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SEMG detection electrode design

In Electronic Infomation Category: S | on April 19,2011

SEMG (Surface electromyogram) that surface EMG signal, when a skeletal muscle contraction, the muscle fiber action potentials produced weak to raise the skin surface, and ADSP2101KP-40 datasheet and collected by surface electrodes to the signal. SEMG detection electrodes is used to detect the active EMG electrodes, it is a non-invasive detection electrodes, simple and ADSP2101KP-40 price and practical, widespread application prospect. SEMG detection electrodes for detection of surface EMG signals is important, especially in the clinical diagnosis, human bionic systems. SEMG signal is relatively weak, generally in the V level, vulnerable to the ECG signal and ADSP2101KP-40 suppliers and the interference frequency noise, so should use the analog filter and notch filter the signal conditioning. In addition, the electrode and the distance between the preamplifier also cause interference too long into the detection system. The design of the electrode, amplifier circuit, filter circuit and a trap circuit to be packaged together and shield protection, reducing the electrode distance and signal conditioning circuitry, thus reducing the interference caused by conductive coupling.

1 noise and interference sources

SEMG signal is relatively weak, between 15 ~ 100V in amplitude, the signal spectral width is generally 10 ~ 500 Hz, and ECG therefore vulnerable to frequency noise. Both skin and tissue attenuation of EMG has also vulnerable to disruption. In the actual test, because from the body surface, air, power and circuit itself, and other factors, makes the detected SEMG signal often carries a very low frequency (near DC) and high frequency (greater than 500 Hz) interference signal.

2 SEMG detection electrodes

SEMG detection electrodes that detect surface EMG electrodes, it is a buffer amplifier and filter circuit with the electrode, the so-called buffer amplifier is a high input impedance, low output impedance of the amplifier, the main achievement impedance conversion can be done to a small volume, and filter circuit attached to the electrodes, to achieve integration, significantly reducing the electrode connections. During EMG testing, the use of surface EMG electrodes to the acquisition of the analog signal into the ADC, and then into the data processing system shown in Figure 1.

Figure 1 SEMG signal acquisition and processing circuit

2.1 SEMG detection electrode structure

Original design is the use of Ag-Agcl disc surface electrodes, through the guide to the front line to guide the signal processing circuit, resulting in the signal frequency is basically all been overwhelmed by the noise interference. In a lot of access to domestic and foreign-related information, refer to the companys U.S. delsYs electrode, combined with the actual situation in the design of the surface EMG detection electrodes. To reduce the effect of noise, the electrode together with the pre-processing circuit sealed, and to shield protection, will be collected analog signals into digital signals transmitted via unshielded twisted pair to the data processing system for processing, greatly enhance the signal noise immunity. Use will be stored in preservation solution in the silver pole rod removed, insert the mount on the PCB, the skin coated with conductive adhesive, and use the bandages with velcro fixed to the muscle surface of the electrode, the contact surface of the cavity design can make electrode more closely aligns the skin, while not easy to move the cause motion artifact.

2.2 SEMG detection electrode design

EMG is very weak, the rate of about 15 ~ 100V, in order to facilitate follow-up treatment (active filter A / D conversion) accurate and effective, to be amplified to 1 000 to 2 000 times, the amplitude reaches 0 ~ 5 V between. Superimposed in order to suppress the input common-mode signals, the device provides a high common mode rejection ratio (CMRR). Between the skin and subcutaneous muscle and skin contact between the electrode and there is resistance, especially in the stratum corneum on the skin clean and dry environment in the resistivity up to 100 k ~ 1 M, thus requiring a high preamp input impedance. Many researchers at home and abroad come through a lot of experiments, SEMG signal of the main energy is concentrated in 50 ~ 150 Hz range, as in the actual detection of air from the surface, the power supply and the circuit itself, a variety of factors, detected SEMG often carries a very low frequency (near DC) and high frequency (greater than 500 Hz) of the interference signal, for the secondary processing circuitry in the SEMG in the need to join the band-pass filter. In addition, 50 Hz power system frequency interference noise in many cases, the magnitude of the EMG signals that will be useful to completely submerged, so how to suppress the 50 Hz frequency interference EMG signal detection noise is the primary problem. The introduction of double-T filter notch filter frequency of 50 Hz interference signals, and its essence is a band stop filter. The principle of SEMG detection electrodes shown in Figure 2.

Figure 2 SEMG detection electrodes principle

Figure 2, the first stage amplifier circuit using ADI Corporation junction FET (JFET) input instrumentation amplifier, and two AD8220 AD8627. The basic principle for the three AD8220 op amp instrumentation amplifier, the amplifier features JFET differential input to single ended output. Junction FET op amp AD8627 as a voltage follower, to improve the common mode rejection ratio, which is a buffer with low input bias current, bias current of less than 1 pA, a very weak signal can be collected. OP2177 with high precision, low offset, low power consumption and other features, the chip integrates two operational amplifiers, the flexibility to form various types of amplifier and filter circuit, so the follower constituted by the OP2177. This circuit, the first stage magnification G1 = 1 +49.4 / (R4 + R5) / / R6, set the G1 is 150, the secondary amplifying and filtering circuit magnification G2 = 10, so the total magnification G = G1xG2 = 150x10 = 1 500, is sufficient to signal amplification to the V class, so in the post-level signal processing and anti-jamming capability greatly enhanced.

2.3 SEMG detection electrodes laboratory equipment

In this design, a three-electrode polarity electrode, the middle of the electrode as reference electrode, used to reduce noise and improve the common mode rejection ratio. Electrodes with cylindrical pole piece of metal bullion. The circuit board shown in Figure 3, size 50 mmx25 mm, experimental side of the fixed plate 3 parallel silver electrodes, silver bullion distance between the 10 mm, length 20 mm, pore size of 1mm. The other side of the test board SEMG detection circuit, the silver bullion are access holes as shown in Figure 3. Detection of surface EMG signal during the time, it should be reasonably placed electrodes on the muscle to prevent the outer edge of the electrode is likely to detect cross-talk from adjacent muscles. Determination of tablets and alcohol when the sand required to remove grease and wipe the skin, the stratum corneum, and conductive paste applied on the skin or saline, the use of paste will stick to the skin with surface electrodes. Surface of the electrode contact with the skin will have a slight displacement of low-frequency noise, so the measurement should be measured by emotional stability in the circumstances and try to keep the body relatively static state. In short, it should be possible to reduce the external interference on the experimental process to effectively extract useful surface EMG.

Figure 3 SEMG detection electrodes circuit board

3 test results

Test is the implementation arm action with the electrode surface EMG acquisition.

Figure 4 shows the arm flexion-extension movement, the SEMG detection electrodes are collected and the resulting EMG spectrum chart. The figure shows EMG amplitude is about 0 ~ 1.5 V, a frequency of approximately 10 ~ 500 Hz. Therefore, the electrode was extracted using the ideal EMG, effectively suppress noise.

Figure 4, surface EMG and the spectrum

4 Conclusion This article is designed

SEMG detection electrodes, the more successfully applied to the practical application of SEMG detection. Test showed that the detection of surface EMG electrode design is reasonable, the electrode design and signal conditioning circuit will be packaged together greatly reduces the electrode connections. This SEMG detection electrodes used to achieve the detection of surface EMG signals, can improve the signal to noise ratio, reducing the noise, effectively extract useful surface EMG. The development of the SEMG detection electrodes may be flawed and inadequate learning process in the future will continue to cooperate with the relevant manufacturers, to communicate, using its advanced production techniques and technologies, the design of the electrode to do further research and improvement.

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