Design and implementation of biological neural electrode amplifier system

0 Introduction

Biosignals have various manifestations, such as physical changes in sound, light, electricity, force, etc., as well as chemical changes in concentration, gas partial pressure, pH value, etc. They are characterized by weak signals, nonlinearity, high internal resistance, many interference factors, etc., which can reflect the life activity state of organisms. Therefore, the collection and processing of biosignals is one of the important means of biological science research.

The application direction of the biological neural electrode amplifier system (hereinafter referred to as the electromyography detection system) is to monitor the patient's brain nerve area during surgery. Although similar instrument systems have been available at home and abroad, most domestic designs are limited to the preamplifier part and lack systematicity; while foreign systems are already relatively mature, but are expensive, and the subsequent processing of the signal has not yet been digitized.

In comparison, this electromyography detection system emphasizes systematization, constitutes a complete instrument, and adopts a digital processing method, which provides space for future digital improvements of the system. At the same time, the cost of the system is about 2,000 yuan, while foreign products, such as Germany's Neurosign100, cost about 100,000 yuan, which is much lower. In addition, since it is independently developed, it has intellectual property rights.

1 Introduction to neural/electromyographic signals

The application direction of the electromyography detection system is to monitor the patient's brain nerve area during surgery. The method used is shown in Figure 1.

First, a continuous nerve-like signal is generated to act on one end of the nerve, and then the electromyographic signal is extracted from the muscle connected to the other end of the nerve for various processing such as amplification and filtering. Finally, it is displayed in the form of LED and sound, which is used to detect in real time whether the nerve is touched during the operation.

Therefore, for the amplifier system to be designed, a neural-like electrical signal needs to be generated as a stimulation signal, and the biological signal to be amplified and processed is an electromyographic signal.
Neural signals have some of the same characteristics as other biological signals of the human body, and also have some unique characteristics. According to neurobiological research, neural signals are a type of electrical signal that resembles a pulse, with a frequency of generally around 1kHz and as high as 10kHz; and should be a pulse current with a pulse width of 0.2ms, a peak value of 0.05mA to 1mA, and a frequency of 3Hz and 30Hz.

2 Circuit Design

The electromyography detection system includes stimulation circuit, amplification and filtering circuit and display processing circuit, and the electrodes are also designed, including probes, etc. The expected results are shown in Figure 2.

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The left side of Figure 2 is the electric signal excitation source part, which is used to stimulate nerves and can generate two current pulses with frequencies of 3Hz and 30Hz. The pulse width is 0.2ms and the peak value is adjustable between 0.05 and 5mA. The right side is the signal part, which is used to process the electric signal generated by the receiving muscle. The specific parameter indicators of the signal processing and display prompt part are as follows: It is divided into two channels, which can independently display and prompt the strength of the received electromyographic signal. The intensity of the electromyographic signal is displayed with an LED light, ranging from 30 μV to 20mV; the intensity of the electromyographic signal is prompted with a buzzer, and the volume is adjustable. The electrode for receiving the electromyographic signal has three pins (V+), (Vref), and (V-), which are used by the receiving and processing part.

2.1 Stimulation circuit

Since the output pulse current is required to have two options of 3Hz and 30Hz, and the pulse width is 0.2ms, and the pulse current is not easy to generate directly, the pulse generating circuit first generates a voltage pulse with two frequencies of 3Hz and 30Hz and a pulse width of 0.2ms. Then it is converted into a current pulse through the V/I conversion circuit. The conversion circuit contains an adjustable knob to control the peak value of the output pulse between 0.05 and 5mA.

2.2 Pre-processing box circuit

The electromyographic signal from the probe is first sent to the preamplifier circuit for voltage signal amplification, and then processed by high-pass filtering to filter out polarization voltage and low-frequency noise. The low-frequency cutoff frequency is 5Hz (the polarization voltage is generated by the different depths of the probe's V+ and V- legs inserted into the muscle). The voltage signal after high-pass filtering is then low-pass filtered to filter out high-frequency noise. The high-frequency cutoff frequency is 1kHz. The voltage signal after low-pass filtering is then notched through the power frequency to filter out the 50Hz power frequency electrical signal. At this point, the processing process of the pre-processing box is completed, and the processed voltage signal is used by the post-stage signal processing and display prompt circuit.

2.3 Signal processing and display prompt circuit

The voltage signal output by the pre-processing box is first amplified to a suitable amplitude by the secondary amplifier circuit, and then drives the LED to display the strength of the electromyographic signal. At the same time, the voltage signal is amplified and sent to the speaker circuit for sound prompts.

In addition to the main circuits mentioned above, to complete an overall system, a power supply circuit and interface circuits are also required. Since each circuit has different requirements for power supply, the power supply circuit will be divided into five parts: the power supply of the excitation circuit, the positive and negative 15V power supply before the isolator, the positive and negative 15V power supply after the isolator, the power supply of the A/D part, the power supply of the LED and the power supply of the speaker.

3 Circuit Testing

After PCB drawing, we made an experimental board. Based on this experimental board, we did a variety of tests to verify the performance of the designed system.

3.1 Testing of the Stimulus Circuit

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(1) When the frequency selection terminal is set to a high level, the output voltage waveform of the 3rd pin of the 555 timer is a pulse wave with a peak value of about 4.16V and a frequency of about 3.1Hz; when the frequency selection terminal is set to a low level, the output is a pulse wave with a peak value of about 4.16V and a frequency of about 30 Hz. This proves that the system's pulse voltage generation and frequency selection functions work normally.

(2) The frequency selection terminal is set to a low level, and a 1kΩ load resistor is connected to the output terminals of the stimulation circuit. The voltage at both terminals is detected at the same time, and the waveform is shown in Figure 3.

Its peak value is about 4.64V, and the frequency is still about 30Hz. The output current peak value is about 4.64mA. When the load resistance is replaced with 300Ω, the output waveform frequency remains unchanged, but the peak value is about 1.39V, and the current peak value is still about 4.64mA, which proves that the voltage/current conversion function works normally. When the pulse voltage is at a high level, a constant current can be output.

(3) When the load resistance is constant, the variable resistor value in the voltage/current conversion circuit is changed, and it is found that the peak value of the output waveform changes accordingly. Since a 1kΩ resistor is connected before the variable resistor for current limiting, the maximum output current value is about 4.6mA, and the minimum value is about 0.04mA. Therefore, the output current value selection function in the design requirements can be realized by this variable resistor, and its adjustment range can be limited by the maximum resistance value of the current limiting resistor, the current limiting resistor and the variable resistor. In summary, the excitation signal is generated normally, with a pulse current with a constant peak value.

3.2 Testing of amplifier circuit

A sine wave with a frequency of 100Hz and peak values ​​of 200 μV, 500 μV and 1 mV was input to the input end of the front box. The output waveform was tested step by step. When stable amplification was achieved, the total amplification factor of the front board was about 100.

3.3 Speaker Circuit Testing

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When the frequency selection terminal is set to a high level, that is, 3 Hz is selected, the output waveform of the speaker circuit is selectively truncated, and when the frequency selection terminal is set to a low level, that is, 30 Hz is selected, the waveform is relatively continuous, so the sound effects produced in the two cases are different; at the same time, no matter what the situation is, changing the variable resistor in the speaker circuit can change the peak value of the output waveform, that is, change the output volume.
The test results of the motherboard speaker control part are summarized as follows:

1) The channel can be selected by switch.

2) The frequency of the output sound can be controlled according to the operating frequency of the excitation signal (3Hz/30Hz).

3) The volume of the output sound can be adjusted through an adjustable resistor.

4) The volume can increase as the amplitude of the input signal increases.

3.4 Testing of other functional circuits

(1) Power supply test results: The power supply works normally and outputs positive and negative 15 V power, allowing the entire system to work normally.

(2) Test results of digital tube display circuit: display is normal.

4 Summary and Outlook

Through simulation and actual circuit board testing, it can be verified that the general functions of the system have been completed and the operating performance is good. The specific parameters are as follows:

1) A pulse current with a frequency of 3Hz or 30Hz and a pulse width of 0.2ms is generated, and the current peak value is constant and adjustable between 0.04 and 4.6mA;

2) The overall amplification gain of the system is between 960 and 100007;

3) The volume of the sound prompt increases with the increase of the amplitude of the input signal. The channel can be selected by the switch, and the frequency of the output sound can be controlled according to the operating frequency of the excitation signal (3Hz/30Hz), and the volume of the output sound can be adjusted by an adjustable resistor.

The medical application prospects of electromyography detection system are broad, but since it will be directly applied to the human body, various performance requirements are strict. At present, the main work of system design and testing has been completed, and the system's operating performance is relatively good, but there are still many areas that need to be improved, such as the detection part of the stimulation circuit, the poor function of the power frequency trap circuit, and many other problems that need to be solved.

In addition, the system has extensive room for improvement, such as:

1) The circuit structure currently uses analog circuits, but some modules such as pulse source generation and filter circuits can be designed digitally to make the structure more integrated;

(2) The current application of the system is still limited to the monitoring of surgery. However, due to its large amplification gain range, its application can be continuously expanded through continuous improvement of details in the future. For example, the types of signals collected by the system can be increased and applied to the analysis of different biological signs, and the back-end display data can be more complex, so that the system can be applied to the monitoring of critically ill patients.

Therefore, the project still needs to invest more efforts until the system is further improved to meet the requirements for application in actual medical treatment. This is the development prospect of the electromyography detection system.