Download 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.