bio-amplifiers-basics.pptx

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1. B I O -AMPLIFIERS

2. What is Amplifiers? An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the power of a signal ( a time-varying voltage or current). Biopotential Amplifiers also called Bio-Amplifiers are specifically designed for processing of Bio-electric signals as they are low in amplitude.

3. W H A T I S B I O - A M P L I F I E R S ? I t i s a n e l ec t r o p h y s i o l o g i c a l d e v i ce , a v a r i a t i o n o f the instrumentation amplifier, used to gather and increase the signal integrity of physiologic electrical activity for output to various sources. It may be an independent unit, or integrated into the electrodes.

4. What is Bio-potential Amplifier? Amplifiers are an integral part of Electronic devices and modern Instrumentation for measuring Bio-potentials. As the name indicates, Amplifiers are used to increase the signal strength while maintaining high fidelity. The measurements include voltages that are at low levels and high source impedance.

5. Introduction Bio-electric signals are generally low in amplitude. Amplifiers that are specifically designed for processing this type of Bio-potentials are known as Biopotential Amplifiers. The outputs from such amplifiers are used for analysis and they show up as ECG, EMG or other Bio-electric waveforms. These amplifiers typically process voltages but in some cases they process current.

6. Introduction The amplifier provides high impedance, high CMRR and thereby minimizes loading effects. This is the vital functionality of Biopotential Amplifiers. For Biomedical applications, Bio-amplifiers must meet the below mentioned requirements in order to work incessantly.

7. Introduction To measure Biopotential , electrodes are placed on Human skin as shown in the Fig. 2. The signals from the Electrodes pass on to the Amplifier stage. Amplifier helps in minimizing, eliminating most of the signals interfering with the measurement of Bio-potentials and final readout is obtained.

8. Fig. 2 – Schematic Representation of Biopotential Measurement

9. Basic Requirements for Biopotential Amplifier High input impedance. Typically, they range between 2 MΩ to 10 MΩ. Greater the impedance value, lower the distortion of the signal. Every Bio-amplifier must contain Isolation and Protection circuits for safety purposes especially to prevent patients from macro and micro-electric shocks. Output impedance should be low to drive an external load with minimal distortion. Most Biopotential Amplifiers are differential.

10. Basic Requirements for Biopotential Amplifier Signals are recorded using bipolar electrodes that are symmetrically allocated. CMRR (Common Mode Rejection Ratio) must be high as they ride on a large offset signal and to reduce interference from common-mode signals. The gain must be calibrated for each measurement. An ideal Bio-amplifier must be free from noise and distortion. A constant gain must be maintained throughout the entire bandwidth range.

11. . T h e b i o l o g i c a l a m p l i f i e r s h o u l d h a v e a h i g h i n p u t impedance value. . The range of value l ies between 2 MΩ and 10 MΩ d e p e n d i n g o n t h e a pp l i c a t i o n s . H i g h e r i m p e d a n c e value reduces distortion of the signal. . When electrodes pick up biopotentials from the h u m a n b o d y , t h e i n p u t c i r c u i t s h o u l d b e p r o t e c t e d . 4 . Every bio- amplifier should consist of isolation and p r o t e c t i o n c i r c u i t s , t o p r e v e n t t h e p a t i e n t s from electrical shocks. BASIC REQUIREMENTS FOR BIOLOGICAL AMPLIFIERS

12. W H Y I S B I O A M P L I F I E R REQUIRED? Generally, biological/bioelectric signals have low amplitude and low frequency. Therefore, to increase the amplitude level of biosignals amplifiers are designed.

13. Types of Bio- A m p l i f i e r s 1 . D i ff e r e n t i a l A m p l i f i e r Operational Amplifier Instrumentation Amplifier Chopper Amplifier Isolation Amplifier

14. Mostly used bio-Amplifiers Instrumentation Amplifier Isolation Amplifier

15. Differential Amplifier These are used to amplify the difference between the voltages applied to its inputs. The circuits are of two types. Amplifiers built using Op-Amps. Amplifiers built using either FET’s (Field Effect Transistors) or BJT’s (Bipolar Junction Transistors).

16. Differential Amplifier Fig. 3 – Differential Amplifier Circuit

17. Operational Amplifier These are multistage amplifiers which are interconnected and occupies minimal  space even though it consists of many Transistors, Resistors, and FET’s. They are available in the form of an Integrated Circuit (IC).

18. Operational Amplifier Fig. 4 – Symbol of Operational Amplifier

19. Instrumentation Amplifier It consists of 3-amplifiers in the circuit. The input to the amplifier is the output from the Transducer. A non-inverting amplifier is connected to each of the input of the Differential Amplifier. Non-inverting amplifiers are the ones on the left side of the diagram. The non-inverting amplifiers together form the input of the instrumentation amplifier.

20. Instrumentation Amplifier Fig. 5 – Instrumentation Amplifier Circuit

21. Instrumentation Amplifier The third op-amp is called the difference amplifier and is the output of the instrumentation amplifier. The difference between the two input signals forms the output V out . V 1  and V 2  are the inputs to Op-amp 1 and Op-amp 2 respectively. A typical application of these amplifiers includes Biomedical applications such as Biopotential Amplifier. High gain and high impedance are attained using these amplifiers which are crucial in medical instruments to determine the health condition of an individual.

22. INSTRUMENTATION AMPLIFIER In biomedical applications, high gain and the high input impedance are attained with an instrumentation amplifier. Usually, a 3-amplifier setup forms the instrumentation amplifier circuit. The output from the transducer is given as input to the instrumentation amplifier. Before the signal goes to the next stage, a special amplifier is required with high CMRR, high input impedance and to avoid loading effects. Such a special amplifier is an instrumentation amplifier, which does all the required process.

23. INSTRUMENTATION AMPLIFIER

24. CHOPPER A M P L I F I E R

25. CHOPPER AMPLIFIER Noise and drift are the two major issues encountered when recording Bio-potentials. Noise is caused by the movement of the patient or due to the recording device. A DC Amplifier hits a sudden peak in the output when the input is zero. A Chopper Amplifier samples the problem of drift in DC amplifiers.

26. CHOPPER AMPLIFIER Fig. 6 – Chopper Amplifier Circuit

27. I S O L A T I O N A M P L I F I E R

28. ISOLATION AMPLIFIER These are also known as Pre-amplifier isolation circuits. It provides electrical isolation for the safety of the patient. It prevents accidental shocks and increases the input impedance of the patient’s monitoring system.

29. h o s p i t a l . 1 . I s o l a t i o n a m p l i f i e r s a r e k n o w n a s P r e - amplifier isolation circuits. An isolation amplifier increases the input impedance of a patient monitoring system. It also helps to isolate the patient from the device. Using the isolation amplifier prevents accidental internal cardiac shock. It provides up to 1012 Ω insulation between the patient and the power line in the ISOLATION AMPLIFIER

30. The electrical signals are o b t a i n e d w i t h e l e c t r o d e s . T h e s i g n a l s r e c e i v e d g o e s t o the amplifier block, where signals amplification occurs. A f t e r a m p l i f i c a t i o n , t h e s i g n a l enters the modulation block. When either i t goes to the i s o l a t i o n b a rr i e r , o pt i c a l c a b l e or transformer can be used. Isolation Amplifier

31. I f i n c a s e o f o pt i c a l c a b l e , m o d u l a t o r o u tp u t travels to LED. T h e L E D c o n v e r t s e l e c t r i c a l s i g n a l s i n t o l i g h t energy. If the transformer acts an isolation barrier, m o d u l a t o r o u tp u t c o nn e c t s t h e p r i m a r y w i n d i n g o f the transformer. Energy from primary transfers to the secondary w i n d i n g b a s e d o n t h e m u t u a l i n d u c t i o n p r i n c i p l e . At the next stage, secondary output enters the demodulation block. F i n a ll y , t h e a m p l i f i e d d e m o d u l a t e d s i g n a l i s obtained. Isolation Amplifier

32. Various stages which represents Biopotential Amplifier is shown in the Fig. 8. The Electrodes (Bipolar) are placed on the patients skin which provide transition between the ionic flow of currents in biological tissue and electronic flow of current in the Amplifier. The measurement of Bio-potentials is critical and due to relative movements of electrode and tissue, it gives rise to electrode offset potential and electrode/tissue impedance. Thus, two interference signals are generated which are successfully eliminated at later stages of the amplification. How does Biopotential Amplifier Work

33. The signal from the electrodes pass on to the pre-amplifier stage which helps in minimizing, eliminating most of the signals interfering with the measurement of Bio-potentials. High Pass Filter and Low Pass Filter eliminates interference signals like electrode Half-cell potentials and Pre-amplifier offset potentials. It also reduces noise amplitude. Bio-signal should not be distorted or attenuated and hence Filters are used. How does Biopotential Amplifier Work

34. Fig. 8 – Block Diagram of Stages of Biopotential Amplifier

35. In the Isolation Amplifier stage, galvanic decoupling of the patient from the measuring equipment is served. It prevents Galvanic currents from deteriorating Signal to Noise Ratio and provides safety to the patient from electrical hazards. Transformer, Optical or Capacitive Couplers are used in Analog Isolation Amplifiers, to transmit signal through the isolation barrier. On the other hand, Digital Isolation Amplifiers use Voltage and frequency converter to digitize the signal before it is transmitted. Recording of the Bio-potentials in the last stage is done with electrical systems which produce strong electrical and magnetic fields. Hence the system is capacitively coupled and the current flows to the ground electrode. How does Biopotential Amplifier Work

36. They are majorly used in medical instrumentation systems such as ECG, EMG, CT scan equipment, Patient hospital monitor. They are also used in Electromyogram integrator’s, Cardio tachometers, Vector Cardiograph. They are used in Bio-telemetry, Holter Recorder and other devices to determine the specific health condition of a patient. Applications of Biopotential Amplifier

37. Monitored to understand heart health. Displays ECG waveform. Instrumentation amplifiers give accurate testing and measurement. They do not require input impedance to be matched. This is the reason for using these amplifiers for testing and measuring a wide variety of equipment. Biopotential Amplifiers are very easy to use and stable. These are ideal for long term usage. They don’t necessarily depend too much on various factors that influence the output at the later stages. The Instrumentation Amplifiers work with just input. The Biopotential Amplifiers are highly scalable. Even a small input can be amplified to a greater extent at the input level. Advantages of Biopotential Amplifier

38. Sometimes, there could be minor distortion or noise in the output. The system often depends on special cables to remove the noise. Superimposing of original is the only concern when the noise gets transmitted for a long-range. Disadvantages of Biopotential Amplifier