Design method of collecting ECG signals based on amplifier circuit

1 Characteristics of human ECG signals
ECG signals are biomedical signals with the following characteristics:
(1) The signal has the characteristics of near-field detection. If it is a small distance away from the human body, the signal is basically undetectable;
(2) ECG signals are usually relatively weak, at most in the order of mV;
(3) They are low-frequency signals, and their energy is mainly below a few hundred Hz;
(4) Interference is particularly strong. Interference comes from both inside the organism, such as myoelectric interference and respiratory interference, and from outside the organism, such as power frequency interference and other external crosstalk introduced by poor grounding when picking up the signal;
(5) The interference signal overlaps with the frequency band of the ECG signal itself (such as power frequency interference).
2 Design requirements of acquisition circuit
In view of the above characteristics of ECG signals, the design analysis of the acquisition circuit system is as follows:
(1) Signal amplification is a necessary step, and the signal should be increased to the amplitude requirement of the A/D input, that is, at least the order of "V";
(2) The influence of power frequency interference should be weakened as much as possible;
(3) The baseline drift caused by breathing, etc. should be considered;
(4) The signal frequency is not high, and the passband usually meets the requirements, but factors such as input impedance, linearity, and low noise should be considered.

3.1 Design of preamplifier circuit Due
to the characteristics of human ECG signals and the strong background noise, the impedance between the electrode and the skin is large and the range of variation is also large when collecting signals. This puts forward higher requirements on the preamplifier circuit (first stage), that is, the preamplifier circuit should meet the following requirements:
high input impedance; high common mode rejection ratio; low noise, low drift, small nonlinearity; suitable frequency band and dynamic range.
For this reason, the instrument amplifier AD620 of Analog Company is selected as the preamplifier (preamplifier). The core of AD620 is a three-op-amp circuit (equivalent to integrating three OP07 op-amps), and its internal structure is shown in Figure 1.

The amplifier has a high common mode rejection ratio (CMRR), good temperature stability, wide amplification bandwidth, small noise factor and easy adjustment, making it an ideal choice for biomedical signal amplification. According to the design principle of small signal amplifiers, the gain of the pre-stage cannot be set too high, because too high a pre-stage gain will be detrimental to the subsequent circuit's processing of noise.
Based on the above analysis, the pre-stage amplifier circuit is designed according to Figure 2 and first simulated using Multisim 2001.

The simulation process uses a 0.5 MV, 1.2 Hz differential signal source as the simulated ECG input to simulate the circuit's amplification process, and the result meets the requirements.
3.2 Secondary amplifier circuit (signal amplification)
The second-stage amplifier circuit is mainly for the purpose of increasing the gain, and the ordinary AD OP07 can meet the requirements.
3.3 High-pass filter (eliminating baseline drift)
Adding a simple high-pass filter link to the circuit part will achieve twice the result with half the effort in isolating the DC path and eliminating baseline drift. This part of the circuit is placed between the pre-amplifier and the signal amplifier circuit. A simple passive high-pass filter circuit is shown in Figure 3.

Its characteristic frequency (turning frequency) is calculated as:

After high-pass filtering, the baseline drift caused by breathing and other factors below 0.03 Hz can be greatly weakened, and the low-frequency end of the ECG signal will take this frequency accordingly.
3.4 Compensation circuit (offsetting various noises in the human signal source)
The compensation circuit is introduced to offset the interference in the human signal source (including power frequency interference). The method of introducing the compensation circuit: establish a common-mode negative feedback between the feedback end of the preamplifier circuit and the ground end of the signal source. In order to increase the feedback depth of the circuit, the feedback signal is amplified (still using OP07) and connected to the reference end of the signal source, so that the power frequency interference can be offset to the maximum extent. This circuit form introduced can be vividly called a "feedback floating tracking circuit" according to its structure and function.
3.5 Block diagram structure of the entire circuit system
The principle block diagram and signal flow of the entire circuit system are shown in Figure 4.

3.6 Schematic diagram of actual circuit system The
final integrated circuit is shown in Figure 5. In the figure, unit U1 is the AD620 preamplifier; U2 is the feedback floating tracking part; U3 is the second-stage amplifier output part.
The gain of the circuit is estimated as follows:
First-stage amplifier:

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4 Experimental verification of circuit performance
The circuit was built according to Figure 5, and the virtual instrument LabVIEW 8.2 system was used to collect the ECG signal output by the circuit through NI's USB-6009DAQ. The results are shown in Figure 6 (the same coordinate scale is used for comparison).
In Figure 6, Figure 6 (a) is the signal waveform collected without the feedback floating part. It can be seen that the interference is very large, and the main interference is the 50 Hz power frequency interference; Figure 6 (b) is the ECG waveform collected after adding the feedback floating circuit part. There is still some ripple interference near the baseline, but it is much better than the waveform obtained without feedback floating in Figure 6 (a). It can be seen that adding the feedback floating circuit is very helpful in reducing the interference signal in the human body. As for the residual power frequency interference, it can be further suppressed by using relevant filtering technology in the subsequent part of the system. The further filtering of power frequency interference will not be discussed here.

5 Conclusion
The signal amplifier with AD620 and OP07 as the core is used to amplify the ECG signal. The circuit has low power consumption and high sensitivity. Theoretically, it only needs 3 V power supply, which can be provided by an external battery. It is easy to realize ECG signal acquisition and processing based on mobile devices (such as laptops). It is a practical ECG signal front-end acquisition and amplification circuit (further optimization of the signal can be conditioned by software after acquisition).