Design of a weak bioelectric signal acquisition circuit

ZHOU Mingjuan,WANG Yuyuan,RAN Li

(1. Shaanxi Railway Institute,Weinan 714000，China；2. Xi’an University of Technology,Xi’an 710048，China)

Abstract：A surface electromyography (sEMG) signal acquisition circuit based on high-order filtering is designed.We use a two-stage adjustable amplifier and a high-order Sallen-Key bandpass filter to solve the problems of non-adjustable amplification gain and low filtering order in traditional acquisition circuits.The experimental results show that the designed sEMG signal acquisition device can eliminate power frequency interference effectively,the stopband drop of the filtering part reaches approximately -100 dB/dec,which can effectively extract useful signals between 20-500 Hz,and the amplification gain reaches 60 dB.

Key words：surface electromyography (sEMG);two-stage amplification;high-order filtering;interference suppression;power frequency noise

0 Introduction

Electromyography (EMG) signal is a complex bioelectric signal,and also the source of the electrical signal that produces muscle strength.When the limbs are in motion,the muscle fibers receive a series of nerve impulses,which causes the muscles contracting,so that the limbs complete the actions set by the brain.Therefore,the EMG signal contains a lot of information related to the movement of the limbs,and reflects the functional state of nerves and muscles.If guided,amplified,displayed and recorded by the surface electrode,a one-dimensional voltage time series signal will be obtained,that is,the surface EMG (sEMG) signal.EMG analysis and recognition not only are required for basic research,but also have important clinical significance[1].Therefore,reliable and stable collection of EMG signals is the prerequisite and basis for further research.

In recent years,with the development of electronic technology and the in-depth study of bioelectric signals,a large number of relevant acquisition equipment and instruments for electromyography signals have emerged[2].However,most of the instruments on the market have complicated circuits and high costs,which is not conducive to popularization.Therefore,we design an sEMG signal acquisition circuit based on high-order filtering,which can effectively eliminate various interference noises in signal acquisition process and the results are accurate and low in cost.

1 Characteristics of sEMG and analysis of acquisition interference sources

1.1 Physiological basis of sEMG signal generation

Since the EMG signal itself is a random signal,there is no unified analytical expression.Usually,we use linear system model,lumped parameter model,non-stationary model and bipolar model to describe it.The first three models are for the unipolar situation,but in actual situations,in order to more accurately extract the information of the sEMG signal and effectively reduce the impact of other muscle tissues,we use bipolar model.The model is expressed as

Z(t)=y(t)-y(t-Δ),

(1)

where Δ is the transmission delay,that is,the time the action potential travels from the first electrode to the second electrode.It is noteworthy that the distance between the two detection electrodes cannot be too far,and the connection line should be parallel to the muscle fiber axis.

1.2 Characteristics of sEMG

The sEMG signals collected by different participants have large individual differences,and the sEMG signals collected from different parts of the same participant also have differences.Although the parameters of the sEMG signal will be affected by various factors,there still exist certain rules as follows[3]:

1) sEMG signal is extremely weak,and its amplitude is generally in the range of 0-5 mV,so it is easily submerged by noise;

2) sEMG signal is alternating,the greater the muscle strength,the greater the amplitude;

3) sEMG signal frequency is low,mainly in the range of 20-1 000 Hz,and the main energy is concentrated between 50-150 Hz;

4)Strong noise background.Due to the complex composition of the EMG signal guided by the electrode,many interference signals together constitute a strong noise background;

5) The sEMG signal changes all the time and is non-stationary[4].

1.3 Interference analysis during sEMG acquisition

From the above analysis,it can be known that the EMG signal itself has the characteristics of randomness,instability and nonlinearity.Therefore,in the signal acquisition process,it is easy to be interfered by noise factors.In other words,we must fully consider the noise interference faced in the acquisition process,and suppress noise interference as much as possible.The possible ways to introduce noise are as follows:

1) The biggest interference in the sEMG signal collection process is the power frequency interference generated by the power grid[5]due to radiation of the devices connected to the power supply.Because the interference frequency is exactly in the energy-concentrated frequency band of the collected signal and the amplitude is one to three orders of magnitude of the useful signal,how to remove this interference is the key to our collection circuit design.

2) Space electromagnetic field is also one of the main interferences.Because the test environment is full of electromagnetic fields,noise will inevitably be mixed with useful signals through component pins and equipment connection lines.

3) When the subject is testing,the induced current generated by itself will also be mixed in the useful signal and added to the front end of the amplifier together,causing common mode interference.

4) The interference of various bioelectric signals.

In addition,the interference from detection equipment (such as power supply),stimulus artifacts,motion artifacts,high-frequency signal,etc.,will affect signal acquisition.Therefore,the signal acquisition system design must fully consider these influencing factors to ensure the accuracy and reliability of the acquisition results.

2 Design of signal acquisition circuit

Considering the characteristics of sEMG signals and various interference problems mentioned,we designs an sEMG signal acquisition device for good anti-noise performance,including two electrodes,a preamplifier,a high-pass filter,a low-pass filter,a bandstop filter,a secondary amplifier,a terminal equipment and so on.The overall system block diagram is shown in Fig.1.

Fig.1 Block diagram of sEMG signal acquisition system

The workflow is as follows:A pair of differential electrodes are placed along the direction of the muscle fiber to guide the skin-surface potential difference.The signal is preliminarily amplified by the pre-amplification circuit,and then some low-frequency interference,environmental noise and mixed noise are removed through the band-pass filter.The other physiological signals are also filtered out,and the power frequency interference is removed through the notch circuit.Finally,the processed signal is amplified to the required voltage level by the two-stage amplifying circuit for subsequent processing.

2.1 Pickup electrode

The sEMG signal completes the coupling between the body surface and the amplifier circuit through the pickup electrode.Therefore,the material,structure,placement position and contact degree of the electrode have a great influence on the signal pickup.Here,we use silver-silver chloride (Ag-AgCl) electrode sheets as pickup electrodes.In order to reduce noise and improve the common mode rejection ratio,the overall structure of the electrode adopts a bipolar type,the distance between the two detection electrodes is determined to be 20 mm,and a reference electrode is also added,which helps to form a differential system[6].In addition,before installing the electrode,we clean the area to be tested with alcohol,apply conductive paste,etc.,so that the electrode can effectively contact the skin surface of the muscle,and help the pickup electrode to obtain a good sEMG signal.

2.2 Preamplifier

The sEMG signal obtained through the pickup electrode will be directly sent to the preamplifier.However,since the sEMG signal is extremely weak and susceptible to interference,the preamplifier needs to meet the following requirements[7]:

1) High common mode rejection ratio (CMRR).Interference signals in the acquisition process are mostly input in the form of common mode signals.Therefore,the higher the CMRR,the stronger anti-interference ability of the system.

2) High input impedance.The amplifier is required to have a high input impedance to minimize the signal attenuation on the internal resistance.

3) High gain.Due to the small amplitude of the sEMG signal,the amplifier gain should be increased as much as possible.

4) Low noise.We should choose a suitable preamplifier gain to ensure good low-noise performance of the system.

5) Low drift.The baseline drift of the amplifier will affect the results,and the use of a differential input helps to achieve low drift performance.

Based on the above analysis,we choose amplifier AD620,which has advantages of high accuracy,low offset voltage,low offset drift,low noise,low input bias current and low power consumption,and is very suitable for battery-powered portable applications.It adopts differential input,and only needs an external resistor to adjust the amplification gain from 1 to 1 000.The gain can be calculated by

(2)

whereGmeans amplification gain,andRGis adjustable resistance.It is worth noting that in order to ensure that the signal is not excessively amplified before filtering[8],a two-stage amplification circuit is adopted.Setting the amplification gain of the first stage amplification circuit to 200 times,the electromyographic signal is amplified to hundreds of volts,which is convenient for subsequent processing.The schematic diagram of the preamplifier circuit is shown in Fig.2.

Fig.2 Preamplifier schematic diagram of sEMG signal acquisition circuit

2.3 Bandpass filter

The bandpass filter is formed by cascading a high-pass filter and a low-pass filter to filter out various high and low-frequency interference as well as other mixed bioelectric signals,whereas useful signals are not filtered out[9].The cut-off frequency of the filter is set according to the characteristics of the sEMG signal and the frequency characteristics of other interference signals.

During the signal acquisition process,the muscle contraction will cause the friction between the detection electrode and the skin,which may will produce movement artifacts with the frequency less than 20 Hz as well as significant drift in the signal.Therefore,we set the cut-off frequency of the high-pass filter at 20 Hz to filter out this interference and eliminate the noise introduced by the inherent instability of the sEMG signal[10].

Furthermore,considering that the sEMG signal is a low-frequency signal,its frequency spectrum is mainly distributed in the range of 20-500 Hz,and most of the energy is concentrated in the range of 50-150 Hz,therefore,the cut-off frequency of the designed low-pass filter is 500 Hz,so as to achieve the purpose of suppressing high-frequency interference.

It is worth noting that the designed bandpass filter is different from other acquisition devices because it uses the high-order filter.Since traditional sEMG signal acquisition devices generally adopt fourth-order Butterworth filters when designing band-pass filters,the lower order results in a slow drop in the filter stopband,which in turn causes attenuation of interference.The filter stopband drops slowly because of the lower order,which cannot effectively suppress high frequency interference.The designed high-order filter is composed of a cascade of a fifth-order low-pass filter and a fifth-order high-pass filter[11].Both the low-pass filter and high-pass filter are composed of a Sallen-Key third-order filter and a Sallen-Key second-order filter.Taking the odd-time first and even-time later for connection can prevent high-frequency leakage signals from mixing into the output signal.In fact,the Sallen-Key filter is also a Butterworth filter with a gain of 1,and the topology is also very easy to adjust.Changing the Sallen-Key filter gain also changes the amplitude-frequency characteristics and type of the filter.The schematic diagram of the bandpass filter is shown in Fig.3.

Fig.3 Schematic diagram of bandpass filter

When designing a filter,the accuracy of RC component is closely related to the required frequency characteristics.Here,the combination of capacitors is given a priority because the resistance value is relatively easy to obtain.Finally,the resistance is calculated and selected.In the schematic diagram shown in Fig.3,the parameter relationship of the low-pass filter is expressed as

(3)

The parameter relationship of the high-pass filter is expressed as

(4)

According to the principle of the circuit schematic shown in Fig.3,by adjusting the parameters,the amplitude-frequency characteristic curve of the filter after simulation is obtained,as shown in Fig.4.

Fig.4 Amplitude-frequency characteristic curve of bandpass filter

From the above simulation results,the bandstop attenuation rate of the tenth-order band-pass filter can reach approximately -100 dB/dec,which has excellent attenuation performance for interference signals outside the passband and other physiological electrical signals.

2.4 Power frequency notch filter

One of the biggest interferences in the signal acquisition process is the power frequency interference generated by main power supply,poor grounding of the testing equipment and the tester himself[5],which is directly mixed with the sEMG signal.Therefore,how to remove it as much as possible is the key.We establish as a mathematical model for analysis.Supposing thatV1andV2are the two input signals of differential detection (the distance between the two electrodes is 2 cm in the actual test),Vnis power frequency interference,and the amplifier has an ideal subtraction function,the output signal at this time is

Vo=G((V1+Vn)-(V2+Vn))=G(V1+V2),

(5)

whereVois output signal.Thus,the power frequency interference can be eliminated.While designing the circuit,we adopt the active doubleTband rejection filter to eliminate the interference,the schematic diagram is shown in Fig.5.

Fig.5 Schematic diagram of notch filter

In the doubleTnetwork,the minimum attenuation of the notch point is closely related to the symmetry of the two branchesRandC.Therefore,the symmetrical relationship betweenRandCmust be strictly guaranteed.The relationship between the parameters in the circuit diagram is expressed as

(6)

The center frequency is

(7)

It is worth noting that the 50 Hz power frequency interference is located in the energy concentration band of the collected signal,we must filter the interference as much as possible and retain the useful signal[12],which requires the notch filter to have a highly adjustableQvalue (quality factor).Therefore,we add a potentiometer to change theQvalue.The relationship between theQvalue and the adjustment coefficientkof the potentiometer is

(8)

whereQis the quality factor,andkmeans the adjustment coefficient of the potentiometer.The simulation result is shown in Fig.6.

Fig.6 Amplitude-frequency characteristics of notch filter under different Q values

It can be seen that the larger the value,the smaller the influence of the notch filter on the signals with other frequencies.In addition,a small frequency change can reduce the attenuation.

2.5 Secondary amplifier

The principle of the secondary amplifier is similar to that of the preamplifier.In our work,the gain of the secondary amplifier is set to be 5 times.The amplification gain of the entire acquisition device can reach 1 000 times,which means that the sEMG signal is effectively amplified by 60 dB.

Finally,the signal is sent to the terminal by a dedicated shielded wire for subsequent processing.

3 Experimental results and analysis

We verify the effectiveness and feasibility of the designed acquisition device through experiments as well as the reliability of the acquired signal through spectrum analysis.In order to ensure a good experimental effect,before the experiment is carried out,it is necessary to perform related treatments on the skin to be tested and to select an appropriate electrode placement position.

3.1 sEMG signal spectrum analysis of fist and arm extension

The sEMG signal acquisition experiments of fist and arm are carried out in turn,and the results are shown in Fig.7.

(a) Fist (b) Arm

It can be seen from Fig.7 that the electrical signal waveform envelopes generated in the two modes are roughly the same.However,because the muscle contraction intensity is different in the two exercise modes,the signal amplitude collected by the extended arm mode is greater than that by the fist mode.

After fast Fourier transform (FFT),the signal spectra are shown in Fig.8.

(a) Fist(b) Arm

It can be seen from Fig.8 that the frequency distribution of the sEMG signal in the two modes has typical noise characteristics,and the main energy is concentrated in the range of 0 to 500 Hz,and it is more obvious in the range of 50 to 200 Hz.This is in line with the frequency characteristics of the EMG signal[13-14].In addition,the comparison result shows that in the extended arm mode,the signal amplitude is greater than that in the fist mode.Therefore,the reliability of the signal acquisition device designed is proved.

3.2 sEMG signal spectrum analysis under electrical stimulator

Using a certain brand of electrical stimulation therapy device to stimulate muscles,the results are as follows.

1) When the DC stimulation voltage is 1 V,the result is shown in Fig.9.

(a) Time domain waveform under 1 V stimulation

2) When the DC stimulation voltage is 5 V,the result is shown in Fig.10.

(a) Time domain waveform under 5 V stimulation

It can be seen that the amplitudes of the signal vary with the stimulation voltages with different intensities.The greater the voltage,the greater the signal amplitude.From the frequency domain analysis results,it can be seen that although the stimulation voltages are different,the frequency distributions of the sEMG signal are basically the same.

4 Conclusions

This paper presents an sEMG signal acquisition device based on high-order filtering.It has an adjustable high-gain amplifier,high input impedance and high common-mode rejection ratio.The experimental results show that it can effectively filter out all kinds of noise and power frequency interference.In addition,it has simple structure,convenient portability,low power consumption and low cost.Therefore,it has a good practical value.