Outline
Requirements A standard ECG Problems frequently encountered Amplifiers for various biopotential
signals
Requirements Large input impedance; small output
impedance Frequency response High gain Protection Differential amplifier
High CMRR (common mode rejection ratio)
Quick calibration
Problems
Frequency distortion Saturation or cutoff distortion Ground loop Open lead wires Artifact from large electric
transients Interference
Large electric transient
Defibrillation Motion of the electrodes Built-up static electric charge Older equipment: different offset
voltage from one lead to another
Interference
Electric power system
Magnetic induction EM interference
Shunting a small capacitor (200pF) EMG interference
Interference from electric power systems
2211 ZiZivv ddBA
21 dd ii
VknAvv BA 120)20)(6(
)( 211 ZZivv dBA
Interference from electric power systems (cont’d)
Gdbcm Ziv
mVkAvcm 10)50)(2.0(
)(21 ZZ
Z
ZZ
Zvvv
in
in
in
incmBA
)( 12
incmBA Z
ZZvvv
VMkmVvv BA 40)5/20)(10(
Interference from magnetic induction
Shielding Keep away from magnetic-field
regions Reduce the effective area of the
single turn coil
Amplifiers for various biopotential signals
EMG amplifier Amplifiers for intracellular
electrodes EEG amplifier
EMG amplifier
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
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)
Even large input impedance due to large source one
Geometry results in a relatively large shunting capacitance Use positive feedback to produce negativ
e capacitance
Compensating positive feedback (cont’d)
ivif
i vAdtiC
v1
fveq
ieq
i
ifv
i
CAC
dtiC
v
dtiCA
v
)1(
1
)1(
1
However……
• gain is frequency dependent
• may be unstable because of positive feedback
• tends to be noisy
Top Related