Helping Hand: EMG prostheticsBiomedical applications of filters and amplifiers

Biomedical Applications

http://www.touchbionics.com/products/active-prostheses/i-limb-ultra/

The ElectroMyoGram (EMG)

How to measure the EMG

http://lowerextremityreview.com/article/unstable-shoe-designs-functional-implicationshttp://www.vibrationtrainingdevice.com/VG-Evolution-I-Vibrogym.htm

Skin electrodes Filters and Amplifiers

EMG Main Idea

• 3 total electrodes

• 1 positioned over an “electrically quiet” place = ground, Vg

• 2 signal electrodes measure potential on muscle relative to ground.

• Instrumentation amplifier computes the difference:

Vout = (V1 – Vg) – (V2 – Vg) = V1 – V2

V1-Vg

V2-Vg

G (V2-V1)

Gain-Bandwidth Product

Gain x Bandwidth = constant

Measuring muscle generated voltages on the skin

PROBLEM: SKIN RESISTANCE

SOLUTION: OP-AMP BUFFER

Voltage lost going across skin: Iskin x Rskin

Buffer blocks current flow, therefore no voltage lost across skin!

Buffer output voltage “follows” the input

Muscle-generated signal

Measurement Device

Skin-Tissue Interface = DC battery

Metal electrode touching skin looks very similar to an electrochemical cell

Unwanted DC offset voltage generated

Full Signal ChainBuffer/Active Electrodes

Computer Processing

Account for skin/electrode resistance

HPFAccount for dc offset (“battery”) offset at electrode-skin interface

Differential Amplifier

Make voltage signal usable to computers that usually require >1V signal

LPFGet rid of unwanted high frequency junk

Sources AJ Fridlund and JT Cacippo. Guidelines for Human Electromyographic Research.

Pscyhophysiology 23(5) 567- 589, retrieved from: http://www.sprweb.org/articles/Fridlund86.pdf

Intan amplifiers: http://intantech.com/amplifier_intro.html INA126 http://www.ti.com/lit/ds/symlink/ina126.pdf Delsys: delsys.com