ECG-Amplifier - TUM

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ECG-AmplifierMB Jass 2009Daniel Paulus / Thomas MeierOperation amplifier (op-amp)Properties DC-coupledg gagain eelectronicec o c voltageo age aamplifierp eHighInverting / non-inverting input and single outputOutput of the op-amp is usually controlled by negative feedbackDifferential amplifier (just the difference between V and V- isamplified)Circuit diagram symbol V V VoutVS VS-: non-inverting input: inverting input: output: positive power supply: negative power supply1

Ideal op-ampProperties for an ideal op-amp Zero output impedance (Rout 0)I fi i open-loopInfinitelgainiInfinite input impedance (Rin )Zero offset voltageZero noiseCommon mode gain Ideal op-amps amplify only the voltage difference in its inputs Real op-amps amplify also voltage that is common to both inputs(common mode gain) Minimizing this common mode gain (i.e. maximizing the commonmode rejection ratio, ’CMRR’) is important for most applications2

Non-inverting Amplifier Very high input impedance Small output impedance Gain: Ua (1 R2)UeR1 Amplifies Ue regarding to theground and not the differencebetween two different voltagesInstrumentation amplifier Type of differential amplifier (usually 3 op-amps) Input buffers Output controlled by negative feedbackCharacteristics Very low DC offsetLow driftLow signal noiseVeryy highg open-loopppggainVery high common-mode rejection ratioVery high input impedance3

Description of the INA 118INA118: Low power, general purpose instrumentation amplifieroffering excellent accuracy 3 op-amps (small size) Current-feedback input provides high gain even at highfrequenciesINA118Electrical Features Low offset voltage (50 µV)LLowtemperature ddriftif (0(0.5 µV/K)V/K)High common-mode-rejection (110dB) at high gainLow quiescent current (350 µA at 1.35V supply) Single external resistor sets any gain from 1 to 10,000 Internal input protection can withstand up to 40V withoutdamage4

Instrumentation amplifier INA118 Circuit of the Instrumentation amplifier used in our ECG-circuit Gain: G 1 50k RGPerformance curves (INA118)Gain of the amplifier plottedagainst the frequencyCCommon-moded rejectionj ti as afunction of the frequency5

ElectrocardiogramProperties of the electric signals on the skin: ACC ssignalg a with babandwidthd d ofo 00.0505 too 10000 Hz,, sosometimese es up too 1 kHz About 1 mV peak-to-peak amplitude External noise at higher frequencies 50/60 Hz interference Common-mode voltages (common to all electrodes)Circuit schematic of an ECGPC6

ECGThe common-mode voltage is comprised of two parts:1)2)50- or 60-Hz interferencesDC electrodel t d offsetff t potentialt ti lOther noise or higher frequencies within the biophysical bandwidthcome from: Movement artifacts that change the skin-electrode interfaceMuscle contraction or electromyographic spikesRespirationElectromagnetic interferencesNoise from other electronic devices that couple into the inputECG Some noise can be cancelled with a high-input-impedanceinstrumentation amplifier (INA) removes the AC line noise common to both inputs amplifies the remaining unequal signals present on the inputsLet’s detect an ECG with ourconstruction7

FilterDevice for manipulating the frequency content of a given signalFourierInverse FourierPassive analog filter: RC networkUinUoutideal1 kΩ8 μFrealrealidealImpedance of capacitor: Z 1/(ωC) - fco (RC)-18

Active analog filterreal Complex circuit Once it’s set up, it’s hardto changeidealFourier Transform Direct manipulation of frequency content in the frequency domain issuitable for post processing only! (Whole signal has to be stored inmemory) Possible solution for in-stream processing: windowingProblems: High computational cost Frequency spectrum limited bywindow sizeManipulation in Fourier space9

ConvolutionPossibility: Operating in time domain Bring transfer function to time domain by inverse Fourier Transform(Impulse response) Cut function to realiseconvolution of impulseresponse & signal as sum Y (t ) T ( ) X (t )d Implementation: FIR Simple implementation:convolution implemented as sum h is the filter kernel of length qqY ( z ) h ( m) X ( z m)m 0Major drawback:very long filters necessary to achieve goodperformance10

Implementation: IIRRecursive implementation:both input and responseare consideredY ( z) q 1 p d i X ( z i ) g jY ( z j ) a0 i 0j 0 Advantage:Less coefficients required incomparison to FIR filters toget same perfomanceDigital Filter: Transfer functionrealideal11

Advantages/DisadvantagesAdvantagesDisadvantages Digital filters can realise FIR filters need to be large tofrequency responsesbe effective (many operations)practically not achievable with IIR filters may be unstableanalog filtersbecause of feed back loop No tolerance of circuit The ADC stage createscomponentsdeterministic quantisation error No complex circuitry (just awhich is due to digital storagenumber of coefficients needed)and computation limitations samplel ratet limitationli it ti ((analoglfilters do not have to sample!) Filter design is not a simpletask12

ECG-Amplifier MB Jass 2009 Daniel Paulus / Thomas Meier Operation amplifier (op-amp) Properties DC-coupled Higg ga eec o c o agea p eh gain electronic voltage amplifier Inverting / non-inverting input and single output Output of the op-amp is usually controlled by negative feedback

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