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

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

6. Voltage and freq. ranges for common biopotential signals

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

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

9. Interference from electric power systems

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

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

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

13. Amplifiers for various biopotential signals EMG amplifier Amplifiers for intracellular electrodes EEG amplifier

14. Amplifiers for various biopotential signals different spectrum and amplitude constraints

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

16. 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

17. 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

18. Compensating positive feedback

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

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