7th Lecture. Dimitar Stefanov. Recapping. Three types electrodes are used for sensing of EMG signals: indwelling (intramuscular) electrodes (single fiber electrodes, monopolar electrodes, concentric electrodes) Wire electrodes surface electrodes – non-invasive recordings.
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Potential of surface electrode (V)
Differential voltage waveform
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.
(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.
Important parameters of the EMG amplifiers:
Problem with the electrodes: polarization
Half-cell potential (HCP) is called the potential difference between the metal of the electrode and the bulk of 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.
insulated lead wire
Ag lead wire
sintered Ag and AgCl
(Ag and AgCl powder mechanically pressed)
Silver – silver chloride electrodes
Low noise electrodes
greater mechanical stability
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
Amplitudes of the EMG signal :
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
Electrode paste – decreases the resistance between the electrode and the skin.
EMG amplifiers should possess high input resistance
The capacitance between the electrode and the skin causes frequency distortions.
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 mid-frequency region (half-power).
Example: linear gain 1000, or 60 dB; gain at the cutoff frequencies – 57 dB (3dB less than that at the mid-frequencies).
Most of the EMG signals are concentrated in the band between 20 and 200 Hz.
Influence of the choice of components. f1andf2to 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 components.
A perfect subtraction never occurs.
Common mode rejection ratio (CMRR) components.
CMRR is measured in dB.
In good quality EMG amplifiers CMRR should be 10,000 (80 dB) or higher.
Processing of EMG components.
Diagram of several common EMG processing systems and the processing results
Biopotential amplifiers processing results
Block diagram of a biopotential amplifier processing results
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.
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 used in different applications
DC instrumentation amplifiers
AC used in different applicationsinstrumentation 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 used in different applications
Isolation provides a complete galvanic separation between the input stage (patient) and the other part of the measure equipment.
Surge protection of the bioamplifiers used in different applications
Protection of the amplifier from damage due to surge input potentials.
Input guarding used in different applications
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.