Genel amaçlı biopotansiyel kuvvetlendirici

Abstract

insan vücudunun çalışması sırasında ortaya çıkan biyolo jik elektrik potansiyeller çeşitli dönüştürücüler vasıtasıyla elektriksel büyüklüklere dönüştürülebilir. işte bu çalışma - mızda yüzey elektrotları vasıtasıyla elde edilen EKG işaret - lerinin işlenmesi üzerinde durulmuştur. Biyolojik işaretle - lerin elde edilip işlenmesinde en önemli unsur insan güvenli ğidir, ölçüm devrelerinden gelebilecek tüm elektriksel etki ler insan devresinden yalıtılmalıdır. Bu amaçla kullandığımız 284J izolasyon kuvvetlendiricisi çok yüksek bir yalıtım sağlamaktadır. Ayrıca elektrotların ve sistemin kaçak kapasitif etkilerinden meydana gelebilecek bo zucu etkileşimleri en aza indirmektedir. Analog Ölçüm devre sinin ekranlanması sonucu işaretin bozulması engellenmiştir. Devrede kullanılan filtreler EKG işaretinin frekans ara - lığında çalışan alçak geçiren filtreler ile 50 Hz. girişimi - ni bastıran çentik filtredir. Filtrelerde ve giriş katındaki enstrumantasyon kuvvetlendiricide düşük güç harcamalı, yüksek giriş dirençli TL 064 işlemsel kuvvetlendiricisi kullanılmış tır. Kullanılan H8 mikro işlemcili kontrol kartı analog EKG işaretinin analog-sayısal çevriminden sonra bilgisayar orta - mına transferini sağlarken dakikadaki kalp atım sayışımda göstermektedir. işlemci; üzerinde bulunan EPROM'a kaydedilen program vasıtasıyla çevrim, hesaplama, test işlemlerini yapa bilmektedir. Bilgisayar ortamında çalıştırılan bir program bilgisayarın paralel iskelesinden transfer edilen sayısal bilginin bilgisayar monitöründen EKG eğrisi olarak takibine olanak sağlar. Çalışma içerisinde daha önceden gerçekleştirilen benzer sistemlerede yer verilmiş ve EKG işaretin oluşumu hakkında da açıklamalarda bulunulmuştur.

Electrical and electronic devices used in the medical field to obtain information abaut the physological parameters of a patient are generaly known as electronic medical equip - ment or electromedical equipment. It is very important to be conversant with the most frequent concepts of physiology such as heart fuction, blood pressure, ECG and all the problems concerning their measurement. The human body can be classified in three sections: Head, limbs and trunk contains the principal organs such as lungs, kidneys, liver, heart. Most measuring data for electromedi - cal equipment is therefore derived from the heart functions such as pulsation, as ECG arterial and venous blood pressure. The heart is of aproximately the same size as a fist. It is vaguely cone-shaped with its point directed to the left from the position in the middle of the chest. The heart may be compared to a double-action pump as its function is to assure appropriate blood circulation throught organs as well as the lungs. The heart beat is not transmited from the auricles to the ventricles directly by cardiac muscle because of a ring of fibrous tissue separeting the auricles from the ventricles. The wave of excitation spreading through the heart wall is accompanied by electrical changes. Active cardiamuscle is electrically negative relative to resting cardiac muscle ahead of the one of excitation. The electrical currents produced are conducted to the surface of the body and can be picked up amplified and processing by a special instrument: The Electrocardiograph (ECG). The excitation which starts at the SA node is represented on the ECG recording as the P wave (the contraction of the auricles starts a fraction of a sec. after the P wave begins) which lasts for 0.06-0.11 sees. The QRS wave is caused by the spread of excitation through the muscle of the ventricles which lasts for a similar period to the P wave. The actual contraction of muscle of ventricles starts a fraction of a second after Q-R begins. The T wave is indicates restitution or repolarization of the ventricles. Potential differences are determined by placing electrodes on the surface of the body and measuring voltage between them, being careful to draw little current. If two electrodes are located on diffe rent equal-potantiel lines of the electrical field of the heart, a nonzero potantiel difference measurement. Different pairs of electrodes at different results due the spatial dependence of the electric field of the heart. Thus it is important to have certain standart positions for clinical evaluation of the ECG. The limbs make fine guideposts for locating the ECG electrodes. Generaly three types connection methods use in ECG recording: 1- Einthoven method is also referred to as the standart derivation of measurement whereby the potantiel between the right and left arm. Low noise and low gain floating intru - 2- Goldberger method: The difference between this method and the former one lies in that voltage measured between one point and the average of other two points. 3- Wilson method: This method the heart's potantiel is measured at different points set abaut a fixed point on the chest, the fixed point being set by interconnecting the three Einthoven connections R, L, and F. The primary function of the ECG acquisition system is to amplify the electrical signal from the heart and reject envi- romental and biological noise and artifacts. A differantial amplifier is commonly employed for that reason. The electri cal signal from the heart is considered a differantial signal and is amplified. The power-line interference picked up the body is considered a common mode signal and is rejected. Heart signals can be masked by muscle noise, electroche mical noise, residual electrode voltages and 50 Hz. power line pickup. To achieve high performance monitoring, model 284J isolation amplifiers design provides high CMRR in the dc to 100 Hz. bandwidth. Instrumentation amplifier which has many additional desirable charecteristics. The instrumenta tion amplifier design is now commonly used in many biomedical applications. The final amplifier stage limits the amplifier response to the desired frequency range and supplies the amplified signals to the monitoring instruments for display, recording or digizition by a computer based data acquisition system. The ECG processing system must be suitable for use on patients in hazardous enviroments such as cardiac intensive care. Consequently, the amplifier is designed to limit the risk current flowing into the patient. This is achieved by cureful design of the front end of the amplifier and isola - tion of the circuit from the leakage paths to the ground. VI Diff. Amplifier - >- Modulator Demodulator 7f Filter - > Isolated Power Supply Input Gain voltage ad j. Demodulat or< ' Modulator Vo Ground Vi Figure-1 Isolation amplifier Another important consideration is protection of the device electronics. Insettings such as cardiac intensive care or surgery, def ibrilators may be used while the ECG monitor is connected to the patient. Additionaly, many ECG acquisation systems feature circuits for dedection of lead failure, rejection of the pacemaker spikes and restoration of the baseline when the amplifier output drifts into satura tion. Our design consider analog processing circuit, digital processing unit with power supply and finaly IBM AT compatib le computer unit. Input signals are taken from the electro - des. Electrodes are used to leads to the body surface. As skin resistance varies from patient to patient, a conductive and absorptive paste is applied to the skin to reduce its resistance. Low noise and low gain floating instrumentation amplifi er used in the input port. Instrumentation amplifier has a very high input resistance and high CMRR (80-90 dB). This circuit protects the patient from dangerous currents that could be genarated in electrocardiograph. The JFET input operational amplifiers of the TL064 series are designed as low power versions of the Texas low power ampplifier series. They feature high input impadence, wide bandwith, high slew rate (3.5 V/us) and low input offset and bias currents. vix Low pass filter eliminate high frequency and than apply isolation amplifier and QRS dedection circuit. When the com mon voltage exceeds the power supply voltages of the instru - mantation amplifier, the source ground must be isolated from the amplifier ground. Isolation amplifiers are used to ac - hieve this. Isolated power is supplied to the floating input stage. This is achieved by the transformer coupling of ac power from the non isolated part, with no conductive connec - tion between the parts. High CMRR(80-90 dB), high input re - sistance (50 kMohm), ± 8 Volts isoleted power supply given by model 284J isolation amplifiers. Non isolated parts consider a notch filter and referance calibration amplifier. Notch filter eliminating 50 Hz. interference. Instrumantation Amplifier Electrodes PC JC L ->- Filter Lowpass Isolation - >A Amplifier-284J i I I j H8/532 A/D Converter I/O Port "1 CPU J Figure-2 General purposes biomedical signal amplifying H8 microcomputer chips supplied by Hitachi. This pro - cessor an internal 16 bit architecture and 16 bit acccess to on chip memory enhance the CPU's data processing capability and provide the speed needed for realtime control applicati ons. The on chip supporting functions include RAM, ROM, a se rial communication interface, timers, A/D conversion, and I/O ports. Analog ECG signals directly connecting H8/532 A/D converter port and than this signal converting to digital signal. Internal program transfering data with I/O ports to the computer. Processor card consider three digits display, chips devices, communication circuits, analag input circuit and power suply circuit for measurement circuit and processor card. Power supply made by 78XX and 79XX voltage regulator ıc chips. Analog input circuit a little programmable output gain instrumantation amplifier. CA3193 used an active attennuator in the output amplifier's feedback loop. The active attenua tion presents a very low impedance to the feedback resistors therfore minimizing the CMRR degration. vm Internal program store in H8 microcomputer chip. Chip is contain EPROM. Program caunting and calculating EKG pul - ses. Displays showing EKG pulses numbers instantaneous and per minute totals. When the processor reseting program start a test. If test complated, program wait a start command con verting and processing analog electrocardiography signal. Computer program consist of a curve drawing program and communication protocol program. This programs making with "Turbo C" programing language. System tests are making with ODAM ECG similator devices. This similator supply spikes and 60-120 puls/min. EKG simulation signals.