lab 2 2010 instrumentation - bioelectrode impedance

lab 2 2010 instrumentation - bioelectrode impedance - IV...

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IV Bioelectrode impedance Background: The impedance of a bioelectrode is approximated by the formula: Z R R j fC R s d d d = + + 1 2 π The formula predicts that Z decreases when frequency increases owing to the frequency “f” in the denominator. On the graph below, two ECG electrodes are positioned on the arm of a subject. A signal generator (frequency f ) drives the circuit composed of an external resistance R ext , the two electrodes and the arm tissue in between. Voltages V 1 and V 2 are measured on the oscilloscope for different driving frequencies f . For this circuit, we have: 2 2 Z V I = ( 29 I V V R V V R V V j R ext ext ext = - = - = - + 1 2 1 2 1 2 0 θ cos sin and therefore, ( 29 2 2 1 2 Z V R V V j ext = - + cos sin where V 1 and V 2 are the magnitudes of the voltages measured on the scope and θ is the phase difference between V 1 and V 2 . Angle θ is negative (reactive impedance). The number "2" comes from the use of two electrodes. The modulus of Z is estimated from measurements of V 1 , V 2 and θ . Procedures: 1. Assemble the measurement circuit repeated here. Resistance R ext is inserted in your protoboard. The two electrodes are placed on the inside of the forearm, with about 10 cm distance center to center. Rub the skin with alcohol before placing the electrodes.
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lab 2 2010 instrumentation - bioelectrode impedance - IV...

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