Transcript lecture7
7th Lecture Dimitar Stefanov Recapping Three types electrodes are used for sensing of EMG signals: 1. 2. 3. indwelling (intramuscular) electrodes (single fiber electrodes, monopolar electrodes, concentric electrodes) Wire electrodes surface electrodes – non-invasive recordings Potential of surface electrode (V) Differential voltage waveform Velocity of propagation of the m.a.p. – 4 m/s There is a delay between the EMG and muscle contraction (30-80 milliseconds). In case of isometric muscle tension, a linear dependency between the muscle tension and the rectified EMG output is observed. Fatigue – (1) If we assume that the EMG is stimulation rate remains constant then the muscle tension deceases in case of fatigue. (2) The shape of the m.a.p. is altered in case of fatigue. (3) tremor occurs. EMG signal: • contains certain level of noises • has specific spectral density function. Important parameters of the EMG amplifiers: 1. 2. 3. 4. Gain and dynamic range Input impedance Frequency response Common mode rejection. Problem with the electrodes: polarization • The electric conductivity of the body involves ions as charge carrier. • Electrodes can be considered as electrical conductors in contact with the aqueous ionic solutions of the body. • The interaction between electrons in the electrodes and ions in the body can affect the EMG signal Half-cell potential (HCP) is called the potential difference between the metal of the electrode and the bulk of the electrolyte. •HCP depends on the ionic concentration •HCP can be measured when no electric current flows between an electrode and the electrolyte Problem with the electrodes: polarization Polarization – arises in case when current flows between the electrode and the solution. Perfectly polarizable electrodes – no actual current crosses the electrode- electrolyte interface Nonpolarized electrodes – allow the current to pass freely in electrode-electrolyte interface. Silver – silver chloride electrode (Ag/AgCl)– it possesses characteristics which are similar to a perfect nonpolarizable electrode. Silver – silver chloride electrodes AgCl film Ag metal insulated lead wire Low noise electrodes greater mechanical stability Ag lead wire sintered Ag and AgCl (Ag and AgCl powder mechanically pressed) Equivalent circuit of a biopotential electrode Ehc – half-cell potential Rd and Cd – represent the impedance associated with the electrode-electrolyte interface Rs – series resistance. Biopotential electrode impedance as a function of frequency EMG amplifiers Amplitudes of the EMG signal : • Surface EMG electrodes - maximum amplitude of 5 mV peak-to-peak • Indwelling electrodes – amplitude of up to 10 mV • Single m.a.p. electrodes – amplitude of 100 mV Noise level of the amplifier is the amplitude of the higher frequency random signal on the output of the amplifier when the electrodes are shorten together. Noise level of the amplifier should not exceed 50 mV, (preferably 20mV). Amplifier gain – the ratio of the output voltage to the input voltage Input impedance of an amplifier of biosignals The resistance of the electrode-skin interface depends on: thickness of the skin layer, the cleaning of the skin prior to the attachment of the electrodes, the area of the electrode surface, temperature. Electrode paste – decreases the resistance between the electrode and the skin. Input impedance of an amplifier of biosignals EMG amplifiers should possess high input resistance The capacitance between the electrode and the skin causes frequency distortions. Frequency response of the EMG amplifier Frequency bandwidth All frequencies present in the EMG should be amplified at one and the same level. Bandwidth – the difference between upper cutoff frequency f2 and the lower cutoff frequency f1. The gain of the amplifier at f1 and f2 is 0.707 from the gain of the gain in the midfrequency region (half-power). Amplifier gain: Example: linear gain 1000, or 60 dB; gain at the cutoff frequencies – 57 dB (3dB less than that at the mid-frequencies). The EMG amplifier should amplify equally all EMG frequency components. Most of the EMG signals are concentrated in the band between 20 and 200 Hz. Recommended range of the EMG amplifiers: • from 10 Hz to 1000 Hz – when the signal is collected with surface electrodes; • from 20 Hz to 2000 Hz – when the signal is collected with indwelling electrodes. Interferences: • Hum from power line (60 Hz in the USA and 50 Hz in Europe)in the middle of the EMG spectrum • Movement artifacts – their frequency lies in the 0 to 10 Hz range – don’t cause big problems • Noise from low quality cabling systems – interfere with the baseline of the EMG signal; can be eliminated by good low frequency filtering (by setting of f1 to about 20 Hz). Influence of the choice of f1 and f2 to the output signal Common mode rejection The human body acts as antenna to pick up any electromagnetic radiation that is present. Radiation: from domestic power lines, fluorescent lighting, and electrical machinery. Single-ended amplifier Differential amplifier A perfect subtraction never occurs. Common mode rejection ratio (CMRR) CMRR is measured in dB. In good quality EMG amplifiers CMRR should be 10,000 (80 dB) or higher. Processing of EMG Example: 1. Half of full-wave rectification (absolute value) 2. Linear envelope (low-pass filtering of the rectified signal) – main decision here is the choice of the low pass filter! 3. Integration of the signal from (2) over the period of the muscle contraction – area under the curve 4. Integration of the signal from (2) for a fixed time, reset to zero, and repeating the integration cycle – such scheme represents the trend of the EMG amplitude with time 5. Integration of the signal from (2) to a present level, reset to zero, and repeating the integration cycle – represents the level of the muscle activity (high or low muscle activity). Diagram of several common EMG processing systems and the processing results Biopotential amplifiers Basic amplifier requirements: 1. The physiological process to be monitored should not be influenced in any way by the amplifier 2. The measured signal should be not distorted 3. The amplifier should provide the best possible separation of signal and interferences 4. The amplifier should offer protection of the patient from any hazard and electric shock 5. The amplifier should be protected against damages due to high input voltages. The input signal to the amplifier consists of 5 components: 1. Desired biopotential 2. Undesired biopotentials 3. A power line interference signal and its harmonics 4. Interference signals generated by the tissue-electrode interface 5. Noise. Block diagram of a biopotential amplifier FET transistors Galvanic decoupling of the patient Motion artifacts – the contact between the electrode and the tissue changes during the relative motions between the electrodes and the tissue. Measures for decreasing the motion artifacts: • High input resistance of the amplifier • Usage of non-polarized electrodes (Ag/AgCl) • Reduction of the source impedance by usage of electrode gel. Artifacts due to electric and magnetic fields – Example. Amplitude/frequency characteristics of the bioamplifiers used in different applications Special circuits which built the biopotential amplifier Instrumentation amplifiers DC instrumentation amplifiers AC instrumentation amplifiers AC amplifiers eliminate the electrode offset potential, permit high gain and permits higher CMRR. The capacitors between the electrodes and the input stage of the amplifier cause charging effects from the input bias current. Isolation amplifier Isolation is realized in the following technologies: • Transformer isolation • Opto-isolation. Isolation provides a complete galvanic separation between the input stage (patient) and the other part of the measure equipment. Surge protection of the bioamplifiers Protection of the amplifier from damage due to surge input potentials. •Diodes •Zener diodes •Gas-discharge tubes Input guarding Technique for increase both the input impedance of the amplifier of biopotentials and the CMRR Instrumentation amplifier providing input guarding Driven-right-leg circuit reducing common-mode interference.