Biomedical applications of filters and amplifiers

Skin electrodes Filters and Amplifiers

EMG Main Idea 3 total electrodes 1 positioned over an “electrically quiet” place = ground, V g 2 signal electrodes measure potential on muscle relative to ground. Instrumentation amplifier computes the difference: V out = (V 1 – V g ) – (V 2 – V g ) = V 1 – V 2 V1-Vg V2-Vg G (V2-V1)

Gain x Bandwidth = constant

PROBLEM: SKIN RESISTANCE SOLUTION: OP-AMP BUFFER Voltage lost going across skin: I skin x R skin  Buffer blocks current flow, therefore no voltage lost across skin!  Buffer output voltage “follows” the input Muscle- generated signal Measurement Device

 Metal electrode touching skin looks very similar to an electrochemical cell  Unwanted DC offset voltage generated

Buffer/Active Electrodes Computer Processing Account for skin/electrode resistance HPF Account for dc offset (“battery”) offset at electrode-skin interface Differential Amplifier Make voltage signal usable to computers that usually require >1V signal LPF Get rid of unwanted high frequency junk

 AJ Fridlund and JT Cacippo. Guidelines for Human Electromyographic Research. Pscyhophysiology 23(5) , retrieved from:  Intan amplifiers:  INA126  Delsys: delsys.com