医学电子学学习.pptx
Frequency Ranges of Various Biopotential Signals第1页/共33页Common biopotential signalsAs shown in Table 2.1,common biopotential signals span the range dc to 10 kHz.Under ideal conditions,a biopotential amplifier with wideband response would serve most applications.However,the presence of common-mode potentials,electrode polarization,and other interfering signals often obscure the biopotential signal under investigation.As such,the frequency response of a biopotential amplifier should be tuned to the specific spectral content expected from the application at hand.第2页/共33页Frequency Ranges of Various Biopotential Signals第3页/共33页Frequency Ranges of Various Biopotential Signals第4页/共33页WIDEBAND BIOPOTENTIAL AMPLIFIERThe biopotential amplifier circuit described by the schematic diagrams of Figures 2.2 and 2.3 covers the complete frequency range of commonly recorded biopotentials with high CMR.第5页/共33页-3dB bandpassSpectral analysis is the most common way of determining the bandwidth required to process physiological signals.For a first estimate,however,the rigors of spectral analysis can be avoided simply by evaluating the durations of high-and low-frequency components of the signal.Koide 1996 proposed a method for estimating the-3dB bandpass based on acceptable distortion.第6页/共33页A stereotypical intracellular signalThe duration of the highest-frequency component,tHF,is estimated from a stereotypical signal to be the minimum rise or fall time of a signal variation.The duration of the lowest frequency component,tLF,on the other hand,is measured from the tilt of the baseline or of the lowest-frequency component of interest.Koide illustrated this with an example.Figure 2.1 shows a stereotypical intracellular potential measured from the pacemaker cells in a mammalian heart SA node.In this example,tHF=75 ms and tLF=610 ms.Using the formulas of Table 2.2,the amplification system must have a 3-dB bandpass of 0.0026 to 41.3 Hz to reproduce the signal with negligible distortion(1%).Acceptable distortion,usually considered to be 5%or less for physiological signals,would require a narrower-3dB bandpass,of 0.013 to 18.7 Hz.第7页/共33页Figure 2.1 A stereotypical intracellular potential measured from the pacemaker cells in a mammalian heart SA node has a minimum rise time of tHF=75 ms and a tilt of tLF=610 ms.The-3dB bandpass needed to reproduce this signal with 1%distortion is of 0.0026 to 41.3 Hz.第8页/共33页Approximate-3dB Frequencies Required第9页/共33页Wideband dc-coupled biopotential amplifier Figure 2.2第10页/共33页Figure 2.2This wideband dc-coupled biopotential amplifier front end covers the complete frequency range of commonly recorded biopotentials.A Burr-Brown INA110AG ICIA is dc-coupled to the electrodes via current-limiting resistors R22 and R23 and IS-1-3.3DP faultcurrent limiters.Capacitors and diodes are used to protect the amplifier from high-frequency currents,such as those used in electrosurgery and ablation procedures as well as from high-voltage transients such as those that may be expected from defibrillation and electrostatic discharge.第11页/共33页第12页/共33页Figure 2.3The output of the ISO107 isolation amplifier is fed to IC2B,which has its gain selectable through switch SW3.The circuit built around IC2A nulls dc offsets automatically when SW1 is closed.The features of this biopotential amplifier make it an ideal choice for recording cardiac monophasic action potentials(MAPs)using electrodes in direct contact with the heart.第13页/共33页AC-COUPLED INSTRUMENTATION BIOPOTENTIALAMPLIFIER FRONT ENDThe circuit of Figure 2.5 embodies the classic implementation of a medium-impedance(10-M)instrumentation biopotential amplifier based on the popular AD521 ICIA by Analog Devices.The gain of this circuit is adjustable between 10 and 1000 and maintains a CMR of at least 110 dB.第14页/共33页第15页/共33页Figure 2.4Dc and very low frequency potentials are prevented from propagating beyond the front-end amplifier through a technique commonly referred to as dc rejection.Here,IC4C,together with R11 and C17,are used to offset IC1s reference to suppress a baseline composed of components in the range dc to 0.48 Hz.第16页/共33页AC-COUPLED INSTRUMENTATION BIOPOTENTIALAMPLIFIER FRONT ENDThe heart of the circuit is IC1,the monolithic IC instrumentation amplifier.Biopotentials are ac-coupled to the amplifiers inputs through C1 and C2.Although instrumentation amplifiers have differential inputs,bias currents would charge stray capacitances at the amplifiers input.As such,resistors R1 and R2 are required to provide a dc path to ground for the amplifiers input bias currents.These resistors limit the impedance of each input to 10M referred to ground.The high-pass filter,formed by the ac-coupling capacitor and the bias shunt resistor on each of the ICIAs inputs,has a 3-dB cutoff frequency of 0.12 Hz.第17页/共33页AC-COUPLED INSTRUMENTATION BIOPOTENTIALAMPLIFIER FRONT ENDThe gain of IC1 is determined by the ratio between R3 and R4.Using a 20-k multiturn potentiometer,and given that the value of the range-setting resistor R3 is 100 k,the differential gain of the amplifier can be trimmed between 5 and 1000.The output offset of the amplifier can be trimmed through R5,which,at any given gain,introduces an output offset equal and opposite to the input offset voltage multiplied by the gain.Thus,the total output offset can be reduced to zero by adjusting this potentiometer.The instrumentation amplifier provides a low-impedance output(0.1)with a permissible swing of 10 V and can source or sink up to 10 mA.第18页/共33页Figure 2.5第19页/共33页Figure 2.5This is a classic medium-impedance(10-M)instrumentation biopotential amplifier based on the popular Analog Devices AD521 ICIA.The gain is adjustable between 10 and 1000 and maintains a CMR of at least 110 dB.The 40-kHz signal bandwidth makes this front end suitable for recording EMG or ECG signals or as a general-purpose high-impedance ac-coupled transducer amplifier.第20页/共33页BOOTSTRAPPED AC-COUPLED BIOPOTENTIAL AMPLIFIERDirect ac coupling of the instrumentation amplifiers inputs by way of RC high-pass filters across the inputs degrades the performance of the amplifier.This practice loads the input of the amplifier,which substantially lowers input impedance and degrades the CMRR of the differential amplifier.Although unity-gain input buffers can be used to present a highinput impedance to the biopotential source,any impedance mismatch in the ac coupling of these to an instrumentation amplifier stage degrades the CMR performance of the biopotential amplifier.第21页/共33页Figure 2.6第22页/共33页Figure 2.6This bootstrapped design yields an ac-coupled differential amplifier that retains all of the superior performance inherent in dccoupled instrumentation amplifiers.Ac voltages from the outputs of the ICIAs differential input stage are fed to the inverting inputs of their respective amplifiers via C3 and C4.This causes the ac voltage drop across R1 and R4 to be virtually zero.Ac current flow through resistors R1 and R4 is practically zero,while dc bias currents can flow freely to ground.第23页/共33页PASSIVE FILTERSThe simplest filters are those that comprise only passive components.These filters contain some combination of resistive(R),capacitive(C),and inductive(L)elements.The inductive and/or capacitive components are required because these elements present varying impedance to ac currents at different frequencies.第24页/共33页Figure 2.7第25页/共33页Figure 2.7In this biopotential amplifier,biopotentials are amplified by a Burr-Brown INA128U instrumentation amplifier.R5,R6,R9,R10,and C22 implement a low-pass filter with a cutoff of approximately 3.6 kHz.The biopotential amplifiers main low-pass filters are implemented by two cascaded RC passive filters with selectable cutoff frequency.IC4 buffers the signals between the cascaded sections.The two RC sections are identical,therefore setting a pole at the same frequency.However,the effect of the second RC can be suppressed by disconnecting its capacitor through switch SW2.第26页/共33页Figure 2.8第27页/共33页Figure 2.8The high-pass filters for the amplifier of Figure 2.7 are implemented in essentially the same way as the low-pass sections.Each high-pass section has capacitors(C50 and C53)which oppose current flow with an impedance that varies inversely with frequency and a resistor of selectable value that shunts the load.The RC sections are identical,therefore setting a pole at the same frequency.However,the effect of the second RC can be suppressed by shorting C53 through SW5.Op-amp IC13 buffers the signal between the stages.第28页/共33页Figure 2.9第29页/共33页Figure 2.9Two notch filters are used to reduce power line interference that may be picked up by the amplifier of Figure 2.7.The filter built around IC15 and IC17 has a notch at around 50 Hz,while the other(built around IC16 and IC18)has a notch at 60 Hz.第30页/共33页Figure 2.10第31页/共33页Figure 2.10This circuit can be used to full-wave-rectify the signal at the output of the notch filters of Figure 2.9.This signal-processing operation is often used in electromyography(EMG)to yield a signal proportional to the force generated by a muscle.The full-wave rectifier can be bypassed through SW3.第32页/共33页感谢您的观看！第33页/共33页
