, and data processing is impossible. The post-stage filter amplifying circuit is composed of an active low-pass with a large gain, and the gain is adjustable. Different people's ECG signals are different in intensity. Considering the attenuation, the gain is generally set to 150 times. The transfer function of the system is: Both low-pass filters require low voltage drift, low temperature drift, and low noise, and Analog Devices' rail-to-rail input and output dual op amps, the AD8607, are well suited to these requirements. Since the second stage amplifier is the inverting input, the resulting signal is inverted, which can be processed in software. From the point of view of protecting the patient and improving the common mode rejection of the system, the common mode signal must be inverted and amplified, and then fed back to the human body, so that the system and the human body together form a voltage parallel negative feedback network, which is commonly said The right leg drive circuit. The processed ECG signal is AD-converted by the ADuC7020, and the ADC sampling mode controlled by the timer is selected. At the end of an A/D conversion, the ADC interrupt is triggered and the digital signal is processed in the interrupt service routine. Figure 3: Basic steps of data processing. The key to processing ECG digital signals is the accurate identification of QRS complexes in ECG signals. A normal person's QRS complex has a width of 0.06 to 0.10 seconds and is not affected by heart rate changes. In view of the sharpness of the R wave, we judge whether the peaks and valleys of the signal meet the requirements through a sliding time window and confirm whether it is in the time window. The threshold of the signal amplitude is a double variable threshold method, that is, a peak threshold and a valley threshold are set for the waveform. If the peak and valley thresholds change significantly over a suitable period of time, reset the peak and valley thresholds. Below we will discuss QRS complex positioning and heart rate calculation. The signal for the initial period of time is only inverted and filtered. This is to recover the inverted ECG signal and avoid the initial fluctuation of the signal to affect the threshold. Then, within a certain period of time, the peak threshold Thpeak and the valley threshold THtrough are set based on the sampled data, and then the QRS wave is positioned. Finally, follow the steps below (see Figure 3) for data processing. Read the new sampling point Ni: 1) Determine whether the sampled signal point amplitude is greater than the peak threshold Thpeak. If not, go back to step 1). 2) If the condition is met, start counting n=1 and record the value of n as peakTIme1. Slide the start side of the time window here. 3) Continue to sample Ni+1, n+1 for each sample. 4) Determine whether the new sampling point Ni+1 is smaller than the valley threshold. If not, go back to step 4). 5) If the sampling point is less than the valley threshold Thtrough, the n value of the point is recorded as troughTIme1. 6 It is judged whether the difference between the signal point and the peak TIme1 of the two times satisfying the amplitude requirement is within the time window, that is, whether it is smaller than the window width THtime. If it is not less than THtime, return to step 1). 7) If satisfied, this signal is considered to be a QRS complex. Peaktime1 is positioned as an R wave. With the positioned R wave, you can calculate the heart rate by following the steps below: 1) Find the first R wave and record the n value as peaktime1; 2) Find the second R wave and record the n value as peaktime2 ; 3) Calculate the heart rhythm according to the following formula. Practice has proved that the accuracy of the heart rhythm calculated by this algorithm is high, the calculation is simple and easy to implement, and finally the ECG signal waveform and heart rate value are observed on the PC. Solid-state Capacitors / Motor Starting Capacitors Solid - state capacitors are all called: solid - state Aluminum Electrolytic Capacitors.It with the ordinary capacitance (that is, the liquid aluminum electrolytic capacitors) the biggest difference is that use different dielectric material, liquid aluminum capacitor dielectric material as the electrolyte, and solid-state capacitor dielectric material is conductive polymer materials.Solid-state capacitors / Motor starting capacitors Solid-state capacitors / Motor starting capacitors,Solid-State Capacitors,Solid-State Small Size Capacitors,Solid-State Low Impedance Capacitors,Long Life Solid-State Capacitors YANGZHOU POSITIONING TECH CO., LTD. , https://www.cnfudatech.com
