Improvement of ECG amplifier for wearable devices

The amplifier occupies half of the volume of the entire ECG amplifier circuit, so the design of the amplifier becomes the key to the design of the ECG sensor in wearable devices. This article will provide some ideas for improving the amplifier circuit to achieve higher accuracy, lower power consumption, and smaller ECG modules. The author compares the advantages of different circuits and the performance of different components by means of comparison. Some ideas for improving ECG amplifying circuit are obtained. The amplifier occupies half of the volume of the entire ECG amplifier circuit, so the design of the amplifier becomes the key to the design of the ECG sensor in wearable devices. This article will provide some ideas for improving the amplifier circuit to achieve higher accuracy, lower power consumption, and smaller ECG modules. The author compares the advantages of different circuits and the performance of different components by means of comparison. Some ideas of improving ECG amplifying circuit are obtained, and some methods of improving ECG amplifying circuit by differential amplifying circuit, changing circuit components and adding filter structure are summarized.


Introduction
Electrocardiogram is one of the main basis for determining heart disease.Although the commonly used electrocardiogram detection machine has the ability to detect ECG signals and diagnose disease types, the high price of the equipment and the characteristics of not easy to carry bring a lot of inconvenience to the electrocardiogram detection.Therefore, in recent years, various research institutes have been developing more portable and accurate ECG amplifiers.The amplitude of the ECG signal is generally between 50μV-5mV [1].Since the amplitude of the human ECG heartbeat is very small, a large amplification ratio is needed to amplify the tiny signal into a signal that can be used for observation and diagnosis.This article will investigate how to improve existing wearable ECG sensors to reduce their size and power consumption.Currently, the ECG amplifying circuit consists of a differential amplifier in series with a post amplifier.Differential amplifiers are essential throughout the ECG amplifying circuit because of their both-sides input, both-sides output, completely symmetrical circuit structure that maximizes noise cancellation in the circuit.There are several key design parameters that need to be discussed.Because the physiological electrical signal is very weak, the ECG amplitude is only 50μV ~ 5mV, and the internal resistance of the human body is also very great, and because the human body is a good conductor, so the signal collected by the sensor is doped with a variety of electromagnetic interference, the interference is mostly manifested as a common mode signal.Therefore, the amplifier circuit requires extremely high impedance and amplification gain to display the ECG signal more accurately.And the value of CMRR needs to reach at least 100dB to ensure that the noise will not have a great impact on the accuracy of the results [2].
Moreover, due to the impurities contained in the material of the circuit device, the current will generate thermal noise due to the thermal effect when passing through, and due to the impurities contained in the resistance and other devices, it will lead to the circuit noise, so the replacement of better materials, update the developed devices will be a great help for the accuracy of the circuit.While in the power supply, because the circuit will have a 50Hz power frequency interference, so, it is essential to add a filter structure in the circuit.Therefore, adding a 50Hz band-stop filter to the circuit can also play a role in improving the accuracy of the circuit.

Different improvements of ECG amplifier
To reduce the death caused by cardiovascular diseases and provide more convenient cardiac status monitoring for people with cardiovascular diseases, the market is constantly developing more accurate, less power consumption, more portable ECG sensors.ECG sensors are widely recognized for their accuracy in wearable devices.If it needs to be applied in the wearable devices market wider, it needs to consider optimizing its power consumption and volume.Since the amplifier takes up half the volume of the entire ECG sensors' circuit, the design of the amplifier circuit is the key to the ECG in a wearable device.Now, however, the ECG amplifiers which are equipped in the smart devices in the market are still unreliable, it usually can't distinguish whether the electrical signal goes wrong, it usually gives out the result: the result has an indeterminacy.So, aiming to improve the reliability, this study focus on the hardware of the ECG amplifier, improving the accuracy of the hardware to get the signal of better quality, so the AI can better analyse the signal and give out a satisfying result in the end.As many research has realized that the OTA in the circuit is of great importance, which can be used to make different-pair based inverter to improve power-noise efficiency in the first stage.Studies of these related efforts reveal the importance of low-noise, low-power preamplifier design and optimized device geometry to produce complete cardiac screening SOCs suitable for use in wearables and the Internet of Things (IoT) [3].Therefore, as a basic part of the low voltage, low power analogy circuit design is particularly important.While there are some factors that can influence the ECG amplifier.

CMRR
So how to improve the common-mode rejection ratio of differential amplifier circuit becomes the first step of designing differential amplifier circuit.The first is the familiar design of symmetrical circuits.Because the symmetrical structure can ensure that the circuit has a better ability to resist common-mode interference, it can effectively improve the CMRR of the amplifier.At the same time, increasing the emitter resistance is also a good way to improve the CMRR.For ECG amplifiers that require a higher CMRR, we can also improve the differential mode gain, which can also improve the CMRR.These two schemes have an economic advantage over the last one.In addition, changing the components used in the circuit is also a way to improve the CMRR [4].The most effective is to replace the newer generation of products.For example, if choose the AD8422BRZ produced by ADI company as the front-end amplifier in the differential amplifier circuit.Under the condition of 10 times gain, its CMRR can reach to 114dB.Its CMRR can even reach to 150dB at the condition of 100 times gain [5].
Because the differential amplifying circuit requires two perfectly symmetrical amplifying circuits, it occupies a large volume [6].However, due to its low cost, superior performance and stability brought by simple principles, differential amplifiers are very widely used in ECG sensor amplifiers.
Here the author gives part of a sample circuit of differential amplifying circuit (Figure 1).

Materials
Another way to reduce noise is to change the resistance used in the circuit.The current noise of the resistor is directly related to the material, process, and type (nonlinear magnitude) of the resistor.The nonlinearity of the resistor reflects the degree of the internal structure of the resistor material is not dense and discontinuous (heterogeneous), so it reflects the quality of the resistor to a certain extent.Through experiments, it is found that most resistors have some nonlinearity.For a nonlinear resistor, the RMS value of current noise can be expressed as： Where, K is a constant that depends on the material of the resistor element and its manufacturing process, U is the DC voltage drop at both ends of the resistor,  is the frequency width, and is the signal frequency.The above equation shows that the current noise   is proportional to the square of the direct voltage drop U across the resistor and inversely proportional to the frequency within a fixed  bandwidth.As a result, under the same conditions, the use of better resistance which has a lower value of K can reduce the noise generated in the circuit [7].Noise will affect the performance and precision of the circuit, so choosing better resistance materials can improve the reliability and performance of the circuit.In addition, better resistive materials generally have higher stability and durability, can be used over a wider range of temperatures, and can reduce drift of resistance values due to temperature changes.The essential reason is that better resistance materials usually have a more uniform and stable structure, resulting in a more uniform distribution of current inside the resistor and less thermal noise as the current passes through.In addition, better resistive materials generally have higher purity, which can reduce the impact of impurities and defects on the performance of the resistor, and thus the ability to reduce noise in the circuit.
The research targeted on how to improve the reliability of the circuit.So, we compare the different materials in their effect on the circuit component and how these components change in different conditions and working hours.The main part of the circuit is the OTA in the amplifiers, if we design a more stable OTA with better materials which can let the amplifiers' parameters stabilize within an allowable error range, as a result, the circuit will do better in getting a clear ECG signal.So, materials like wide-band gap silicon carbide (SiC) and gallium nitride (GaN), which are called the third generation of semiconductor materials, what's more, Ga N is suitable for high frequency, high efficiency, high temperature, high pressure and broadband high power microwave devices.So, use material like Ga N can greatly improve the reliability of the circuit.

Filters
Because the circuit has a 50Hz power frequency interference, at the same time, the circuit will be affected by the surrounding electric field, magnetic field, temperature, resulting in signal distortion.Therefore, adding a filter into the circuit is a necessary structure to obtain a clearer and more stable ECG signal.As interference signals above 200Hz are strong and those below 0.05Hz are relatively weak, the filter structure needs to be able to filter out signals below 200Hz.Moreover, since the power supply frequency is 50Hz or 60Hz, the noise at this frequency is extremely significant.Therefore, the filter that can filter the signal of 50-60Hz is also very important.Therefore, after filtering out noise above 200Hz, a 50Hz band-stop filter is needed to filter out power supply noise.The commonly used Chebyshev filter is characterized by rapid attenuation from pass to stopband, however, much bigger and complex than the double T-type band-stop filter, so a 50Hz or 60Hz double T-type band-stop filter can be used in the circuit, and its centre frequency is shown below (Figure 2).Its circuit is listed below [8].By adjusting R5, the feedback depth can be adjusted, so as to adjust the quality factor.When K increases, the Q value of the filter is larger, and the frequency selectivity is better.The adjustment of Q value is independent of the centre frequency of the trap, which means that when R5 increases, the 50Hz frequency is suppressed greater.
Double T-type band-stop filter isn't the best solution to decrease the noise [9].As it can only decrease the centre frequency of double T notch is not adjustable, and a special asymmetric resistancecapacitance network can form a band-stop filter which has an adjustable centre frequency.That means it can adjust its centre frequency to adjust to the unstable power frequency interference, which means that it will be more accurate in decrease the power frequency interference, of cause, the more complex Chebyshev notch filter will have a better performance [10].
The content involved in this paper is conducive to the research of wearable ECG sensor, and provides some suggestions for the improvement of wearable ECG sensor.I believe that the next evolution in wearable ECG integrated circuits is the OTA component [11].OTA components are the largest footprint and power consumption components in the entire circuit, so the ability to reduce their size and power consumption means that ECG amplifiers can be optimized for size and power consumption.The smaller OTA allows more accurate circuits to be applied to the ECG, allowing for circuits of better accuracy that may be discarded due to space issues to be used in the ECG amplifier circuit.This relies on the invention of new materials, which can be used to create smaller, lower power amplifiers to improve overall performance.

Conclusion
This paper explains how to improve the accuracy of ECG amplifier circuit from three aspects: circuit structure, device application and filter.In the front-end differential amplifier circuit, generally speaking, we will adopt the structure of double-terminal input and double-terminal output, and the double-terminal output differential amplifier must make full use of the circuit symmetry ratio and emitter resistance, that is, in the differential amplifier circuit, we need to adopt a completely symmetrical structure and improve the resistance value of the emitter resistance, so as to obtain a greater common-mode rejection ratio.
When the research focus shifted to the second point: the components used in the circuit, the author found that because the amplitude of the ECG signal is very small, the components made of different materials have a great impact on the circuit noise in the actual operation of the circuit.A purer and less temperature-affected alloy will reduce the thermal noise caused by the thermal effect of the circuit in the actual circuit.
The third point mentioned in this article is to add a band-stop filter to the circuit.Because the circuit will have 50hz power frequency interference, it is essential to add a filter structure to the circuit.Therefore, adding 50Hz band-stop filter to the circuit can also play a role in improving the accuracy of the circuit.
Above all, apply a completely symmetrical structure, use more stable elements and add a band-stop filter can make the ECG amplifier more accurate.
This paper provides some suggestions for the design and improvement direction of ECG amplifier.The improvement direction of ECG sensor accuracy is provided from three main angles: circuit, component， and filter.It is beneficial to the research of sensors in wearable ECG and provides some suggestions for the improvement of wearable ECG sensors.I believe that the next development direction is the OTA space, which is transitioning to how to further integrate and reduce its power consumption.This relies on the invention of new materials, based on the lifetime output of smaller, lower power amplifiers.This paper provides some suggestions for the design and improvement direction of ECG amplifier.The improvement direction of ECG sensor accuracy is provided from three main angles: circuit, component and filter.It is beneficial to the research of sensors in wearable ECG and provides some suggestions for the improvement of wearable ECG sensors.I believe that the next development direction is the OTA space, which is transitioning to how to further integrate and reduce its power consumption.This relies on the invention of new materials, based on the lifetime output of smaller, lower power amplifiers.

Figure 1 .
Figure 1.The circuit of differential amplifier.

Figure 2 .
Figure 2. Circuit of band-stop filter.By adjusting R5, the feedback depth can be adjusted, so as to adjust the quality factor.When K increases, the Q value of the filter is larger, and the frequency selectivity is better.The adjustment of Q value is independent of the centre frequency of the trap, which means that when R5 increases, the 50Hz frequency is suppressed greater.Double T-type band-stop filter isn't the best solution to decrease the noise[9].As it can only decrease the centre frequency of double T notch is not adjustable, and a special asymmetric resistancecapacitance network can form a band-stop filter which has an adjustable centre frequency.That means it can adjust its centre frequency to adjust to the unstable power frequency interference, which means that it will be more accurate in decrease the power frequency interference, of cause, the more complex Chebyshev notch filter will have a better performance[10].The content involved in this paper is conducive to the research of wearable ECG sensor, and provides some suggestions for the improvement of wearable ECG sensor.I believe that the next evolution in