Anti-jamming design of single chip microcomputer in ECG detection system
With the rapid development of embedded technology, single-chip microcomputers have been used more and more in various medical instruments, so that the degree of intelligence and measurement accuracy of medical instruments are continuously improved. But at the same time, new problems have arisen, including interference problems. Light interference will affect the output, making it impossible for medical staff to make a correct judgment; it will make the instrument unable to work properly. How to effectively suppress interference is a problem that must be considered and solved in the design process of medical instruments. The ECG signal is a weak electrical signal of the human body surface. The detection system not only requires effective removal of noise interference in the measurement signal, but also requires the system itself to have better resistance to external interference. This article will take a self-designed ECG detection system as an example to introduce a series of anti-interference measures used in the system.
1 System external interference sources
The structure of the ECG detection system is shown in Figure 1. Among them, the single-chip used in the system is AT89C51, and other modules include reset circuit, keyboard, LCD and so on.
Figure 1 Schematic diagram of ECG detection system
Generally, the interference in the detection system mainly comes from: power supply interference, signal channel interference, and spatial radiation interference. In the case of ECG detection systems, signal channel interference and power supply interference are the main causes of system interference. Once the interference enters the analog signal input channel of the system and is superimposed on the useful signal, the data acquisition error will increase, especially for the weak signal such as ECG. The influence is more serious. In addition, the utility power grid is used. In the normal 50Hz sinusoidal waveform, there are many high-voltage spike signals superimposed on them, which will seriously affect the normal operation of the system and cause large interference to the ECG signals. Therefore, how to take appropriate methods to eliminate the interference source, suppress the coupling channel, and weaken the sensitivity of the circuit to noise interference is a special problem to be paid attention to when designing the system. It is usually necessary to take anti-jamming measures combined with hardware and software.
2 hardware anti-jamming measures
2.1 Power supply anti-interference measures
The power supply of the system can be as shown in Figure 2. The mains can be suppressed to some extent by the AC voltage regulator (selectable switching power supply with strong anti-interference ability), π filter circuit and DC voltage regulator circuit. State interference, removing high frequency interference. In addition, a high-frequency capacitor (typically 0.01-0.1μF) can be connected to the power input of each integrated chip to reduce the impact of the integrated chip on the power supply.
Figure 2 power circuit
2.2 Grounding Technology
In this system, the analog circuit and the digital circuit coexist, so all the analog ground and the digital ground should be connected separately, and finally the analog ground and the digital ground in the circuit are gathered together with the power ground. At the same time, the analog area is isolated from the digital area by the ground line, so that the analog circuit and the digital circuit can be prevented from interfering with each other through the ground loop.
2.3 Filter circuit
For systems with known signal characteristics, suitable filtering circuits can be designed to eliminate some of the interfering signals. As in this system, a dual T-type filter circuit [1] can be designed to remove 50 Hz power frequency interference in the signal.
2.4 PCB board wiring anti-interference design
When layout of the PCB, the interconnected devices should be arranged as much as possible; as far as possible, a device with a large amount of heat, such as a clock generator, should be placed in a position where the heat dissipation condition of the circuit board is good, so as to facilitate heat dissipation. When wiring the PCB board, the power and ground lines should be as thick as possible; as far as possible, the direction of the power and ground lines should be consistent with the direction of data transmission, which will help to enhance the anti-noise capability; Should be as close as possible to the power supply end. If it is far away, it will increase the equivalent series resistance of the high frequency capacitor and affect the filtering effect.
In addition, the system is a system in which analog signals and digital signals coexist, so digital and analog signal partitioning is required. At the same time, since the TTL level of A/D and AT89C51 is a strong signal relative to the ECG signal, the system is a system in which a strong signal and a weak signal coexist. Therefore, the system layout should consider how to avoid interference of a strong signal with a weak signal. This is to distinguish between the layout and direction of the two signal lines.
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