Abstract Of ECG circuit by:
1- Abdallah Ishbeata.
2- Mohammad Kalbouneh.
- The aim of the project is to design ECG circuit that pick-up the signal from the heart using electrodes actually we simulate the circuit in simulation software to sure that the circuit can work practically, and we design circuit that detect any failure in the electrodes connection.
- The main challenges include amplifying the desired weak signal in the presence of noise from other muscles and electrical source so there is requirement needed in amplifier circuit to avoid any problem may occur during the measurement I will mention it later.
We cannot find the words that can covey the depth of our feeling towards many people who helped us.
All what we can say thanks very much and we will always remember and be grateful for them. This work would have never come to be without the guidance of our supervisors and the support of our families.
We would like to express our deep thanks to DR. AMJED AL FAHOUM for Supervisions.
Last, we would like to express our appreciation and deep gratitude to our parents and friends for their support and encouragement.
1.1 Heart beat
Is the number of heartbeats per unit of time – typically expressed as beats per minute (bpm) – which can vary as the body’s need to absorb oxygen and excrete carbon dioxide changes, such as during exercise or sleep, The measurement of heart beat is used by medical professionals to assist in the diagnosis and tracking of medical conditions. It is also used by individuals, such as athletes, who are interested in monitoring their heart beat to gain maximum efficiency from their training.
1.2 Heart and Functions:
The heart is the organ responsible for pumping blood throughout the body. It is located in the middle of the thorax, slightly offset to the left and surrounded by the lungs. The heart is composed of four chambers; two atriums and two ventricles. The right atrium receives blood returning to the heart from the whole body. That blood passes through the right ventricle and is pumped to the lungs where it is oxygenated and goes back to the heart through the left atrium, then the blood passes through the left ventricle and is pumped again to be distributed to the entire body through the arteries.
This is a list of events that occur in the heart on each heart beat. Figure1 shows heart behavior and part of the generated signal.
Also known as QRS complex:
1. Atrium begins to depolarize.
2. Atrium depolarizes.
3. Ventricles begin to depolarize at apex. Atrium re polarizes.
4. Ventricles depolarize.
5. Ventricles begin to re polarize at apex.
6. Ventricles re polarize.
Figure 1- Electrical Activity of the Heart.
Figure 2 shows a typical heart signal. In this signal, the heart muscles generate different voltages. The P wave represents the atrium contraction. QRS complex and the T wave represents the ventricles actions. Each time that this signal is present, a heart beat is generated.
Figure 2.Typical heart signal.
ECG circuit design
The first stage of the ECG circuit include instrumentation amplifier it is the most important part in the circuit it should provide high gain to amplify the weak of ECG signal and be able immunity the noise (common mode signal ) and other signal in electromagnetic spectrum.
Noise from the environment will easily swamp the tiny pulse signal from the heart. The leads connecting the electrode to the amplifier will act like an antenna which will inadvertently receive unwanted radiated signals. Such signals are for example the 50Hz from power lines and emf’s from fluorescent lights will add a tiny sinusoidal wave which is generally quite difficult to filter away, but in our project we will not concern this type of noise (50Hz) since our range of signal is from 0.5-5 Hz. Noise and interference signals acquired in this type of system are caused by the electric installation. The signals from the heart are too small and it is necessary to amplify the signal and reduce the common-mode voltage on the system. Other aspects that generate noise are muscle contractions, respiration, and electromagnetic emissions from electronic components.
To address the issues above, the following measure will be taken:
A high gain instrumentation amplifier with a high Common Mode Rejection Ratio (CMRR) will be used to receive the desired signal.
A band pass filter will be implemented to remove the noise. Because most of the noise types discussed are of high frequency while the desired signal is relatively low.
Peak detection circuit to detect the failure in electrodes connection.
Oscillator to generate signal with frequency approximately 5okhz that pass through the instrumentation amplifier when the loss connection occurred.
Analog to digital circuit to processing the signal using computer techniques.
Figure 3: block diagram of the circuit
2.4 instrumentation amplifier (INA 128):
The signal acquisition is the first consideration when an HRM is implemented. But the signal is too small and contains a lot of added noise. As we said above the signal extracted from the heart has amplitude of approximately 0.5mV.
Since, it is necessary to amplify the signal and filter the noise, and then extract the QRS complex.
An instrumentation amplifier is usually the very first stage in an instrumentation system. This is because of the very small voltages usually received from the probes need to be amplified significantly to be proceeding stages.
We can summarize the reasons to use instrumentation amplifier:
1- Get differential signal.
2- High input impedance.
3- High CMRR.
Let us take some review about instrumentation amplifier:
Figure 4: general instrumentation op amp.
This intimidating circuit is constructed from a buffered differential amplifier stage with three new resistors linking the two buffer circuits together. Consider all resistors to be of equal value except for Rgain. The negative feedback of the upper-left op-amp causes the voltage at point 1 (top of Rgain) to be equal to V1. Likewise, the voltage at point 2 (bottom of Rgain) is held to a value equal to V2. This establishes a voltage drop across Rgain equal to the voltage difference between V1 and V2. That voltage drop causes a current through Rgain, and since the feedback loops of the two input op-amps draw no current, that same amount of current through Rgain must be going through the two “R” resistors above and below it. This produces a voltage drop between points 3 and 4 equal to:
The regular differential amplifier on the right-hand side of the circuit then takes this voltage drop between points 3 and 4, and amplifies it by a gain of 1 (assuming again that all “R” resistors are of equal value). Though this looks like a cumbersome way to build a differential amplifier, it has the distinct advantages of possessing extremely high input impedances on the V1 and V2 inputs (because they connect straight into the non inverting inputs of their respective op-amps), and adjustable gain that can be set by a single resistor. Manipulating the above formula a bit, we have a general expression for overall voltage gain in the instrumentation amplifier:
The INA128 and INA129 are low power, general purpose instrumentation amplifiers offering excellent accuracy. Their versatile 3-op amp design and small size make them ideal for a wide range of applications. Current-feedback input circuitry provides wide bandwidth.
Even at high gain (200 kHz at G = 100). A single external resistor sets any gain from 1 to10, 000. INA128 provides an industry standard gain.
Equation; INA129’s gain equation is compatible with the AD620. The INA128/INA129 is laser trimmed for very low offset voltage (50mV), drift (0.5mV/°C) and high common- mode rejection (120dB at G ³ 100). It operates with power supplies as low as ±2.25V, and quiescent.
Current is only 700mA—ideal for battery operated systems. Internal input protection can withstand up to±40V without damage. The INA128/INA129 is available in 8-pin plastic DIP, and SO-8 surface-mount packages, specified for the –40°C to +85°C temperature range. The INA128 is also available in dual configuration, the INA2128.
- LOW OFFSET VOLTAGE: 50mV max.
- LOW DRIFT: 0.5mV/°C max.
- LOW INPUT BIAS CURRENT: 5nA max.
- HIGH CMR: 120dB min.
- INPUTS PROTECTED TO ±40V.
- WIDE SUPPLY RANGE: ±2.25 to ±18V.
- LOW QUIESCENT CURRENT: 700mA.
- 8-PIN PLASTIC DIP, SO-8.
Figure 5: circuit design of INA 128.
From our design we choose RG= 1kΩ since the gain we get 51 v/v actually the gain not very large for reason that although the instrumentation amplifier has high common mode rejection ratio but the noise still effect to the output of the circuit according to this equation:
so this signal that depends on the frequency that will pass through instrumentation op amp.
Figure 6: voltage noise VS frequency.
2.6 filtering stage:
The required band width for ECG signal (0.5 hz- 30 Hz) for normal heart human so we chosen the bandwidth of the circuit near to this range, now if we choose the bandwidth (0.5-120Hz) notching filter required in design to remove 5ohz noise from power line grid .
Figure 7: band pass filter.
2.7 dc offset stage:
Actually we use dc offset because some component of (QRSTU) waves in negative portion so if we want to convert our signal to digital form we need to make dc offset or we can use bi polar analog to digital converter.
Figure 8: dc offset circuit.
Figure 9: ECG signal after apply dc offset.
3.1 Peak detector circuit:
Peak detectors are used when you have a rapidly changing AC input signal, and you want to obtain the peak voltage the signal reaches. Peak detectors are really simple to make – just a diode and a capacitor in their simplest form. In our design where I use a peak detector to hold on to a peak signal that generated from oscillator which is appear when there is failure connection at the input of instrumentation amplifier at the first stage, simply the peak detector circuit contains capacitor at the anode of diode terminal for charging process and second capacitor at cathode terminal for discharging process, the charging and discharging process depend on τ= RC .
The comparator compare between the voltage at second capacitor and threshold voltage to detect the peak of 50 kHz signal.
Figure 10: peak detector circuit.
3.2 testing the circuit in failure connection of electrodes:
According to the impotency of the circuit in medical application we need to detect any failure may occur during diagnostic, the most important problem that the doctor needed to know if there is any loss in electrode connection to solve it and take the best measurements from devices.
Figure 11: failure connection in right arm.
Figure 12: failure connection in right leg.
3.3 oscillator circuit:
Oscillator circuit is required to generate signal for current circuit to detect the failure in electrode connection the amplitude of generated signal depends on power supply of the circuit, the frequency of generated circuit can calculate it according to the values of resistors and capacitor in the circuit.
Note : We change the oscillator circuit at the simulation with this circuit at Figure 13 when we connected our circuit that we designed on the bread board .
Figure 13: oscillator circuit .
4.1 ECG signal simulator:
SPICE simulation, the language by which Multisim emulates circuit design behavior, does not run in real-time. This means that if a real-world signal is acquired by a Lab VIEW instrument it cannot be directly injected into simulation, since the measurements will be running at different rates (real time vs. simulated time).
In the growing world of Biomedical engineering, the need to be able to quickly design circuitry which interfaces to a human signal is a common design problem. An amplifier for example can be designed in a circuit simulation package, however the signals which will interface to that particular circuit will be simple simulation stimuli (such as a sine wave, square wave etc…). To truly test a biomedical amplifier, you need to be able to interface the design to a real signal.
Using this unique Lab VIEW instrument you are able to define a human electrocardiogram (ECG) signal, and have amplified it through a circuit created in Multisim.
Figure 14: ECG signal
4.2 steps of simulation:
This Lab VIEW instrument, built custom for NI Multisim produces a raw electrode ECG waveform. Since this instrument is built in Lab VIEW we have a number of analysis, and signal altering functions available to us. In this case we are able to add common mode voltage, as well as noise components to alter the signal to improve the test and validation of our circuit.
Download the archived file to the desktop from Here
1- Close NI Multisim if open.
2- Open the attached 5925_ECG Amplifier.zip file.
3- Select My Computer.
4- Browse to C:\Program Files\National Instruments\Circuit Design Suite 10.1\lvinstruments.
5- Save the ECG Electrode.llb file from the zip file to this folder.
4.3 using the instrument ECG signal:
This instrument will use the example circuit that is in the attached 5925_ECG Amplifier.zip file folder.
1- Open Multisim by selecting Start > All Programs > National Instruments > Circuit Design Suite 10.1 > Multisim 10.1.
2- Go to File > Open.
3- Browse to the ECG Amp with ECG Signals.ms10 file from the 5925_ECG Amplifier.zip file.
4- You will see a complete amplifier based upon two common Op-Amps.
5- In Multisim select the Lab VIEW icon in the instrument toolbar (as seen below).
6- Select ECG Signals. This is the name of the ECG Electrode instrument (as seen below).
Figure 15: using ECG signal.
Sheet 1: over all circuit.
Part 1: ECG circuit and failure detection.
Sheet 3: 3D view.
2- Wikipedia (ecg circuit )
3- All datasheet site download (www. Alldatasheet.com)
4- JOGN G.WEBSTER ,”Medical Instrumentation, Application And Design”.
6- Jessica ambourn, “Portable ECG Logger”, October 2003.
7- Leece Sofoklis Nikiforos,” Heart Rate Monitor and Data Acquisition System”
8- http://www.google.jo/search?sourceid=chrome&ie=UTF-8&q=ECG+ Instrumentation .
Download this file ECG Circuit Analysis and Design
You can find the circuit , documentation and 5925_ecg_amplifier.
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