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Sunday, 16 July 2017

Isolation Amplifiers

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Isolation amplifiers are used to provide electrical isolation and an electric safety barrier to the patients during measurements. Some patients are highly susceptible to electrical shock hazards. Under certain conditions, these shock hazards can cause very severe effects to the patient. So it is required to protect the patient. The isolation amplifiers can provide very high insulation in the range of about a lack mega ohms! There exist a common mode voltage which are the potential difference between instrument ground and signal ground during measurement. The data acquisition components need to be protected from this common mode voltages because as the magnitude of it increases, the chance of instrument destruction increases. Even if the magnitude of the common mode voltage is low, there is a possibility of noisy representation of the signal under investigation.

Isolation amplifiers are very useful when we need to amplify low level signals in multichannel applications. Also they eliminate measurement errors caused by ground loops.

There are two broad classifications of isolation amplifiers. Some isolation amplifiers provide input-to-output isolation without channel-to-channel isolation. It offers only one isolation barrier for a multichannel instrument. Some other type of isolation amplifiers provide both input-to-output isolation and channel-to-channel isolation.

There are two isolation amplifier specifications.
1. The amplifier isolation breakdown voltage: It is defined as the absolute maximum common mode voltage that the isolation amplifier can handle without damage.
2. The Amplifier CMRR: It is defined as the degree to which the common mode voltage will disrupt the normal mode component measurement.

In addition to these, the frequency of the common mode voltage is also important. Higher frequency common mode voltages can create difficulty for many isolation amplifiers due to the effect of parasitic capacitance.

The basic block in figure which illustrate the working of an isolation amplifier is shown below.

Input amplifier amplifies the input  and the amplified output is applied to a modulatior which modulates the signal by using AM, PWM or any other technique. The isolation barrier is generally an energy converter where the electrical energy of the modulator is converted to some other form of energy (a non-electrical energy). Then the signal is modulated and finally amplified by the output amplifier.

The standard symbol of an isolation amplifier is shown in figure.

Isolation Amplifiers using Optical Isolation

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Normally we use optical isolation barrier in order to reduce the effect of EMI (Electromagnetic Interference) as the optical fibers are not susceptible to EMI.  In Isolation amplifier using Optical isolation, the transducer converts the physiological information into electrical form which is amplified by the isolation amplifier. The amplified output is applied to a voltage to frequency converter. It is done because after voltage to frequency conversion the signals will be in digital form which is ideal for optical transmission. The FOT (Fiber Optic Transmitter) transmits the signal through the optical fiber cable which can provide a high degree of isolation. At the receiving end, the signal is received by a FOR (Fiber Optic Receiver) which feeds the signal to FVC (Frequency to Voltage Converter) to get the original signal.

Bioelectric Amplifiers

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As the name implies, the bioelectric amplifiers are used to amplify the bioelectric signals. The bioelectric signals measured from various body parts are having an amplitude ranging from mVs to ┬ÁVs. So it is necessary to amplify the extremely low amplitude signals for the analysis of the biological data. It is for this purpose that the bioelectric amplifiers are used. Usually we use operational amplifiers as bioelectric amplifiers due to its high gain and other versatile features.

The bioelectric amplifiers have some properties.

1. The gain of the bioelectric amplifier may be low, medium or high depending on the type of amplifier and the signal to be amplified. For example the low gain amplifiers are used for the measurement of action potential, medium gain amplifiers are used for the amplification of ECG waveform and the high gain amplifiers are used for EEG signal amplification.

2. The bioelectric amplifiers may be ac coupled or dc coupled.

3. The frequency response of a bioelectric amplifier range from very low frequency to high frequency range.

4. Bioelectric amplifiers have differential input and single ended output.

5. High CMRR and extremely high input impedance.

Two important parameters of bioelectric amplifiers are noise and drift. We have to avoid the effect of both these parameters on bioelectric amplifiers. Drift is the change in output due to the change in temperature. Noise is the thermal noise generated in electronic devices. Both these problems can be avoided by proper design to make the bioelectric amplifier more effective.

LVDT - Advantages and Disadvantages

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As shown in figure, LVDT consists of a transformer with one primary winding and two secondary windings. Here the secondary windings are connected in such a way that their induced voltages oppose each other. An alternating current is driven through the primary, causing a voltage to be induced in each secondary proportional to its mutual inductance with the primary. An iron core is located in between the primary and secondary windings.If the core is at the central position, the voltages in the two secondary windings are equal and hence the output is cancelled. When the core is displaced in one direction, the voltage in one coin increase as the other decreases causing the output voltage to increase from zero to maximum. This voltage is in phase with the primary voltage. When the core moves in the other direction, the output voltage also increases from zero to maximum, but its phase is opposite to that of primary. Thus the magnitude of the output voltage will be proportional to the distance moved by the core and it is why the device is called linear. The phase of the voltage indicates the direction of the displacement.

Advantages of LVDTs

1. The sliding core does not touch inside the tube and hence it can  move without friction.
2. As friction is avoided, the output will be accurate in almost all cases.
3. There is no sliding or rotating contacts which improves the efficiency.
4. Very high resolution
5. Simple in construction
6. Easy to align and maintain
7. Light in weight

Disadvantages of LVDTs

1. Large displacements are required for differential output
2. Sensitive to stray magnetic field
3. It is sensitive to temperature variations.