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Biopotential Amplifier

Biopotential Amplifier

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Biopotential Amplifier

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

  2. Outline • Requirements • A standard ECG • Problems frequently encountered • Amplifiers for various biopotential signals

  3. Requirements • Large input impedance; small output impedance • Frequency response • High gain • Protection • Differential amplifier • High CMRR (common mode rejection ratio) • Quick calibration

  4. Problems • Frequency distortion • Saturation or cutoff distortion • Ground loop • Open lead wires • Artifact from large electric transients • Interference

  5. Voltage and freq. ranges for common biopotential signals

  6. Large electric transient • Defibrillation • Motion of the electrodes • Built-up static electric charge • Older equipment: different offset voltage from one lead to another

  7. Interference • Electric power system • Magnetic induction • EM interference • Shunting a small capacitor (200pF) • EMG interference

  8. Interference from electric power systems

  9. Interference from electric power systems (cont’d)

  10. Interference observable! Voltage and freq. ranges for common biopotential signals

  11. Interference from magnetic induction • Shielding • Keep away from magnetic-field regions • Reduce the effective area of the single turn coil

  12. Amplifiers for various biopotential signals • EMG amplifier • Amplifiers for intracellular electrodes • EEG amplifier

  13. Amplifiers for various biopotential signals • different spectrum and amplitude constraints

  14. EMG amplifier • Amplitude depends on the electrode used and signal • Frequency spectrum wider than ECG • Less motion interference due to higher frequency band

  15. 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

  16. 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 negative capacitance

  17. Compensating positive feedback

  18. Compensating positive feedback (cont’d) • However…… • gain is frequency dependent • may be unstable because of positive feedback • tends to be noisy

  19. EEG amplifier • Low level of signal; Higher gain • Small electrodes; higher input impedance • Higher CMRR • Low noise amp