Design of low-noise low-power ECG amplifier circuit with high integration level

Electrocardiogram (ECG) is a common tool in medicine and is an important indicator for diagnosing many diseases. Nowadays, aspects such as low noise, low power, and high integration have become important indicators for ECG circuit design. Also, due to the low amplitude of ECG signals, ECG circuits need to have high gain to ensure accurate signal detection and analysis. Second, ECG signals are usually subject to interference from other parts of the body, so a high common mode rejection ratio (CMRR) is required to suppress these interfering signals. In this paper, we propose an ECG circuit composed of an amplifier with a drive right leg circuit (DRL) using pseudo-resistors as well as capacitors to improve the common instrumentation amplifier. The simulation using industrial-grade simulation software LTspice shows that the circuit has a voltage of 1.8 V, a bandwidth of 17 mHz-1 KHz, a gain of 55 dB, and an INR of 3.4 μVrms, and because the signal amplifier part of this circuit design consists of only pseudo-pseudo-resistors, capacitors, and mos tubes. Therefore, it has a smaller size and a higher degree of integration. In addition, the circuit has less power loss for the same amplification design. Overall, this design has excellent characteristics such as low noise, low power, and small size.


Introduction
Electrocardiograms (ECG) are a widely used diagnostic tool, they measure the electrical signals produced by the heart, and by analyzing these signals abnormalities can be identified and diseases diagnosed, forming the backbone of medical devices for the treatment of cardiovascular diseases.With advances in technology, ECG circuits have evolved considerably since their invention more than a century ago.ECG circuits have become smaller, more precise and more energy efficient.Today, ECG circuits are not only used in traditional medical settings, ECG signals are also analysed and used for various purposes, such as emotion recognition and biometric identification [1].Much excellent work has been done in ECG information acquisition before, and low cost and high reliability have become the main goals of ECG circuit design nowadays [2].
In this paper, we present an amplifier circuit designed using pseudo-resistors as well as capacitors to replace the resistors in a common instrumentation amplifier circuit.The use of pseudo-resistors can achieve sufficient linearity and effectively reduce the size of the circuit [3].The use of capacitors to control the gain of the circuit can effectively avoid the noise introduced when resistors are used.Also, the design proposed in this paper includes a driving right leg circuit, a structure that has been shown to be effective in reducing the interference caused by common mode voltages in the amplifier [4].Based on the above design ideas, this paper designs an ECG circuit with high gain, low noise, and small size.
The contribution of this paper is to reduce the overall size of the ECG circuit, making it easier to integrate.It also reduces the power of the circuit and makes the circuit have low noise.This design can be applied to some of the wearable devices and has good practicality.
The second part of the article explains the design structure of this circuit in detail.The third part shows the simulation results based on the industrial-grade simulation software LTspice and compares and discusses with the previous design.The fourth section summarizes the full design and provides an outlook on future improvements that need to be made.

Instrument amplifier circuit design
Instrumentation amplifier is a precision differential voltage amplifier, which is widely used in medical instruments.The instrumentation amplifier has the characteristics of high open circuit gain, high input impedance, and high CMRR [5].A common instrumentation amplifier consists of three amplifiers internally and can be divided into two parts as shown in Figure 1 below.The first part is an input buffer stage consisting of two amplifiers connected in parallel, and the second part is a differential amplifier.For a common instrumentation amplifier circuit, the first stage can be used to suppress the common mode voltage gain and provide a high differential voltage gain.The differential amplifier of the second stage is mainly used to amplify the input small signal.Based on the principle of common instrumentation amplifier, this paper proposes an ECG instrumentation amplifier circuit with low noise, high gain, and small product characteristics.This design uses capacitors as well as pseudo-resistors to replace the resistors in the two stages inside the common instrumentation amplifier.In this design, capacitors will be used to control the gain of the circuit, which avoids the use of resistors and thus the introduction of noise [6].The use of pseudo-resistors and capacitors will reduce the overall size of the circuit and the internal noise and improve the accuracy of the gain setting.At the same time, this design will continue the excellent characteristics such as high CMRR of the common instrumentation amplifier.The design schematic of the circuit is shown in Figure 2. As shown in Figure 2, the three capacitors in series in the first stage will replace the three resistors in the corresponding positions in Figure 1.In this circuit, the capacitor will set the gain instead of the resistor.The reason for using two series-connected 1p capacitors on the right side is to facilitate the common mode voltage from the amplifier for driving the right leg circuit as described later.The three pseudo-resistors in series on the left side act as voltage equalizers and are used to ensure that all three capacitors in series on its right-side work properly.The resistors in the feedback branch of the differential amplifier circuit in the second stage are also replaced by capacitors, so that this differential amplifier will be in the form of a capacitive feedback amplifier.In this form, the DC offset from the skin electrode can be suppressed [7].The internal structure of the pseudo-resistor in all figures consists of pmos tubes with L of 30 μm connected in series.Figure 3 illustrates this structure.

Operational transduction amplifier (OTA)
The OTA model commonly used in the medical field is usually two-stage.Therefore, this design will also use two-stage operational transconductance amplifier as a way to obtain better linearity and higher open-loop gain.The current source in the first OTA is 50 nA and the current source in the second OTA is 3 nA, but the rest of the structure is the same.This can effectively reduce the noise of the circuit because the larger current can reduce the transmitted noise.Also, pmos is used in all OTAs for the design because pmos introduces less flicker noise compared to nmos [8].The schematic diagram of the operational transconductance amplifier used in this design is shown in Figure 4.

Drive right leg circuit (DRL)
DRL is a common circuit structure added to bio-signal amplifiers, the main function is to reduce common-mode interference.Since the human body can also pick up electromagnetic interference, it is often subject to interference from 50 or 60 Hz interference signals when testing patients using ECG.This interference can mask the biological signal being measured, so active noise cancellation is performed by introducing a DRL, which inverts and amplifies the common-mode voltage taken from the amplifier by using an inverting amplifier and reconnects it to the human body.The right leg is farther away from the human heart, and any signal connected to the right leg will be transmitted in common mode to the two electrodes near the heart, which is why we chose to connect to the right leg.The signal returning to the heart is in opposite phase to the noise, so it can act as a mutual cancel to eliminate the noise.Also, the feedback network of the inverting amplifier in the DRL circuit acts as some low-pass filtering.The design diagram of the driving right leg circuit structure is shown in Figure 5.

Body model
The body model used is shown in Figure 6.Three 200-ohm resistors in the circuit are used to represent the impedance of any two terminals inside the human body.The current source is a Norton equivalent model that is used to simulate a 60 Hz internal body noise.The body model also has three connection segments used to simulate the ECG circuit connected to the left arm, right arm, and right leg, respectively.
There are also three electrodes in the circuit consisting of resistors and capacitors in parallel, and they will provide impedance to protect the body.

Simulation and discussion
The simulation in this paper was performed using a sinusoidal signal with an amplitude of 1 mV, a frequency of 50 Hz, and an AC amplitude of 1 to simulate the human heart signal.The value of VDD in the circuit is 1.8 V and the value of Vref is 0.9 V.The simulation tool used in this paper is LTspice, which is an industrial grade simulation software.
Figure 7 shows the AC analysis scan from 1 mHz to 10 KHz.The AC simulation shows the bandwidth as well as the circuit gain.The results show that the mid-band gain is about 55 dB, the low cutoff frequency is about 17mHz, and the high cutoff frequency is about 1 KHz. Figure 8 shows the noise analysis scan from 1 mHZ to 10 KHz.This simulation shows that the total output noise of the circuit is 1.9711 mV.By dividing this result by the mid-band gain of the circuit, we obtain a total integrated input noise of 3.49 μVrms.Table 1 shows the relevant circuit parameters for the circuit designs that have been performed in this area in recent years.From this table, this design has the lowest power consumption with almost the same gain.Also, this design has excellent work in terms of INR of the circuit.In addition, the design uses all capacitors and pseudo-resistors in the amplifier section, which greatly reduces the size of the circuit, which is also an advantage of this design.
2016 [9] 2019 [10] 2020 [11] 2020 [12] This work The design of this paper is aimed at reducing the power consumption of the amplifier while providing high gain and reducing the size of the circuit.The simulations in this paper are based on LTspice software only and more actual experiments are needed for measurements.Since this circuit has a higher gain, it consumes more power compared to a lower gain ECG circuit.Therefore, even though this circuit has a small size, putting it into a small wearable device will still face the problem of high power.In future designs, improving the internal structure of the OTA or trying to reduce the gain of this circuit can be used to solve this problem.

Conclusion
Overall, the simulation using LTspice has successfully verified that this circuit has high gain, low noise and other performance.The circuit has a low break frequency of approximately 17 mHz and a high cut frequency of approximately 1 KHz, a gain of 55 dB, an INR of 3.4 μVrms, and a power of 52.6 μW.The advantage of this design is its small size and low noise.Secondly, it has a lower power consumption with the same gain.The design successfully uses pseudo-resistors and capacitors instead of resistors in the amplifier, which will help to reduce the power consumption and size in future designs.However, now that ECG is widely used in small wearable devices, this circuit still needs to be improved due to the high-power issue to fit into small wearable devices.In order to make this design more widely useful, the circuit can be improved by designing the internal structure of the OTA in the future.Also, this design can be combined with wireless transmission technology to improve the efficiency and convenience of data transmission, as well as to reduce the cost and complexity of the device.It is hoped that this design can be used in various fields to help determine related diseases and improve the survival probability of patients.

Figure 1 .
Figure 1.Structure of common instrumentation amplifier.For a common instrumentation amplifier circuit, the first stage can be used to suppress the common mode voltage gain and provide a high differential voltage gain.The differential amplifier of the second stage is mainly used to amplify the input small signal.Based on the principle of common instrumentation amplifier, this paper proposes an ECG instrumentation amplifier circuit with low noise, high gain, and small product characteristics.This design uses capacitors as well as pseudo-resistors to replace the resistors in the two stages inside the common instrumentation amplifier.In this design, capacitors will be used to control the gain of the circuit, which avoids the use of resistors and thus the introduction of noise[6].The use of pseudo-resistors and capacitors will reduce the overall size of the circuit and the internal noise and improve the accuracy of the gain setting.At the same time, this design will continue the excellent characteristics such as high CMRR of the common instrumentation amplifier.The design schematic of the circuit is shown in Figure2.

Figure 2 .
Figure 2. Instrument amplifier design schematic.As shown in Figure2, the three capacitors in series in the first stage will replace the three resistors in the corresponding positions in Figure1.In this circuit, the capacitor will set the gain instead of the resistor.The reason for using two series-connected 1p capacitors on the right side is to facilitate the common mode voltage from the amplifier for driving the right leg circuit as described later.The three pseudo-resistors in series on the left side act as voltage equalizers and are used to ensure that all three capacitors in series on its right-side work properly.The resistors in the feedback branch of the differential amplifier circuit in the second stage are also replaced by capacitors, so that this differential amplifier will be in the form of a capacitive feedback amplifier.In this form, the DC offset from the skin electrode can be suppressed[7].The internal structure of the pseudo-resistor in all figures consists of pmos tubes with L of 30 μm connected in series.Figure3illustrates this structure.

Figure 7 .
Figure 7. AC simulation.Figure8shows the noise analysis scan from 1 mHZ to 10 KHz.This simulation shows that the total output noise of the circuit is 1.9711 mV.By dividing this result by the mid-band gain of the circuit, we obtain a total integrated input noise of 3.49 μVrms.

Figure 8 .
Figure 8.Output noise simulation.Table1shows the relevant circuit parameters for the circuit designs that have been performed in this area in recent years.From this table, this design has the lowest power consumption with almost the same gain.Also, this design has excellent work in terms of INR of the circuit.In addition, the design uses all capacitors and pseudo-resistors in the amplifier section, which greatly reduces the size of the circuit, which is also an advantage of this design.Table1.Performance comparison.