Biopotential amplifiers FYS 4250 Kap.6 Biopotential amplifiers Amplifiers - important part of modern instrumentation Low voltage signals Designed for signal strength amplifying + maintaining high fidelity = Biopotential amplifiers Normally voltage amplifiers, can be power amplifiers as well Basic requirements: 1. High input impedance (=minimal loading of measured signal)> 10 Ohm 2. Input circuit must provide protection to the organism being studied 3. Must operate in that portion of the frequency spectrum in which theh biopotentials that they amplify exists 4. Normally bipolar electrodes= high common mode voltage -> amplifiers need high common mode rejection ratio (minimize interference due to the common mode signal) 5. Must facilitate quick calibration

Heart vector = dipole ECG can be approximately represented as a vector quantity. -> Need to know both location of detected signals and time dependency Simple representation -> dipole = a vector directed from the negative charge to the positive charge and magnitude proportional to the amount of charge multiplied by the separation of the two charges. Through the cardiac cycle, magnitude and direction of M will vary bc dipole field varies

Vector Lead vector = is a unit vector that defines the direction that a constant-magnitude cardiac vector must have in order to maximise the voltage in a single pair of electrodes. A pair of electrodes or combination of several electrodes through a resistive network that gives equivalent pair = a lead For a cardiac vector M, voltage induced in a lead represented by a1 is the va1 Va1=M·a1 (dot produkt/skalar produkt) or va1 = |M|cos What is the contribution from M to a2? (=0) NB! Must use two leads that lie in the same plane as the cardiac vector to describe M

Einthoven triangle Three basic leads make up the frontal-plane ECG in electrocardiography Voltage source, Kirchhoffs voltage law => I-II+III=0

Wilson central terminal Potential appearing on one electrode with respect to the average of the signals seen at two or more electrodes

Augmented leads The resistace R shunts the circuit between the central terminal and the electrode -> reduced amplitude. Remove connection between measured limb and central terminal = augmented leads-> Increase amplitude of the signal.(50%) Possible to estimate position of cardiac cycle by comparing the six leads and fint the one with the greatest amplitude

Unipolar breast leads ECG in the transverse plane, electrode placed in the esophagus

ECG apparatus Protection circuit: High voltages are not harming the circuit Lead selector: Find the necessary electrodes for the selected leads and connect them to the circuit Calibration signal: 1 mV calibration signal introduced for each recorded channel Preamplifier: High impedance and high common mode-rejection ratio Isolation circuit: A barrier to the current from the power line Driven right leg: Driven reference potential Driver amplifier: Amplifies ECG to a recordable level. AC-coupled input to neglect preamplified offset voltages

Limited bandwidth

Voltage transients

Power, EMG ”noise” b. Muscle induced

Shielding, Faraday’s cage

Power-line noise NB! Can be present even when the apparatus is not turned on, no current is necessary to establish the elctrical field. Small virtual capacitors between the power lines and lead wires Id1 and Id2 do not go into the ECG -> high input impedance, but through the body to ground U=RI Va-Vb=Id1(Z1) * id2(Z2) Id1=id2 Va-Vb=id1(Z1-Z2)= 6nA 20KOhm= 120 uV

Common-mode voltage U=RI Vcm=idb * ZG=0.2 uA *50 kOhm= 10 mV (Can be up to 50 mV)

Magnetic coupled noise

Protection of input amplifier Problem: Electrosurgery, high voltages

Voltage limiters At voltages less than Vb (breakdown), the device allows little current to flow until -Vb where the current is short circuited to ground. b. Parallel silicon diodes with a breakdown voltage of approximately 600 mV (forward biased) Main advantage: small breakdown voltage c. Zener diodes, higher breakdown voltage, forward = 600mV, reverse is 2-20V d. Breakdown voltage of 50-90 V

”Driven right leg” Why R0? To limit the current flowing back to the patient. In cases of high voltage, amplifier saturates and the right leg is no longer driven= no earthing.

Equivalent scheme for driven right leg circuit

Biopotentail amplifiers

Capacitance amplifiers For intracellular electrodes or microelectrodes. (Small size = very high source impedance and shunting capacitance) -> Extremely high input impedance of the amplifier Shunting capacitance of the elctrode affects frequency-response characteristics Low gain very high input impedance noninverting amplifier, positive feedback The gain Av is controlling the sign of the circuit

Biopotential preamplifier

Signal averaging

Fetal ECG

Separation of fetal ECG

Digital ECG monitor

Detection of errors

ECG telemetry