Week 3_S2 2024_ Lecture slides .pdf

BMET3802/9802 Biomedical Instrumentation Week 03: Biopotential amplifiers Dr Sandhya Clement Lecturer S...

BMET3802/9802 Biomedical Instrumentation Week 03: Biopotential amplifiers Dr Sandhya Clement Lecturer School of Biomedical Engineering The University of Sydney  I would like to acknowledge the Traditional Owners of Australia and recognise their continuing connection to land, water and culture. I am currently on the land of the Cadigal people of the Eora Nation, and I pay my respects to their Elders, past, present and emerging. I further acknowledge the Traditional Owners of the country on which you are on and pay respects to their Elders, past, present and future. The University of Sydney Questions from last week Lecture Equilibrium potential in mV for potassium ion for a cell at 37 degrees if the inside and outside conc for the ion is 150 and 5 mmol/L -96.01 mV Design a first order low pass filter to pass signals that are having a frequency less than 5 kHz and attenuate all others. Assume you are using a standard resistance of 1 k ohm. Capacitance is 31.84 nF (fc=1/2πRC) What happens if you interchange resistance and capacitance? High Pass Filter The University of Sydney This lecture: Signal Conditioning using op amp. – Biopotential amplifiers – Op Amp Circuits – Introduction and characteristics. – Inverting and Noninverting Amplifiers. – Unity gain Amplifiers – Instrumentation Amplifiers – Active Filters – Low Pass Filter – High Pass filters – Band Pass Filters – Band Stop Filters – Overview of Butterworth Filters The University of Sydney Page 4 Please note that most of the Lecture content are prepared and most of the Figures in this week’s lecture slides have been taken from the following textbooks: Yawale, S., & Yawale, S. (2021). Operational amplifier : theory and experiments. Springer. https://sydney.primo.exlibrisgroup.com/permalink/61USYD_INST/1c0ug48/alm a991032201081405106 Bronzino&Peterson: https://www-taylorfrancis- com.ezproxy.library.sydney.edu.au/books/mono/10.1201/9781351228671/me dical-devices-human-engineering-joseph-bronzino-donald-peterson The University of Sydney Page 5 Biopotential Amplifiers The University of Sydney Page 6  Why Amplifiers – Biosignals acquired often are – Very low voltage signal in the range of 1μV and 100 mV. – Often have a high source impedance. – Also, associated with noise. – Biopotential amplifiers are – Specifically designed for biomedical Bronzino, Joseph D., and Donald R. Peterson. Medical devices and human application. engineering. CRC Press, 2018. – They selectively amplify the biosignal and reject the noise. – guarantee protection from damages through voltage and current surges for both patient and electronic equipment. The University of Sydney Page 7 Basic Amplifier requirements in Biomedical application: – The biosignal measured should not be influenced by any means by the amplifier. – There should not be any kind of distortion to the measured signal. – The amplifier should be able to provide the best possible separation between the signal and interferences. – The amplifiers have to offer protection to patients from any electrical shock. – The amplifier itself has to protect against the results from high input voltage (This may occur during the application of electrosurgical instruments or defibrillators) The University of Sydney Page 8 Measurement of Biopotential – There are five different components associated with the input of the amplifier – Desired biosignal – Undesired biosignal – A power line interference signal (50 Hz or 60 Hz) – A generated interference signal by tissue/electrode interface – Noise – The desired biosignal appears as a voltage between two input terminals is called a differential signal (in Fig, Vbiol). – Line frequency has nil or very small change in amplitude between the two measuring electrodes and hence it is called a common mode signal. – In a real application, the common mode signal (in Fig, Vc.) is measured between the input and ground. The University of Sydney Page 9 Stages of Biopotential amplifier – The electrode determines the large portion of the measured signal. – The pre-amplifier sets the stage for the quality of the signal. – There is a noise generated by the amplifier (amplifer noise) and connection between the biological source and amplifier (thermal noise). The University of Sydney Page 10 Noise – The square of the rms value of the total noise associated with the system is calculated using the equation – The internal amplifier noise depends upon the internal noise voltage (en), noise current (in), and amplifier bandwidth(B=f2-f1). – The rms value of the thermal noise voltage (Erms) is calculated using the equation Where k is the Boltzman constant, T=absolute temperature, Rs=total source resistance (includes the resistance of the biological source and all transition resistance between source and amplifier input) and B is the Bandwidth in Hz. The University of Sydney Page 11 Various stages – The current technology uses differential amplifiers with voltage noise of less than 10 nV and current noise less than 1pA per √Hz. – The High pass and low pass filters eliminate electrode half cell potentials, pre- amplifier offset potential, and reduce the noise amplitude. Higher order Bessel filters are chosen. – Isolation amplifier stage serves as a galvanic decoupling of the patients from the equipment to prevent electric shock. – The various stages based on Operational Amplifiers (Op-Amp) satisfy all the necessary requirements for acquiring the high-quality bio signals The University of Sydney Page 12 Operational Amplifiers (Op amps) The University of Sydney Page 13 Introduction to Op amp Internal structure – Op amp is an integrated circuit (IC) preliminary designed for analog computations such as addition, subtraction, differentiation, integration, etc. – Op amps are broadly classified into two as general purpose (eg: LM741) and special purpose op amps (eg:LM380). – Op amps have two input terminals and one https://www.researchgate.net/post/Can-we-reveal-the- output terminal. brilliant-ideas-behind-the-741-op-amp-circuit-solution- of-genius – Most of the op amp circuit uses dual power supply. Symbol +Vcc Equivalent Circuit Inverting Input V1 Output Vo Vo =AVd =A(V2 – V1 ) Noninverting Input V2 where A (AOL)is the open loop gain. -Vcc The University of Sydney Page 14  Block diagram of Op amp – Stage1: dual input balanced output differential amplifier: it provides voltage gain and setup high input impedance. Also, provides two input terminals and high CMRR. – Stage 2: dual input unbalanced (one output) amplifier: two inputs of this stage are connected to the two outputs of the stage 1 amplifier. It provides a larger voltage gain. – Stage 3: Level shifter: direct coupling of the previous stage brings a DC offset (DC is not zero) to the output, this stage brings this DC value to zero to prevent undesired current in the load. – Stage 4: Emitter follower: Act as a buffer (input=output), offering high input and low output impedance which avoids unavoidable loading. The University of Sydney Page 15 CMRR(Common mode rejection ratio) – CMRR is the ratio of the differential gain (Ad) to the common mode gain (Ac) – CMRR is the ability of op-amp to amplify the differential mode signal and reject the common mode signal. – In Biomedical instruments, a differential mode signal is the desired signal from the electrodes. – Common mode signal is the signal which appears equally in both the input terminals of the op-amp, often interferences and noise. – High CMRR is always desirable for the amplifiers used in biomedical instrumentation. The University of Sydney Page 16 Slew rate – Slew rate (SR) is the change of output voltage or current per unit time – Fast response is required for almost all biomedical instruments. – Fast response depends on the change in output voltage with respect to the change in input voltage. – The min slew rate requirement for an op amp can be calculated using the equation The University of Sydney Page 17 Characteristics of Op amp Frequency response Ideal Op amp values – Infinite gain A=∞ – Infinite input impedanceZi (Ri) =∞ – Zero output impedance Zo = 0 Practical Op amp values – Infinite bandwidth, BW=∞ – For input offset voltage,Vd = 0 (V1 – – Infinite gain A≥ 104 V2 = 0) , Vo = 0 – Infinite input impedance Zi ≥ 106Ω – Infinite CMRR, CMRR=∞ – Zero output impedance Zo ≤ 500Ω – Slew rate= ∞ – Infinite bandwidth, BW= order of MHz – Input and offset bias current=0 – Input offset voltage < 10 mV – Slew rate =10 V/µsec – CMRR ≥ Zo dB – Input bias current is low – Input offset current—low < 0.2 nA The University of Sydney Page 18 Feedback in Op amp Feedback is the technique used to control the gain of the amplifier. Two types: Positive and Negative feedback – Positive feedback: the output signal fed back to the input terminal is in phase with the input signal (0° or 360º), signals get A is the open loop added up, gain increases, but more gain and β is the distortion.eg: oscillators feedback factor – Negative feedback: the output signal fed back to the input terminal is out of phase with the input signal (180º), signals get subtracted, gain decreases, reduce noise and increase stability. Eg: instrumentation amplifiers. – Voltage series negative feedback – Voltage shunt negative feedback – Current series negative feedback – Current shunt negative feedback The University of Sydney Page 19 Non inverting op amp VIN VOUT – It is a voltage series amplifier that amplifies the input signal. – The input voltage is provided to the non inverting terminal. – The feedback is given to the inverting terminal. R1 RF – Output voltage is calculated as RF + R 1 VOUT = VIN OR R1 VOUT= [1+(RF/R1)]VIN = AFVIN=(1/ β) VIN – Closed loop gain can be adjusted by R1 and RF. – Updated Input impedance Zif=Zi(1+βA) – Updated Output impedance https://www.falstad.com/circuit/ The University of Sydney Page 20 Voltage Follower (Unity gain follower) – Basically, a non-inverting amplifier without any external resistors connected in the feedback loop. – V0=Vi – Hence gain= (Vo/Vi)=1 – It offers high input impedance and low output impedance. – These circuits are commonly used for impedance matching (buffer). – To connect the high output impedance circuit to a low output impedance circuit. The University of Sydney Page 21 Find out the reason? The University of Sydney Page 22 Inverting op amp VOUT – It is a voltage shunt amplifier that amplifies the input signal. – The input voltage is provided to the inverting terminal. – The feedback is given to the inverting terminal. VIN – Output voltage is calculated as VOUT = -VIN RF R1 R1 RF – Closed loop gain can be adjusted by R1 and RF. – Updated Input impedance Zif=R1 – Updated Output impedance The University of Sydney Page 23 Multistage op-amp voltage gain A1 A2 A3 A=A1*A2*A3 OR A(db)=A1 (db)+ A2 (db)+A3 (db) The University of Sydney Page 24 The University of Sydney Page 25 Differential V amplifier = -V R f OUT1 1 1 R – Basic building blocks of biopotential amplifiers f + R1 – Amplifies the difference between Rg theRtwo input signalsRwith 2 = V2 VOUThigh a stable gain, input impedance and = f lowV2output impedance. R2 + Rg R1 R1 – The output voltage V0UT is Rf VOUT = (V2 -V1 ) R1 – To obtain the following equation, the design must satisfy the https://en.wikipedia.org/wiki/File:Op-Amp_Differential_Amplifier.svg criteria, Rg R f = R2 R1 – In the frequency response, Break frequency (fo) is the break point frequency (fo) at which the voltage gain starts to falls off or the point at which frequency starts roll-off (−3 dB point) The University of Sydney Page 26 Instrumentation amplifier – The important stage of a biopotential amplifier is the pre- amplifier – The pre-amplifier does the following – Sense the voltage between two electrodes – Reject the common mode signal – Minimise the effect of electrode polarization overpotentials. – The input impedance of the amplifier must be as high as possible to achieve the above requirements. – A single op amp differential amplifier discussed in the previous slide can not achieve this high input impedance. – Hence a standard instrumentation amplifier is used for this Instrumentation purpose. amplifier IC – The two input op amps provide high differential gain with a reduction to the common mode signal. – The output from both these amplifiers is fed to the output differential amplifier to further improve the CMRR. The University of Sydney Page 27 Instrumentation Amplifier Circuit from the Week 4 lab The University of Sydney Page 28 Op amp Filters (Active Filters) The University of Sydney Page 29 General Filters – Filters are the frequency selective circuits that attenuate or pass a band frequency. – There are many kinds of filters such as – Digital filters – Analog filters – Passive filters – Active filters – Audio filters (AF) – Radio frequency filters (RF) – Video frequency filters – Microwave filters – Ultrahigh frequency filters (UHF). The University of Sydney Page 30  Active Filters – Active filters use active components like transistors and op amps to filter the signals. – In addition to the filtering, the op amp filters offers a gain to the filtered signal. – Active filters are classified into two four different types based on their roll-off as – Butterworth filter – Chebyshev filter – Bessel filter Roll-off is the slope of the filter’s response in the – Elliptical filter transition region between the pass-band and stop- band The University of Sydney Page 31 Commonly used Filter Circuits – The commonly used filter circuits are – Low Pass Filter (LPF) – High Pass Filter (HPF) – Band Pass Filter (BPF) – Band Reject Filter (BRF)/Notch Filter – All pass filter (APF) Cutt off frequency of a filter is the frequency at which the gain of the filter is reduced by 3db. Hence cut off frequency is also called a -3db frequency. The University of Sydney Page 32 Low Pass Butterworth Filter (First order) – The input signal is connected to the noninverting terminal through a single RC circuit. – Gain magnitude ( ) and phase angle (Ф) equations are To design a filter, consider fH=1/2πRC – At f< fH – At f= fH – At f>fH Fc Please note fH and Fc are same here Remember Af= 1+(RF/R1) The University of Sydney Page 33 Second order Low Pass Filters – Filters with more than one combination of R and C for a given filter type (low-pass or high-pass) are higher order filters and attenuate high frequencies (for a low-pass filter) or low frequencies (for a high-pass filter) more rapidly with changing frequency than does a first order filter. – Roll-off rate for a second order is -40db/decade – Cutt-off frequency is – Gain of the amplifier is calculated as The University of Sydney Page 34 High Pass Butterworth Filter – The input signal is connected to the noninverting terminal through a single RC circuit. – The cut-off frequency is determined from R and C as fL=1/2πRC – Gain magnitude ( ) is obtained from the equation as – At f< fL – At f= fL – At f>fL The University of Sydney Page 35 Band Pass Filter – This filter passes a band of frequency and attenuates all other frequency – Band Pass is a cascaded connection of LPF and HPF – Cut off freq of HPF (fL)< Cut off freq of LPF (fH) – Centre frequency fc=√fH fL – Quality factor Q= fc/BW – If Q>10, narrow band pass filter – If Q< 10, wide band pass filter The voltage gain in a band pass filter is the product fc of the voltage gain of high pass and low pass filters The University of Sydney Page 36 Band Stop Filter – Band stop filter is the parallel combination of LPF and HPF – It is also known as Band Rejection Filter – Cut off freq of LPF (fH)< Cut off freq of HPF (fL) – Null frequency fnull=√fH fL – Quality factor, Q= fL –fH / fnull – Notch filter is the Band stop filter with a narrow stop band. The University of Sydney Page 37 The University of Sydney Page 38 Other Op amp circuits The University of Sydney Page 39 Isolation amplifier – Isolation amplifiers are used to provide isolation to the patient from the instrument. – It prevents the electric shock due to the interaction of the patient, amplifier, and other instruments in the surrounding (eg: defibrillator, surgical device) – Isolation barrier provides the complete galvanic separation between input (patient, amplifier) and other devices – This isolation barrier is established either by – Transformer isolation technique – Capacitor isolation technique – Optoisolation technique The University of Sydney Page 40 Surge Protection – Voltage surge between the electrodes during the defibrillator or electrosurgical instrument presents a risk to the biopotential amplifier. – This is important because the damaged pre- amplifier stage applies a dangerous current level to the patient. – To achieve this, a voltage limiting circuit is connected between each measuring electrode and the ground. – If the input voltage is within the allowed range, Protection of the amplifier input against high-voltage transients. e connection this device won’t provide any shunt impedance diagram for voltage-limiting elements is shown in panel (a) with two optional resistors R′ at the input. A typical current-voltage characteristic is shown in panel and hence input impedance remain high. (b). Voltage-limiting elements shown are the antiparallel connection of diodes (c), – When the voltage drop reaches a critical voltage antiparallel connection of Zener diodes (d), and gas-discharge tubes (e). (Vb), surge device impedance changes sharply, and current flows through it, making sure voltage does not exceed Vb. The University of Sydney Page 41 This Week Lab Introduction to d basic filter programming using python. Objectives In this laboratory, you will: Learn to use the NumPy library for computing. Learn to use matplotlib for signal plotting. Learn the basics of filter programming. Gain some degree of understanding in digital signal filtering. The University of Sydney Page 42 End Don’t forget to prepare for tomorrow’s lab The University of Sydney Page 43