Medical Applications Guide (Rev. B - Mouser Electronics

More documents

Recommendations

Info

20 Diagnostic, Patient Monitoring and Therapy➔ECG/Portable ECG and EEG➔ In This SectionECG/Portable ECG and EEG 20Component Recommendations for ECG/Portable ECG and EEG 25Pulse Oximetry 27Component Recommendations for Pulse Oximeter Applications 31Ventilation 33Component Recommendations for Ventilation Applications 38Automatic External Defibrillator (AED) 40Component Recommendations for AED Applications 45Power Management for Diagnostics 43Biophysical Monitoring OverviewThe human medical data acquisition system, in particular the patientmonitoring system, presents the challenge to designers of measuringvery small electrical signals in the presence of much larger commonmodevoltages and noise. Front-end amplifiers perform the essentialconditioning that complements downstream digital processing, which inturn refines the measurement and communicates with other systems.Biophysical measurements include electrical and mechanical signalsfor general monitoring, diagnostic and scientific purposes both inclinic and non-clinic environments. Successfully meeting the signalacquisition challenge requires system designers to have knowledgeof the signal source, good design practice and ICs with appropriatecharacteristics, features and performance.Signal Acquisition ChallengesThe action potential created by heart wall contraction spreads electricalcurrents from the heart throughout the body. The spreading electricalcurrents create different potentials at different points on the body,which can be sensed by electrodes on the skin surface using biologicaltransducers made of metals and salts. This electrical potential is an ACsignal with bandwidth of 0.05Hz to 100Hz, sometimes up to 1kHz. Itis generally around 1mV peak-to-peak in the presence of much largerexternal high frequency noise plus 50/60Hz interference normal-mode(mixed with the electrode signal) and common-mode voltages (commonto all electrode signals).The common-mode is comprised of two parts: 50 or 60Hz interferenceand DC electrode offset potential. Other noise or higher frequencieswithin the biophysical bandwidth come from movement artifacts thatchange the skin-electrode interface, muscle contraction or electromyographicspikes, respiration (which may be rhythmic or sporadic),electromagnetic interference (EMI), and noise from other electroniccomponents that couple into the input. Some of the noise can be cancelledwith a high-input-impedance instrumentation amplifier (INA),like the INA326 or INA118, which removes the AC line noise commonto both inputs and amplifies the remaining unequal signals present onthe inputs; higher INA common-mode rejection (CMR) will result ingreater rejection. Because they originate at different points on thebody, the left-arm and right-arm ECG signals are at different voltagelevels and are amplified by the INA. To further reject 50 and 60Hznoise, an operational amplifier deriving common-mode voltage is usedto invert the common-mode signal and drive it back into the patientthrough the right leg using amplifier A2. Only a few microamps or lessare required to achieve significant CMR improvement and stay withinthe UL544 limit.V IN–(Right Arm)Inverted Common-ModeVoltage (Right Leg Drive)V IN = V IN+ – V IN–V IN+(Left Arm)390 kΩ= 0.15 to 2mV PP390kΩ390kΩ40kΩ40kΩA1It is Possible to OmitA1, Depending onResistances.= (V IN+ + V IN– )/2OPA23351/2+39pF390kΩA2+OPA23352/2++5VG = 5INA32620kΩ750pF200kΩ0.0015µFMedical Applications Guide www.ti.com/medicalTexas Instruments 2Q 20071µFA4+OPA23352/2Integrator(High-Pass Filterin the ECG Path)G = 2005kΩ3.2MΩ1 MΩA3++2.5V PedestalVoltagePower Supply+V S –V SOPA23351/2+5VREF3125100Ω0.1µF+5V+A5ADS8321OPA335+5V100ΩAnalog Circuit Common(Isolated from Earth Ground)Three ECG electrodes connected to patient using CMOS components with 5V single supply. This circuit will operate on a 3.3V supply.To TMS320 DSPor MSP430 MCUV REF = 2.5V10µF0.1µF
Diagnostic, Patient Monitoring and TherapyECG/Portable ECG and EEG21➔Supply VoltageAs in most other applications, the system supply voltage in biophysicalmonitoring continues the trend toward low, single-supply levels. Whilebipolar supplies are still used, 5V systems are now common and trendingto single 3.3V supplies. This trend presents a significant challengefor the designer faced with at least a 300mV DC electrode potentialand emphasizes the need for a precision signal-conditioning solution.While the following discussion concentrates on the single supplydesign, the principles involved apply to bipolar designs as well. A listof recommended single and bipolar supply components can be foundbelow.Frequency ResponseStandard –3dB frequency bandwidth for patient monitoring is 0.05Hzto 30Hz, while diagnostic grade monitoring requires 0.05Hz to 100Hz ormore. The analog front end must be AC coupled to remove artifactsfrom the electrode offset potential.Electrode PotentialBecause electrode potential can in practice reach ±500mV, eliminatingthe effects of electrode potential by AC coupling is essential. ADCrestorator amplifier in a feedback configuration nulls out the DC offset.If the left arm DC offset is +300mV and the right arm electrode is 0VDC, the differential input voltage is 300mV. Because theinstrumentation amp has a gain of 5, 1.5V appears at the outputof the instrumentation amp. With a gain of 50 or more, the outputamplifier would try to drive the signal up to 75V but never doesbecause a feedback integrator applies an equal negative voltage tothe reference point. Using this linear summing effect, the electrodeoffset is cancelled. The result of this DC restorator is to turn theoriginal DC-coupled amplifier into an AC-coupled amplifier. With theDC normal-mode voltage removed, the output stage can amplify theAC ECG signal without becoming saturated.Instrumentation Amplifier Requirements• Stability in low gain (Gain = 1 to 10)• High common-mode rejection• Low input bias current (I B )• Good swing to the output rail• Very low offset and driftOperational Amplifier Requirements• Low noise in high gain (Gain = 10 to 1000)• Rail-to-rail output• Very low offset and drift90 - 264 V AC12-24V 5-10VAC/DCStep Down3kV 1W ISO DC/DCLDOV CC Digital3kV 1W ISO DC/DCLDOV CC AnalogTPS54x0 DCH01xx TPS799xxTPS795xxREF32xxREF31xxREFTMS320F28xxCH1LPFLPFG=10HPFG=10LPFRCADCINA128INA118INA121INA326INA122INA126TLC2254OPA4348TLV2434OPA434xTLV2264ADS834xADS1258DSPDigitalµC SignalIFIsolationMSP430FxxISO72XSN65HVDxxCH32/64LPFLPFLPFG=10 HPF G=10 RC ADCECG/EEG block diagram.Texas Instruments 2Q 2007Medical Applications Guide
Page 1 and 2: TMTechnology for InnovatorsMedical
Page 3 and 4: Consumer and Portable Medical Devic
Page 6: 6➔Consumer and Portable Medical D
Page 13: Consumer and Portable Medical Devic
Page 16 and 17: 16➔Consumer and Portable Medical
Page 18 and 19: 18➔Consumer and Portable Medical
Page 22 and 23: 22➔Diagnostic, Patient Monitoring
Page 48 and 49: 48➔Medical ImagingUltrasound/Port
Page 54 and 55: 54➔Medical ImagingComponent Recom
Page 56 and 57: 56➔Medical ImagingCT ScannersDual
Page 60 and 61: 60➔Medical ImagingMagnetic Resona
Page 62 and 63: 62➔Medical ImagingMagnetic Resona
Page 66 and 67: 66➔Medical ImagingDigital X-Raybl
Page 68 and 69: 68➔Medical ImagingDigital X-RayLo
Page 70 and 71:
70➔Medical ImagingPET ScannersPET
Page 72 and 73:
72➔Medical ImagingComponent Recom
Page 74 and 75:
74➔Medical ImagingPower Managemen
Page 76 and 77:
76 Medical Instruments➔Medical In
Page 78 and 79:
78➔Medical InstrumentsMedical Ins
Page 80 and 81:
80 Resources➔Enhanced ProductTexa
Page 82 and 83:
82➔ResourcesTI Design ToolsBelow
Page 84:
TMTechnology for InnovatorsFindProd
show all

Medical Applications Guide (Rev. B - Mouser Electronics

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?