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

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## Biopotential Amplifier

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**Biopotential Amplifier**Speaker: Sun Shih-Yu 3/20, 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**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 observable!**Voltage and freq. ranges for common biopotential signals**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 negative capacitance**Compensating positive feedback (cont’d)**• 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

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