Biomedical Instrumentation (BME420 ) Chapter 6: Biopotential Amplifiers John G. Webster 4 th Edition

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1 Biomedical Instrumentation (BME420 ) Chapter 6: Biopotential Amplifiers John G. Webster 4 th Edition Dr. Qasem Qananwah BME 420 Department of Biomedical Systems and Informatics Engineering 1 Biopotential Amplifiers Basic function to increase the amplitude of a weak electric signal of biological origin typically process voltages but in some cases also process currents Typical bio-amp requirements high input impedance -greater than 10 MΩ safety: protect the organism being studied careful design to prevent macro and microshocks isolation and protection circuitry to limit the current through the electrode to safe level output impedance of the amplifier should be low to drive any external load with minimal distortion gain greater than 1000 biopotentials are typically less than a millivolt most biopotential amplifiers are differential signals are recorded using a bipolar electrodes which are symmetrically located high common mode rejection ratio biopotentials ride on a large offset signals rapid calibration of the amplifier in laboratory conditions BME 420 Department of Biomedical Systems and Informatics Engineering 2 1

2 Figure 6.16: Voltage and frequency ranges of some common biopotential signals; dc potentials include intracellular voltages as well as voltages measured from several points on the body. EOG is the electro-oculogram, EEG is the electroencephalogram, ECG is the electrocardiogram, EMG is the electromyogram, and AAP is the axon action potential. (From J. M. R. Delgado, "Electrodes for Extracellular Recording and Stimulation," in Physical Techniques in Biological Research, edited by W. L. Nastuk, New York: Academic Press, 1964) BME 420 Department of Biomedical Systems and Informatics Engineering 3 Electrocardiogram The electric dipole, consists of two equal and opposite charges, separated by some (usually small) distance The potential differences arising in the heart (cardiac dipoles) can be represented by electrical vectors Amplitude and direction All basic vector operations can be applied to the cardiac vectors Each depolarizing myocardial cell is in fact a dipole and thus can be represented by a vector = elementary vector The sum of all elementary vectors will create an instantaneous vector the potential differences generated by the heart change from moment to moment during the cardiac cycle Once a single cell is stimulated the depolarization will propagate in every direction: a propagating wave of depolarization will be created Each of these moments can be described by an instantaneous vector (with a different size and orientation) All these vectors can be brought to a single common point: electrical center of the heart BME 420 Department of Biomedical Systems and Informatics Engineering 4 2

3 Figure 6.1: Rough sketch of the dipole field of the heart when the R wave is maximal. The dipole consists of the points of equal positive and negative charge separated from one another and denoted by the dipole moment vector M. BME 420 Department of Biomedical Systems and Informatics Engineering 5 By recording the magnitude and direction of the electrical forces that are generated by the heart by means of a continuous series of vectors that form curving lines around a central point one can record the vectorcardiography The projection of this curve as function of time on an axis corresponding to a lead is actually the ECG in that particular lead M a Cardiac vector Lead vector Figure 6.2: Relationships between the two lead vectors a 1 and a 2 and the cardiac vector M. The component of M in the direction of a 1 is given by the dot product of these two vectors and denoted on the figure by v a1. Lead vector a 2 is perpendicular to the cardiac vector, so no voltage component is seen in this lead. BME 420 Department of Biomedical Systems and Informatics Engineering 6 3

4 BME 420 Department of Biomedical Systems and Informatics Engineering 7 Electrocardiogram Electrodes for recording the potential changes of the heart are placed on the body surface in a standard way Potential changes recorded by specifically connected electrodes is called a lead(a pair of electrodes, or combination of several electrodes through a resistive network that gives an equivalent pair, is referred to as a lead) Each lead will be assigned with an axis and each of the axes will have an orientation The projection of the cardiac vectors as function of time on the axis corresponding to a lead is actually the ECG trace in that particular lead BME 420 Department of Biomedical Systems and Informatics Engineering 8 4

5 Electrocardiogram 1. Two different points on the body (bipolar leads) 2. One point on the body and a virtual reference point with zero electrical potential, located in the center of the heart (unipolar leads) The standard ECG has 12 leads: 1. 3 Standard Limb Leads 2. 3 Augmented Limb Leads 3. 6 Precordial Leads The axis of a particular lead represents the viewpoint from which it looks at the heart. Limb leads Precordial leads Bipolar Lead I, II, III (Standard limb leads) - Unipolar avr, avl, avf (augmented limb leads) V 1 -V 6 BME 420 Department of Biomedical Systems and Informatics Engineering 9 Figure 6.3: Cardiologists use a standard notation such that the direction of the lead vector for lead I is 0, that of lead II is 60, and that of lead III is 120. An example of a cardiac vector at 30 with its scalar components seen for each lead is shown. BME 420 Department of Biomedical Systems and Informatics Engineering 10 5

6 Generation of ECG signal in Einthoven limb leads BME 420 Department of Biomedical Systems and Informatics Engineering 11 Generation of ECG signal in Einthoven limb leads BME 420 Department of Biomedical Systems and Informatics Engineering 12 6

7 BME 420 Department of Biomedical Systems and Informatics Engineering 13 In clinical electrocardiography Electrocardiograph Leads more than one lead must be recorded to describe the heart's electric activity fully several leads are taken in the frontal plane and the transverse plane 1. frontal plane: parallel to the back when lying 2. transverse plane: parallel to the ground when standing standing Frontal plane lead placement (called Eindhoven s triangle) Additional leads Additional leads unipolar measurements potential measured at electrodes with reference to a reference; average of the 2 electrodes Wilson central terminal three limb electrodes connected through equal-valued resistors to a common node augmented leads some nodes disconnected increase the amplitude of measurement using Figure 6.4: Connection of electrodes to the body to obtain Wilson s central terminal BME 420 Department of Biomedical Systems and Informatics Engineering 14 7

8 Example 6.1: Show that the voltage in lead avr is %50 greater than that in lead VR at the same instant. Solution: Considering the connections for avr and VR, we can draw the equivalent circuits. The voltages between each limb and ground v a, v b, v c Figure E6.1 (a) avr (b) VR (c) Simplified circuit of (a) BME 420 Department of Biomedical Systems and Informatics Engineering 15 R should be very high (~5 MΩ) or buffers (voltage followers) are used between each electrode so that the loading of any particular lead will be minimal. Figure 6.5: (a), (b), (c) Connections of electrodes for the three augmented limb leads, (d) Vector diagram showing standard and augmented lead-vector directions in the frontal plane. BME 420 Department of Biomedical Systems and Informatics Engineering 16 8

9 Figure 6.6 (a) Positions of precordial leads on the chest wall, (b) Directions of precordial lead vectors in the transverse plane. BME 420 Department of Biomedical Systems and Informatics Engineering 17 6 limb leads define electrical activity in frontal plane 6 precordial leads define electrical activity in transverse plane The 3 augmented leads compare one limb electrode to the average of the other two. (avr, avl, avf). Leads are made of a combination of electrodes that form imaginary lines in the body along which the electrical signals are measured. BME 420 Department of Biomedical Systems and Informatics Engineering 18 9

10 Figure 6.7: Block diagram of an electrocardiograph BME 420 Department of Biomedical Systems and Informatics Engineering 19 Problems Frequently Encountered 1. Frequency Distortion a. High frequency distortion if band pass filter has 0 25 Hz response b. Low frequency distortion if band pass filter has Hz response In low frequency distortion, the baseline is no longer horizontal, especially immediately following any event in the tracing. Monophasic waves appear to be biphasic. BME 420 Department of Biomedical Systems and Informatics Engineering 20 10

11 Problems Frequently Encountered 2. Saturation or Cutoff Distortion The peaks of QRS can be cutoff because the output of the amplifier cannot exceed the saturation voltage or even the lower portion of the S wave will be cutoff. 3. Ground Loop BME 420 Department of Biomedical Systems and Informatics Engineering 21 Problems Frequently Encountered 4. Open Lead Wire Always make sure that the electrodes are in good contact. Otherwise, usually high potentials can often be induced in the open wire as a result of electric field. Coming from a power line, it causes a wideconstant-amplitude deflection on the ECG trace. BME 420 Department of Biomedical Systems and Informatics Engineering 22 11

12 Problems Frequently Encountered 5. Artifacts From Large Electric Transients Occurred due to cardiac defibrillation Figure 6.8 Effect of a voltage transient on an ECG recorded on an electrocardiograph in which the transient causes the amplifier to saturate, and a finite period of time is required for the charge to bleed off enough to bring the ECG back into the amplifier's active region of operation. This is followed by a first-order recovery of the system. BME 420 Department of Biomedical Systems and Informatics Engineering Interference from Electric Devices Figure 6.9 (a) 60 Hz power-line interference, (b) Electromyographic interference on the ECG. BME 420 Department of Biomedical Systems and Informatics Engineering 24 12

13 6. Interference from Electric Devices For a 9 m cable, i d 6 na, the electrodes impedance can differ as much as 20kΩ. Then, This can be minimized by shielding the leads. Figure 6.10 A mechanism of electric-field pickup of an electrocardiograph resulting from the power line. Coupling capacitance between the hot side of the power line and lead wires causes current to flow through skin electrode impedances on its way to ground. BME 420 Department of Biomedical Systems and Informatics Engineering Interference from Electric Devices Look at this situation. This is noticeable on the ECG and very objectionable on EEG. Figure 6.11 Current flows from the power line through the body and ground impedance, thus creating a common-mode voltage everywhere on the body. Zin is not only resistive but, as a result of RF bypass capacitors at the amplifier input, has a reactive component as well. BME 420 Department of Biomedical Systems and Informatics Engineering 26 13

14 6. Interference from Electric Devices Figure 6.12: Magnetic-field pickup by the electrocardiograph (a) Lead wires for lead I make a closed loop (shaded area) when patient and electrocardiograph are considered in the circuit. The change in magnetic field passing through this area induces a current in the loop, (b) This effect can be minimized by twisting the lead wires together and keeping them close to the body in order to subtend a much smaller area. BME 420 Department of Biomedical Systems and Informatics Engineering 27 Transient Protection Figure 6.13: A voltageprotection scheme at the input of an electrocardiograph to protect the machine from highvoltage transients. Circuit elements connected across limb leads on left-hand side are voltage-limiting devices. Figure 6.14: Voltage-limiting devices (a) Current voltage characteristics of a voltagelimiting device. (b) Parallel silicon-diode voltage-limiting circuit. (c) Back-to-back silicon zener-diode voltage-limiting circuit. (d) Gas-discharge tube (neon light) voltage-limiting circuit element. 28 BME 420 Department of Biomedical Systems and Informatics Engineering 28 14

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16 BME 420 Department of Biomedical Systems and Informatics Engineering 31 Figure 6.15 Driven-right-leg circuit for minimizing common-mode interference. The circuit derives common-mode voltage from a pair of averaging resistors connected to v3 and v4 in Figure 3.5. The right leg is not grounded but is connected to output of the auxiliary op amp. BME 420 Department of Biomedical Systems and Informatics Engineering 32 16

17 Figure E6.3 Equivalent circuit of driven-right-leg system of Figure BME 420 Department of Biomedical Systems and Informatics Engineering 33 Figure 6.18: This ECG amplifier has a gain of 25 in the dc-coupled stages. The high-pass filter feeds a noninvertingamplifier stage that has a gain of 32. The total gain is 25 X 32 = 800. When ma 776 op amps were used, the circuit was found to have a CMRR of 86 db at 100 Hz and a noise level of 40 mv peak to peak at the output. The frequency response was 0.04 to 150 Hz for ±3 db and was flat over 4 to 40 Hz. A single op amp chip, the LM 324, that contains four individual op amps could also be used in this circuit reducing the total parts count. BME 420 Department of Biomedical Systems and Informatics Engineering 34 17

18 Cardiotachometer BME 420 Department of Biomedical Systems and Informatics Engineering 35 Beat- to-beat Cardiotachometer 36 BME 420 Department of Biomedical Systems and Informatics Engineering 36 18

19 37 BME 420 Department of Biomedical Systems and Informatics Engineering 37 Figure 6.22 Typical fetal ECG obtained from the maternal abdomen. F represents fetal QRS complexes; M represents maternal QRS complexes. Maternal ECG and fetal ECG (recorded directly from the fetus) are included for comparison. (From "Monitoring of Intrapartum Phenomena," by J. F. Roux, M. R. Neuman, and R. C. Goodlin, in CRC Critical Reviews in Bioengineering, 2, pp , January 1975, CRC Press. Used by permission of CRC Press, Inc.) BME 420 Department of Biomedical Systems and Informatics Engineering 38 19

20 Figure 6.23 Block diagram of a scheme for isolating fetal ECG from an abdominal signal that contains both fetal and maternal ECGs. (From "Monitoring of Intrapartum Phenomena," by J. F. Roux, M. R. Neuman, and R. C. Goodlin, in CRC Critical Reviews in Bioengineering, 2, pp , January 1975, CRC Press. Used by permission of CRC Press, Inc.) BME 420 Department of Biomedical Systems and Informatics Engineering 39 BME 420 Department of Biomedical Systems and Informatics Engineering 40 20

21 Figure 6.24 Block diagram of a cardiac monitor. BME 420 Department of Biomedical Systems and Informatics Engineering 41 BME 420 Department of Biomedical Systems and Informatics Engineering 42 21

22 Figure 6.25 Block diagram of a system used with cardiac monitors to detect increased electrode impedance, lead wire failure, or electrode fall-off. BME 420 Department of Biomedical Systems and Informatics Engineering 43 BME 420 Department of Biomedical Systems and Informatics Engineering 44 22

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