Design of a low-noise portable ECG monitor

0 Introduction
A monitor is a device or system that measures and monitors the physiological parameters of a patient and compares them with known set values. If a deviation occurs, an alarm can be issued. Portable monitors are small and convenient, with simple structures and stable performance. They can be carried around and powered by batteries. They are generally used for monitoring patients in non-intensive care units and for emergency rescue. Cardiovascular disease is one of the most important threats to human life, and electrocardiogram (ECG) signal is the main basis for diagnosing cardiovascular disease. Therefore, it is of great significance to monitor the patient's ECG activity in real time and design a portable system that automatically collects the patient's ECG signal. The
traditional lead system adopts a universal three-electrode method. The upper electrode on the right chest and the lower electrode on the left abdomen are ECG sampling electrodes, and the lower electrode on the right abdomen is the right leg driving electrode. This connection method is effective and practical, and is conducive to portable use. The portable monitor analysis and processing system can be divided into two parts. One is a pocket monitor carried on the person being examined, and the other is an ECG processing and diagnosis system composed of a microcomputer system. The person being examined records the dynamic ECG signal of a certain period of time by the monitor, and transmits the data to the hospital's ECG processing and diagnosis system through GPRS communication. ECG signals are quite complex weak signals emitted by the human heart. In order to obtain ECG signals with less noise, it is necessary to perform noise reduction on the collected ECG signals.
Features of this design:
(1) There are many methods for reducing the noise of ECG signals. Here we mainly introduce the separation of noise from the signal from the aspect of filtering. The filtering adopts high-pass and low-pass two-stage filtering. The filtering circuit has obvious effect after Workbench simulation.
(2) Different from the previous double-T type 50 Hz notch filter, the designed circuit introduces an amplifier to form positive feedback to reduce the stopband width.
(3) In order to facilitate daily life of human body, this paper designs a lead electrode detachment detection circuit to prevent the movement input electrode from detaching.

1 Characteristics of ECG signals and overall system structure
ECG signals are medical biological signals, which generally have the following characteristics: strong randomness, that is, the signal cannot be described by a certain function, but can only be described by statistical methods from a large number of measurement results; strong noise background, that is, the useful signal to be measured is often submerged in many useless signals. The frequency band range of conventional ECG signals is 0.05-100 Hz, and 90% of the energy components of ECG signals are contained in this frequency band. Since ECG signals are mV-level signals, they are very weak signals for interference environments.
ECG signals are taken from the surface of the human body by skin electrodes and are low-frequency weak bipolar signals. They are submerged in many strong interferences and noises. These interferences mainly include internal interference signals such as electromyographic signals and respiratory wave signals, and the influence of external electromagnetic field interference signals mainly composed of 50 Hz power frequency interference and noise between the electrode and the skin interface. The signal source impedance is about 100 kΩ, the signal is 10μV~5 mV, and the typical value is 1 mV. In addition, the surrounding electromagnetic interference (especially 50 Hz power frequency interference) is relatively large, requiring the amplifier circuit to have high gain, high input impedance and high common mode rejection ratio; to maintain the stability of the signal, it is also required to have small input offset voltage and bias current, small temperature drift; in order to be easy to carry, it is also required to be small in size, low in power supply voltage, and low in power consumption.
To accurately measure the ECG signal, an amplifier with excellent performance must be designed. The core and key of the amplifier is the design of the preamplifier. The entire preamplifier circuit consists of a preamplifier circuit, a notch circuit and a filter circuit. After the ECG signal obtained from the body surface is input through the lead, the ECG signal is amplified by the preamplifier composed of an op amp, and the filter removes the high-frequency interference, and then further suppresses the power supply interference through a 50 Hz notch filter, and then enters the A/D conversion through level shifting, thereby obtaining a digital ECG signal.

2 Circuit structure description, ECG signal sensing, amplification and filtering
2.1 Circuit structure description and simulation
The entire monitor is composed of a preamplifier circuit, a notch circuit and a filter circuit. The medical sensor obtains the ECG signal from the body surface, filters out interference from other frequency bands, and then transmits it to the computer for data analysis after amplification, conditioning and A/D conversion. In terms of portability, an electrode detachment detection circuit is designed to get rid of the shackles of cables so that users can carry it with them. The hardware circuit can be simulated with Workbench software to realize its function, and the filter function used can meet the design requirements after simulation with Matlab and Filterlab software. The filtering method uses a 50 Hz notch, and then passes through two stages of high and low pass filtering, and introduces an amplifier to form positive feedback to reduce the stopband width. [page]

2.2 ECG input electrode
The electrode is crucial to the quality of ECG signal acquisition and recording by dynamic ECG. The electrode should be of high quality with strong adhesion, good air permeability, sweat absorption, good electrode conductivity, and low polarization voltage. In addition, it should have the advantages of less skin irritation, comfortable wearing, and easy disassembly. Usually, a detachable disposable soft electrode with AgCl plated on the surface is used, and a high-quality conductive paste is applied on the electrode.
2.3 Preamplifier
The preamplifier circuit of the portable machine is the key part of the automatic detection of ECG function. It is required that the system can detect the ECG signal without distortion through the surface sensor under strong noise background, amplify it to the appropriate amplitude, and send it to A/D to become a digital signal for computer analysis and processing.
The acquisition of biomedical signals such as ECG signals adopts a modular approach, which is mainly composed of front-end medical sensors, signal filtering and amplification conditioning circuits, and A/D sampling circuits. Among them, the conditioning circuit selects different filters and amplifier circuits according to the different spectrums and amplitude ranges of different biomedical signals. The ECG signal is amplified by the preamplifier, which includes the right leg drive to suppress common-mode interference and the shielded line drive to eliminate lead interference. The gain is set to about 10 times. The preamplifier is designed to use the medical amplifier AD620 produced by Analog Devices. The amplified signal is further amplified after filtering and 50 Hz notch processing, and the post-stage gain is set to about 100 times. Since the maximum amplitude of the ECG signal is a few mV, and the amplitude of the input signal in the A/D conversion is required to be above 1 V, the total gain is set to about 1,000 times.
Among them, the filtering uses a second-order high (low) pass filter circuit to eliminate interference signals such as myoelectricity outside the 0.05-100 Hz frequency band, and the remaining high-order harmonics in the power frequency can also be filtered out. At the same time, an active double-T band-stop filter circuit is used to further suppress 50 Hz power frequency interference.
2.4 Amplification of ECG Signals ECG
signals are low-frequency weak signals under high-intensity noise, and the contact resistance between the electrode and the body surface is generally as high as several megohms, so the preamplifier stage should have the characteristics of high input impedance, high common mode rejection ratio, low noise, high and adjustable gain, low power consumption and strong anti-interference ability. After comparison, the low-cost instrumentation amplifier AD620 from Analog Device was selected. The
specific implementation circuit of ECG signal amplification is shown in Figure 1. The gain of the ECG signal preamplifier stage should not be set too high to avoid serious distortion of the signal when the interference is strong. In order to better eliminate the common-mode voltage, a bootstrap shield drive circuit is designed as shown in Figure 1. A buffer amplifier is used to drive the common-mode potential of the connection point to the shield line, so that the shield line and the core line are at the same potential when the common-mode signal is input, and there is no effect when the differential-mode signal is input. In order to further improve the anti-interference ability of the circuit, the right leg drive circuit is used to fundamentally reduce the interference caused by the space electric field on the human body. This right leg drive is not a right leg drive in the actual sense, because since the emphasis of this system is on portable operation, the electrode is set on the lower right side of the abdomen.

2.5 Electrode shedding detection
Since this system is used in daily life, people are often active, so the input electrode is likely to fall off, causing the system to not work properly. For this reason, a lead electrode shedding detection circuit is designed as shown in Figure 2.

Under normal circumstances, the polarization voltages formed by the positive and negative electrodes on the human skin can offset each other. When one side of the electrode falls off, a larger polarization voltage will be input. Through a comparator, when the comparison voltage exceeds the range, it is considered that the electrode lead has fallen off, and the V0 output level changes from the normal high level to the low level, the lower transistor is turned on, and the buzzer sounds to indicate.

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2.6 ECG signal filtering
BT3 is interfered by various noises. The sources of noise are usually the following: power frequency interference, electrode contact noise, artificial electromyographic interference (EMG), baseline drift, etc. Among them, the 50 Hz power frequency interference is the most serious and the most difficult to eliminate. Other noises can be well eliminated by high-cut low-pass and high-pass low-cut filtering methods.
The ECG signal obtained from the ECG electrode must first pass through the preamplifier circuit. The processed signal has low noise, low drift, low common mode rejection ratio and other performances. At this time, the ECG signal is mainly interfered by power frequency, electromyography and other signals. The ECG signal needs to be notched twice and filtered twice to achieve the purpose of noise elimination. The two notches filter out the 50 Hz power frequency signal and the 100 Hz harmonic signal respectively. The two filters are a 0.05 Hz high-pass filter and a 100 Hz low-pass filter. In this way, a smoother waveform can be obtained.
2.6.1 Notch circuit
The circuit of the notch filter is shown in Figure 3. The circuit is an active filter with a double-T network, and its transfer function is:

Different from the previous double-T notch filter, this circuit introduces amplifier A2 to form positive feedback to reduce the stopband width, so that the amplitude on both sides near the stopband center frequency increases. The quality factor Q can be adjusted by the variable resistor Rw. The values ​​of R and C can be determined by the center frequency f0.

When f0=50 Hz, C and R are 0.068μF and 47 kΩ respectively; when f0=100 Hz, C and R are 0.068μF and 24 kΩ respectively.
Figure 4 is the simulation result of Filterlab 2.0 for the transfer function of equation (1). It can be seen that the notch circuit design meets the requirements.

2.6.2 Bandpass filter circuit

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The bandpass filter circuit is shown in Figure 5, which uses an active filter with feedback. The first half of the circuit is a 0.05 Hz high-pass filter, and the second half is a 100 Hz low-pass filter.

The transfer function of a high-pass filter is:

The selection of the resistor and capacitor values ​​not only has the function of filtering but also has the function of amplification. The amplifiers used in all the above circuits are OPA2137 from TI.
Figure 6 is the filtering simulation result of Matlab. It can be seen from the figure that the signal is well suppressed at 50 Hz, and the filtering effect is very ideal, which can fully meet the requirements of clinical practical application.

The filter is particularly important to the quality of the final signal. Since the performance of the filter is quite sensitive to the errors of components, stable and precise resistors and capacitors need to be selected in the design of this level. Precision potentiometers can be connected in series to obtain better results.

3 Conclusion
The performance of each filter in the circuit is directly related to the parameters of the filter and needs to be correctly calculated. The resistors and capacitors in the notch filter double-T network need to be accurately matched to ensure the symmetry of the double-T network, otherwise the notch depth will be affected. How the variable resistor is adjusted will affect the quality of the waveform, which can be debugged in the experiment.
Figure 7 is the result of the actual circuit test (the vertical axis is μV), and it can be seen that the circuit has completed the noise reduction of the ECG well. Of course, shielding technology can be added in the noise reduction process to further reduce the interference of external signals. The bandpass filter can also be designed as a filter with only one amplifier, making the circuit simpler, but the accuracy may be reduced.

In order to obtain clear and stable ECG signals, the design of the preamplifier and filter in the ECG amplifier is critical, especially the 50 Hz band-stop filter. The ECG amplifier designed in this paper, which is composed of AD620 op amps, can achieve high output voltage gain, low noise, and high sensitivity, ensuring that the ECG signal is clear and stable. The monitor made according to the above design is small in size, low in power consumption, easy to carry, and works normally. The measured output ECG waveform is basically free of distortion, and both P and T waves can be truly displayed. In particular, the circuit has good resistance to 50 Hz notch performance, and parasitic power frequency interference is basically not seen in the signal. The circuit has good stability, and even if the electrode falls off, the baseline has no obvious drift. It meets the requirements of home monitoring and pathological analysis.
As a portable monitoring instrument, simple hardware structure and easy to carry volume are its inherent characteristics. In view of these characteristics, this paper adopts MSP430 microcontroller for ECG signal acquisition, storage and data processing from the perspective of saving energy and cost. In order to better implement the filtering function, a DSP chip with the advantages of fast computing speed and floating-point operation can be used for improvement, so that the collected signal distortion is smaller and the fidelity is higher, and the accuracy of ECG signal collection is greatly improved, but the expensive price of DSP will increase the cost.