An electro cardio gram (ECG) is one of the simplest and fastest tests used to evaluate the heart. Electrodes (small, plastic patches that stick to the skin) are placed at certain spots on the chest, arms, and legs. The electrodes are connected to an ECG machine by lead wires. The electrical activity of the heart is then measured, interpreted, and printed out. No electricity is sent into the body.

A typical flexible biopotential electrode for ECG (electrocardiogram, ECG) recording is composed of certain polymers or elastomers which are made electrically conductive by the addition of a fine carbon or metal powder. The most common type of biopotential electrode is the silver/silver chloride electrode (Ag/AgCl), which is formed by electrochemically depositing a very thin layer silver chloride onto the surface of silver electrode. These electrodes are recessed and imbedded in foam that has been soaked with an electrolyte paste to provide good electrical contact with the skin. The electrolyte saturated foam is also known to reduce motion artifacts which are produced during stress testing when the layer of the skin moves relative to the surface of the Ag/AgCl electrode. This motion leads to the large interference in the recorded biopotential and, in the extreme cases, could severely degrade the measurement.



ECG Machine Block Diagram:

 Electrocardiography is the process of producing an electrocardiogram, a recording of the heart's electrical activity. It is an electro gram of the heart which is a graph of voltage versus time of the electrical activity of the heart using electrodes placed on the skin.
The main blocks of an ECG machine and the function of each block are explained below.



1. Electrodes:

 These are used for the pickup of ECG signals by placing these electrodes at appropriate parts of body.

2. Lead selector:  

Each pair of lead conveys certain information. So for the appropriate waveform or view we have to select an appropriate lead pair. The lead pair can be selected by a lead selector switch which can be switched to different lead pairs according to the type of waveform needed.

3. Pre-amplifier: 

The ECG signal is having very weak amplitude levels. So it is necessary that for proper analysis and plotting purpose, the waveform is to be amplified. The pre-amplifier used here will be an operational amplifier or instrumentation amplifier with high gain. They have High CMRR and extremely high input impedance.

4. Driver: 

 We use a driver motor of suitable specification to drive the paper roller. Normally the ECG waveform is to be plotted on a moving chart paper to find out the irregularities (if there is ) in the P,Q,R,S,T and U regions of the ECG waveform .So the paper movement and the moving speed can be controlled by the driver motor, which supplies the trigger the roller.

5. PMMC Galvanometer: 

PMMC (Permanent Magnet Moving Coil) galvanometer is a special type of device, where the deflection of the coil depends on the amplitude and the polarity of the signal applied to its input. The writing tip of the pen is connected to the chart paper. So the pen will be at rest in the centre of its travel when no current flows in the coil. So the direction of deflection in the coil and the amount of deflections is determined by the amplitude and polarity of the ECG waveform.

6. Hot - tip stylus and stylus heater:  

In most common ECG recording techniques, we use hot tip stylus for thermal writing. It is because; we normally use thermal recorders for the plotting of waveforms. The stylus is kept hot always by the stylus heater power supply. 

7. Recorders: 

As discussed now, usually we use thermal recorders for the representation of ECG waveform. The paper used in thermal recorders is of special material which turns black when heated. The hot tip of the stylus will turn the white paper black whenever it touches. The tip of the stylus moves in accordance with the movement of the coil which in turn is proportional to the amplitude and polarity of the ECG waveform. Since the tip of the stylus is in contact with the thermal chart recorder, a clear representation of the ECG waveform is obtained.

Applications:

• It indicates the rate and rhythm or pattern of contraction of heart
• It gives a clue about the condition of heart muscle and is used to diagnose heart disorders
• An ECG also helps to determine the location and amount of injury caused by heart attack and later helps to assess the extent of recovery



EEG

EEG is a method to record brain activity throw the capture of electric activation.
Different types of EEG electrodes: passive, active, dry and sponge (R-NET).
Passive electrodes are usually made of silver/silver chloride (Ag–AgCl) and many systems typically use electrodes attached to individual wires. These electrodes are applied to the scalp using a conductive gel or paste

The actiCAP active electrode system makes it easier to reduce the impedance of the electrodes. This system also substantially reduces the time taken to prepare test subjects up to the start of the EEG recording. The sensors consist of high-quality Ag/AgCl (sinter) and are perfectly suited for DC acquisition.
The R-Net electrode system consists of sponges and passive Ag/AgCl electrodes that are held in place with a durable and flexible net. Due to the gel-free application it allows for rapid preparation and high recording flexibility, which is advantageous e.g. in time-restricted EEG recording situations.


EEG Machine Block Diagram:

 An electroencephalogram (EEG) is a test that detects electrical activity in your brain using small metal discs (electrodes) attached to your scalp. Your brain cells communicate via electrical impulses and are active all the time, even when you're asleep. This activity shows up as wavy lines on an EEG recording.
The function of each block in the system is described below.



1. Montage selector: 

The EEG signals are transmitted from the electrodes to the montage selector panel. The montage selector of an EEG machine is a large frame which consists of different switches so as to allow the user to select the desired electrode pair.

2. Pre-amplifier 

As the EEG signals are having amplitude levels in microvolt range it is compulsory that they are to be amplified before further processing. We normally use high gain, high CMRR operational amplifiers as preamplifiers due to its versatile features.

3. Filters and amplifiers 

The muscle artifacts (noise) are a major problem regarding the EEG waveform. These noises can make the representation disturbed. So we have to filter out these noise contents. This function is done by a bank of filters in the EEG machine systems, which are selected according to the need. Amplifiers are used here also to improve the amplitude levels of EEG waveform.

4. Analog to Digital Converters (ADC) 

For the detailed analysis of the EEG waveform, we use computers and oscilloscopes. As the computers only accept digital data we have to convert the analog EEG information in to digital form. The function of ADC is to convert the analog EEG signal to digital form. Thus the computer can store the EEG waveform for future reference.

5. Writing recorder and paper drive 

The writing part of an EEG machine is usually consists of an ink type direct writing recorder. The recorder will be a chart paper which is driven by a synchronous motor. For the clear representation of the EEG waveform an accurate and stable paper drive mechanism is provided by the synchronous motor.  Also there are provisions to control the paper speed.