Biopotential Amplifier Speaker: Sun Shih-Yu 3/20, 2006.

20
Biopotential Amplifier Speaker: Sun Shih-Yu 3/20, 2006

Transcript of Biopotential Amplifier Speaker: Sun Shih-Yu 3/20, 2006.

Biopotential Amplifier

Speaker: Sun Shih-Yu3/20, 2006

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

Voltage and freq. ranges for common biopotential signals

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(

Voltage and freq. ranges for common biopotential signals

Interference observable!

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

Amplifiers for various biopotential signals

different spectrum and amplitude constraints

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

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

EEG amplifier

Low level of signal; Higher gain Small electrodes; higher input

impedance Higher CMRR Low noise amp