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Biopotential Amplifier
Speaker: Sun Shih-Yu
3/20, 2006
Outline
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Requirements
A standard ECG
Problems frequently encountered
Amplifiers for various biopotential
signals
Requirements
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Large input impedance; small output
impedance
Frequency response
High gain
Protection
Differential amplifier
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High CMRR (common mode rejection ratio)
Quick calibration
Problems
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Frequency distortion
Saturation or cutoff distortion
Ground loop
Open lead wires
Artifact from large electric transients
Interference
Voltage and freq. ranges for
common biopotential signals
Large electric transient
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Defibrillation
Motion of the electrodes
Built-up static electric charge
Older equipment: different offset
voltage from one lead to another
Interference
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Electric power system
Magnetic induction
EM interference
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Shunting a small capacitor (200pF)
EMG interference
Interference from electric
power systems
v A  vB  id1Z1  id 2 Z 2
id 1  id 2
v A  vB  id1 (Z1  Z 2 )
vA  vB  (6nA)(20k)  120V
Interference from electric
power systems (cont’d)
vcm  idb Z G
vcm  (0.2A)(50k)  10mV
v A  vB  vcm (
Z in
Z in

)
Z in  Z1 Z in  Z 2
v A  vB  vcm (
Z 2  Z1
)
Z in
vA  vB  (10mV )(20k / 5M)  40V
Voltage and freq. ranges for
common biopotential signals
Interference observable!
Interference from magnetic
induction
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Shielding
Keep away from magnetic-field regions
Reduce the effective area of the single
turn coil
Amplifiers for various
biopotential signals
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EMG amplifier
Amplifiers for intracellular electrodes
EEG amplifier
Amplifiers for various
biopotential signals
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different spectrum and amplitude
constraints
EMG amplifier
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Amplitude depends on the electrode
used and signal
Frequency spectrum wider than ECG
Less motion interference due to higher
frequency band
Amplifiers for intracellular
electrodes
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measure the potential across the cell
membrane
Frequency response must be wide
Amplitude in the order of 50 to 100mV;
gain needs not be high
Amplifiers for intracellular
electrodes (cont’d)
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Even large input impedance due to
large source one
Geometry results in a relatively large
shunting capacitance
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Use positive feedback to produce negative
capacitance
Compensating positive
feedback
Compensating positive
feedback (cont’d)
1
vi 
Cf
 i dt  A v
i
1
vi 
(1  Av )C f
 vi 
v i
 i dt
i
1
ii dt

Ceq
 Ceq  (1  Av )C f
However……
• gain is frequency dependent
• may be unstable because of positive feedback
• tends to be noisy
EEG amplifier
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Low level of signal; Higher gain
Small electrodes; higher input
impedance
Higher CMRR
Low noise amp