1、Frequency Ranges of Various Biopotential SignalsCommon biopotential signals As 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 potential
2、s,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.Frequency Ranges of Various Biopotential
3、SignalsFrequency Ranges of Various Biopotential SignalsWIDEBAND BIOPOTENTIAL AMPLIFIER The 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.-3dB bandpass Spectral analysis is
4、 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 2019 proposed a method for estimating th
5、e-3dB bandpass based on acceptable distortion.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 han
6、d,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 t
7、he 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.Figure
8、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.Approximate-3dB Frequencies RequiredWide
9、band dc-coupled biopotential amplifier Figure 2.2Figure 2.2 This 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-
10、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.Fig
11、ure 2.3 The 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 ac
12、tion potentials(MAPs)using electrodes in direct contact with the heart.AC-COUPLED INSTRUMENTATION BIOPOTENTIALAMPLIFIER FRONT END The 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
13、Devices.The gain of this circuit is adjustable between 10 and 1000 and maintains a CMR of at least 110 dB.Figure 2.4 Dc 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
14、 and C17,are used to offset IC1s reference to suppress a baseline composed of components in the range dc to 0.48 Hz.AC-COUPLED INSTRUMENTATION BIOPOTENTIALAMPLIFIER FRONT END The heart of the circuit is IC1,the monolithic IC instrumentation amplifier.Biopotentials are ac-coupled to the amplifiers in
15、puts 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
16、 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.AC-COUPLED INSTRUMENTATION BIOPOTENTIALAMPLIFIER FRONT END The gain of IC1 is determined by the ratio betwe
17、en 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 offs
18、et 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.Figure 2.
19、5Figure 2.5 This 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 E
20、CG signals or as a general-purpose high-impedance ac-coupled transducer amplifier.BOOTSTRAPPED AC-COUPLED BIOPOTENTIAL AMPLIFIER Direct 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
21、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
22、 amplifier stage degrades the CMR performance of the biopotential amplifier.Figure 2.6Figure 2.6 This 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 ICIA
23、s 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.PASSIVE FILTE
24、RS The 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 f
25、requencies.Figure 2.7Figure 2.7 In 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 tw
26、o 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 SW
27、2.Figure 2.8Figure 2.8 The 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 selec
28、table 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.Figure 2.9Figure 2.9 Two notch filters are used to reduce
29、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.Figure 2.10Figure 2.10 This 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.
