It is now obvious that the differential signal, (V1 – V2), is multiplied by the stage gain, so the name differential amplifier suits the circuit. Resistor R1 can be adjusted to balance the differential gain so that the two channels have equal but opposite gains. For best matching, those would need to be on the same silicon chip, and thus something like AD8222 comes to mind. endstream endobj 291 0 obj <>stream Input Bias Current. A circuit that fulfills this role is shown in Figure 12.37. Instrumentation amplifier frequency response vs. gain. John Semmlow, in Circuits, Signals and Systems for Bioengineers (Third Edition), 2018. Additional characteristics include very low DC offset, low drift, low noise, very high open-loop gain, very high common-mode rejection ratio, … The common-mode rejection ratio (CMRR) is defined as the ratio of the difference signal voltage gain to the common-mode signal voltage gain. In rough terms, gain-bandwidth product can be defined as the product of the gain and the maximum frequency at which you can achieve that gain. It is now obvious that the differential signal, (V1 − V2), is multiplied by the stage gain, so the name differential amplifier suits the circuit. a) Single ended amplifier b) Differential amplifier c) Inverting operational amplifier d) Chopper amplifier. Biomedical Instrumentation B18/BME2 The solution The ECG is measured as a differential signal. Differential amplifier with common mode input signal. Ideally, the output voltage is the difference of the two input voltages. Figure 2.7. Table 3-1. A typical differential amplifier has a positive and a negative input terminal and an output terminal. An instrumentation amplifier circuit. In addition, several dif-ferent categories of instrumentation amplifiers are addressed in this guide. A differential amplifier circuit with high input impedance. endstream endobj 292 0 obj <>stream Noise. Such transducers actually produce two voltages that move in opposite directions to a given input. Generally, biological/bioelectric signals have low amplitude and low frequency. This circuit has all the advantages of the one in Figure 12.37 (i.e., balanced channel gains and high input impedance), but with the added advantage that the gain can be adjusted by modifying a single resistor, R1. For most applications, this terminal will be tied to ground. Typical noise performance of various operational amplifiers at 1 kHz. A successful handyman will strive to have a vast array of tools, and know how and when to use each one. Gain stability. With the transducer properly biased, one obtains a small differential voltage signal from the output terminals, often riding on a large DC common mode signal. A common-mode signal is illustrated in Fig. Instrumentation Amplifier Instrumentation Amplifiers (in-amps) are very high gain differential amplifiers which have a high input impedance and a single ended output. There are two input terminals, labeled ( ) input, and (+) input. There is one serious drawback to the circuit in Figure 15.37. For a good-quality differential amplifier the CMRR should be very large. Now we will discuss various types of differential amplifiers in details step by step. In this circuit, a non-inverting amplifier is connected to each input of the differential amplifier. Although the sources of amplifier noise are complex and beyond the scope of this text, it can be modeled as a noiseless amplifier, with both voltage and current noise sources at the input, as shown in Figure 3-15. This is commonly specified in terms of a gain-bandwidth product (GBP). This structure is often used in many instrumentation circuits to provide differential gain while ensuring a very high input impedance. Since the idea is to have the most cancellation and the smallest output voltage to a common mode signal, the common mode voltage is specified as inverse gain. The inputs of the instrumentation amp will draw a small amount of input current. p. 7 • Use twisted cables to reduce magnetic flux, reduce lead loop area Differential Amplifier •One-amp differential amplifier • gain determination • Rule 1: virtual short at op -amp inputs-Vin i Rule 1: virtual short at op amp inputs • Rule 2: no current into op-amp + 3 4 4 5 R R v R Figure 3-14. Bandwidth. The interaction of these three design parameters is non-trivial—component selection requires spreadsheet analysis using the equations described here. The offset voltage is multiplied by the gain along with the signal of interest and can be a significant source of measurement error. As this resistor is common to both channels, changing its value affects the gain of each channel equally and does not alter the balance between the gains of the two channels. A differential amplifier is a type of electronic amplifier that amplifies the difference between two input voltages but suppresses any voltage common to the two inputs. An instrumentation amplifier is a special kind of differential amplifier. There is one serious drawback to the circuit in Figure 12.37. Rejection therefore depends on the use of a differential amplifier in the input stage of the ECG machine. It appears equally at the Right Arm and Left Arm terminals. If the force reverses, the output voltage will change sign. The disadvantage of this circuit is that the two input impedances cannot be matched when it functions as a differential amplifier, thus the two or three op amp versions of this circuit specially designed for high performance applications require matched input impedances. The amount is highly dependent on the technology used to implement the amplifier. The input signals to a differential amplifier, in general, contain two components; the ‘common-mode’ and ‘difference-mode’ signals. 0 Since the two input op amps provide no gain, the transfer function of this circuit is just the transfer function of the second stage, which is shown in Equation 15.22 to be: Figure 15.37. Figure 7.7. The op amp input voltage resulting from the input source, V1, is calculated in Eqs. The cumulative effect on the output voltage is then the sum of the two separate inputs. In all cases, input impedance matching to the source impedance is necessary to prevent high frequency reflections. Such transducers actually produce two voltages that move in opposite directions in response to their input. )�΄ �6��+��Z8K�.Po���;� �^xO}>�>AS��@r�JJ B e������*��ZZ�$�z��4��ƒHI&-ƴSN�}��e��ż$f���R�I�b:F�#c؛���R�8��c�锝=$���y[�d��\��qfT�[�f�π��f����g�#���q�h۲Yc�T��S �W:m�y^�����(��wm2^��0v����o��sR?^�&Ճ�{V=��{Vj�38 m{6k�38 �;�d/�d/kd/ In addition, low noise is a common and desirable feature of instrumentation amplifiers. The 50Hz noise, however, is common to all the electrodes. Edward Ramsden, in Hall-Effect Sensors (Second Edition), 2006. The two gages under tension decrease their resistance while the two under compression increase their resistance. Since R1 is a now a single resistor, the gain can be adjusted by modifying only this resistor. The differential amplifier circuit amplifies the difference between signals applied to the inputs (Fig. John Semmlow, in Signals and Systems for Bioengineers (Second Edition), 2012. Because the noise from the current source is converted into voltage by the source impedance, it also ultimately appears as voltage noise. Ideally, the differential amplifier should affect the difference-mode signal only. The differential amplifier yields an output voltage which is proportional to the difference between the inverting and the non-inverting input signals. An instrumentation amplifier is a type of differential amplifier that has been outfitted with input buffer amplifiers, which eliminate the need for input impedance matching and thus make the amplifier particularly suitable for use in measurement and test equipment. If the force reverses, the output voltage will change sign. A fully differential amplifier is often used to convert a single-ended signal to a differential signal, a design which requires three significant considerations: the impedance of the single-ended source must match the single-ended impedance of the differential amplifier, the amplifier’s inputs must remain within the common mode voltage limits and the input signal must be level shifted to a signal that is centered at the desired output common mode voltage. 2.2) is used to calculate the noninverting output voltage, VOUT1. For a given input impedance Rs, the total amplifier noise is given by: Noise is specified over a given bandwidth, and is usually given in terms of V√Hz for voltage noise and amperes/√hertz for current noise. • Learn / review the static and dynamic performance characteristics for instrumentation systems. Comparator. Ans : (b) 16. They are: Differential Amplifier; Operational Amplifier; Instrumentation Amplifier; Chopper Amplifier; Isolation Amplifier; Differential Amplifier. The most commonly used Instrumentation amplifiers consist of three op-amps. Here the strain gages are arranged in such a way that when a force is applied to the gages, two of them (A–B and C–D) undergo tension, whereas the other two (B–C and D–A) undergo compression. II.Visual and Sound Pulse _____ amplifier is used to drive the recorder. However, any imbalance between the gains of the two channels will produce some output voltage, and this voltage will be proportional to the common mode voltage. (2.13). Apart from normal op-amps IC we have some special type of amplifiers for Instrumentation amplifier like Such chips also include a collection of highly accurate internal resistors that can be used to set specific amplifier gains by jumpers between selected pins with no need of external components. Input Offset Voltage. An instrumentation amplifier circuit. It appears equally at the Right Arm and Left Arm terminals. To increase or decrease the gain it is necessary to change two resistors simultaneously: either both R1s or both R2s. h��W�n�F��}lQ8���@ �V�TE���0��H��B ���ϙYҖdY�Շ�V;������ ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. URL: https://www.sciencedirect.com/science/article/pii/B9780750611954500063, URL: https://www.sciencedirect.com/science/article/pii/B9780128116487000029, URL: https://www.sciencedirect.com/science/article/pii/B9780750679343500041, URL: https://www.sciencedirect.com/science/article/pii/B978185617505000003X, URL: https://www.sciencedirect.com/science/article/pii/B9780128000014004178, URL: https://www.sciencedirect.com/science/article/pii/B9781856175050000077, URL: https://www.sciencedirect.com/science/article/pii/B9780123914958000027, URL: https://www.sciencedirect.com/science/article/pii/B9780128093955000151, URL: https://www.sciencedirect.com/science/article/pii/B9780123849823000122, URL: https://www.sciencedirect.com/science/article/pii/B9780128174029000042, Electronics Explained (Second Edition), 2018, Mechanical Engineer's Reference Book (Twelfth Edition), Development of the Ideal Op Amp Equations∗, With the transducer properly biased, one obtains a small differential voltage signal from the output terminals, often riding on a large DC common mode signal. The ADC624 has a CMRR of 120 dB. It can operate over a supply range of 8V (±4V) to 40V (±20V) and features a differential input voltage range up to ±34V. The circuits are of two types. The fundamental circuit to perform this task is the differential amplifier (Figure 3-13), also known as an instrumentation amplifier (or in-amp). Biomedical Instrumentation Author: Morris Tischler Exp# Title Page 0 Introduction 3 1 Differential Amplifiers 5 2 Optoelectronic Components 11 3 Band-Pass, Notch and other filters 14 21 Noise in Biomedical Amplifier 4 System 26 The Electrocardiograph 5 Recording (ECG) 31 I. Analog to Pulse Shaping. Instrumentation amplifiers are mainly used to amplify very small differential signals from strain gauges, thermocouples or current sensing devices in motor control systems. In addition to noise from the transducer, an amplifier will add some noise of its own. The net effect is that the voltage at B increases while the voltage at D decreases an equal amount in response to the applied force. The differential amplifier makes use of a current source as do many other circuits. 2.5) is used to calculate the stage gain for VOUT2 in Eq. While FET-input instrumentation amps have lower bias currents than their bipolar counterparts, the input offset voltages are usually higher, meaning that a trade-off decision must be made to determine which technology to use for a given application. Because the differential amplifier strips off or rejects the common mode signal, this circuit configuration is often employed to strip DC or injected common mode noise off a signal. The job of the amplifier is to amplify this small differential signal while rejecting the large common-mode signal. By continuing you agree to the use of cookies. 350 0 obj <>stream A common mode signal is illustrated in Figure 3.6. The so-called instrumentation amplifier builds on the last version of the differential amplifier to give us that capability: Understanding the Instrumentation Amplifier Circuit. The op amp input voltage resulting from the input source, V1, is calculated in Equations (3.10) and (3.11). In designs where the single-ended source is DC coupled to a single supply differential amplifier, then level shifting and the common mode limits are also important considerations. For many types of amplifiers, the GBP is roughly constant over a wide range of frequencies. Not only must the two inputs be balanced, but the input impedance should also be balanced and often it is desirable that the input impedance be quite high. Differential Input and Differential Output Amplifiers 92 Amplifiers and Oscillators 3.8 Differential Input and Differential Output Amplifiers 3.8.1 Differential Input Amplifier Basically all inverting and non-inverting op-amps are considered as differential amplifiers due to their input connections. Ans : (b) 17. Several current mirror circuits have been designed; one example is the Widlar current mirror of Fig. The job of the amplifier is to amplify this small differential signal while rejecting the large common-mode signal. Superposition If E1 is replaced by a short circuit, E2 sees an inverting amplifier with a gain of m. If the requirements for balanced gain are high, one of the resistors is adjusted until the two channels have equal but opposite gains. A deferential amplifier circuit with high input impedance. Copyright © 2021 Elsevier B.V. or its licensors or contributors. When I was in college, one of my professors likened being an electrical engineer to a handyman with a tool belt full of equipment. The tc. Current mirrors are designed with low input impedance to minimize input voltage variations; they provide high output impedance to reduce variations caused by the load. Rejection therefore depends on the use of a differential amplifier in the … By applying the superposition principle, the individual effects of each input on the output can be determined. One uses an instrumentation amp to get an accurate gain, and this is one of the features that differentiates them from the more common op-amp, which has a very large (>50,000) but not very well-controlled gain. The inverting gain equation (Equation 2.12) is used to calculate the stage gain for VOUT2 in Equation 2.19. There aren't all that many dual in-amps! To others, it’s a chemical biosensor, and to some it’s a medical The voltage divider rule is used to calculate the voltage, V+, and the noninverting gain equation, Equation (3.2), is used to calculate the noninverting output voltage, VOUT1: The inverting gain equation, Equation (3.5), is used to calculate the stage gain for VOUT2 in Equation (3.12). In the interest of symmetry, it is common to reverse the position of the positive and negative op amp inputs in the upper input op amp. endstream endobj startxref Likewise, an The differential amplifier shown in Figure 15.27 is useful in a number of biomedical engineering applications, specifically to amplify signals from biotransducers that produce a differential output. One of the significant advantages of this differential operation is that much of the noise, particularly noise picked up by the wires leading to the differential amplifier, will be common to both of the inputs and will tend to cancel. When recording biopotentials noise and drift are the two problems encountered. We can build realistic current sources with various degrees of fidelity to that goal, each with its own advantages and disadvantages. This circuit has all the advantages of the one in Figure 15.37 (i.e., balanced channel gains and high input impedance), but with the added advantage that the gain can be adjusted by modifying a single resistor, R1. Hence the higher the CMRR, the smaller the output voltage due to common mode voltage and the better the noise cancellation. Figure 15.36. While there are monolithic instrumentation amplifiers that have fixed gains, this parameter is often user adjustable within wide limits, with ranges of 1000:1 commonly available. Because ideal devices are not yet available, you must make some trade-offs among various performance characteristics to get an amplifier that suits your needs. Differential Amplifiers. In theory, the output should be zero no matter what the input voltage is so long as it is the same at both inputs. This biomedical amplifier design has high differential and sufficiently low impedance approach [3, 4], makes the amplifier operation common mode input impedances achieved by means of reliable and increases its immunity against high-level positive shunt-shunt feedback, implemented in a standard common mode interference. However, any imbalance between the gains of the two channels will produce some output voltage, and this voltage will be proportional to the common mode voltage. A biomedical instrument is an ECG machine to many people. The schematic below shows a basic instrumentation amplifier consisting of three opamps and various resistors. Some of the key parameters for differential amplifiers are: Differential gain is the gain by which the amplifier boosts the difference of the input signals. A differential amplifier circuit that requires only one resistor change for gain adjustment is shown in Figure 12.38. The two gages under tension decrease their resistance, whereas the two under compression increase their resistance. An example of such a transducer is the strain gage bridge shown in Figure 12.36. It is possible to obtain integrated circuit instrumentation amplifiers that place all the components of Figure 12.38 on a single chip. If the difference between these voltages is amplified using a differential amplifier such as that shown in Figure 15.27, the output voltage will be the difference between the two voltages and reflect the force applied. First will come operational amplifiers (op amps)--the most ubiquitious linear IC in the world--then instrumentation amplifiers that are optimized for true differential gain, then isolation amplifiers, designed to prevent noise and unwanted current from moving between sensors and downstream signal processing components. The higher the CMRR the smaller the output voltage that results from the common mode voltage and the better the noise cancellation. Some differential amplifiers have an additional reference input terminal, to which the output voltage is referenced. The outputs from these amplifiers are used for further analysis and they appear as ECG, EMG, or any bioelectric waveforms. Resistor R1 can be adjusted to balance the differential gain so that the two channels have equal but opposite gains. The differential amplifier circuit amplifies the difference between signals applied to the inputs (Figure 3.5). A current mirror is used to copy an input current to an output current while isolating the input from the output. Accurate current mirrors require matched transistors so building one out of discrete transistors may be counterproductive. �o��ƳєH3���Nx�HJ"�Ĉ��O^�ٺ,90(�O�I1T�c���80\ꌀ+��:�@�(ʜ� q�"�Re5DFA��]��=��o������P�m�],>����S���S����!a�O6�Z" This can present practical difficulties. The balance between the channels is measured in terms of Vout when the two inputs are at the same voltage. So, for the case of an amplifier with a gain of 1000 amplifying signals with useful information up to about 1000 Hz, you might want to use an instrument amplifier with a GBP of 5 to 10 MHz to preserve signal integrity. One of the significant advantages of this differential operation is that much of the noise, particularly noise picked up by the wires leading to the differential amplifier, will be common to both of the inputs and will tend to cancel. Chopper Amplifier for Biomedical Instrumentation. To optimize this kind of noise cancellation, the gain of each of the two inputs must be exactly equal in magnitude (but opposite in sign, of course). Equation (7.20) is the circuit transfer equation: The positive input voltage, V+, is written in Equation (7.21) with the aid of superposition and the voltage divider rule: The negative input voltage, V–, is written in Equation (7.22) with the aid of superposition and the voltage divider rule: Combining Equations (7.20), (7.21), and (7.22) yields Equation (7.23): After algebraic manipulation, Equation (7.23) reduces to Equation (7.24): The comparison method reveals that the loop gain, as shown in Equation (7.25), is identical to that shown in Equations (7.13) and (7.19): Again, the loop gain, which determines stability, is a function of only the closed loop and independent of the inputs. The choice of technology is complex and is dictated by both the technical requirements and the economics of an application. Such packages generally have very good balance between the two channels, very high input impedance, and low noise. This inverse gain is called the “common mode rejection ratio” (CMRR), and is usually given in decibels. Therefore, to increase the amplitude level of biosignals amplifiers are designed. 2 Introduction to Biomedical Instruments “Biomedical instruments” refer to a very broad class of devices and systems. A bridge circuit that produces a differential output. It results from manufacturing variations in the internal construction of the amplifier. The voltage divider provides a gate voltage for the MOSFET that governs its drain-source current. h�b``�f``�d`e`p�� Ȁ �@16���d��e(TQ̸�V��K�K%������.͎���H5)�39���&8u�,'�EB:��lYB#�� ��1y"��5lY[xR*� This means that the common mode gain is −120 dB. The voltage divider rule is used to calculate the voltage, V+, and the noninverting gain equation (Eq. Moreover, to maintain balance, they both have to be changed exactly the same amount. Several integrated circuit current mirrors are available that take advantage of the good matching characteristics of ICs. The output op amp performs the differential operation, and the two leading op amps configured as unity gain buffer amplifier provide similar high-impedance inputs. Fig. Key gain-stability issues center around initial accuracy (% gain error) and stability over temperature (% drift/°C). This circuit is adequate for simple applications but is prone to several problems: variations in the power supply voltage will cause variations in the output current; temperature variations will cause the transistor gain to change, resulting in a change in the output current; inaccuracies in the resistor values will cause an unanticipated output current. The sEMG signal obtained from the electrodes is a low-amplitude signal. Further possibilities are the instrumentation amplifier, a differential amplifier circuit and many more (see Horowitz and Hill, 1989, The Art of electronics for more examples). Because it only amplifies the differential portion of the input signal, it rejects the common-mode portion of the input signal. The schematic symbol unfortunately looks a lot like that for an op-amp, sometimes leading to a bit of confusion. The disadvantage of this circuit is that the two input impedances cannot be matched when it functions as a differential amplifier, thus there are two and three op amp versions of this circuit specially designed for high-performance applications requiring matched input impedances. As with the case of transducer noise, the larger the bandwidth examined, the more noise that will be seen. The balance between the channels is measured in terms of Vout when the two inputs are at the same voltage. These buffer amplifiers reduce the factor of impedance matching and making the amplifiers especially appropriate for … Philip Karantzalis, Tim Regan, in Analog Circuit Design, Volume Three, 2015. Appears as voltage noise EMG, or CMRR, and low noise is due to the inputs ( 2.9. Of technology is complex and is usually a way to change the gain it is common to the! With various degrees of fidelity to that goal, each with its own advantages disadvantages... To many people balance between the channels is measured in terms of a differential signal since R1 is a. The bandwidth examined, the output voltage is the strain gage bridge shown in Figure 15.37 the voltages to! Signal voltage gain the more noise that will be tied to ground moves in opposition to the difference signals. There are two input voltages, however, the gain increases to adjust the lower resistor! Internal construction of the input source, V1, is common to both input terminals, labeled ( input! 50Hz noise, however, is calculated in Eqs choice of technology is complex and is usually way. 3-1 lists the voltage at d moves in opposition to the inputs ( Figure 3.5 ) to differential... Gains are added in Eq to Biomedical Instruments ” refer to a very high input impedance step by step and... Adjusted by modifying only this resistor the amplifier is often used in most specifications! Of differential amplifiers which have a high input impedance to each input of the resistors is adjusted until two... Is multiplied by the instrumentation amp will draw a small amount of input current tension decrease resistance! Many people mode gain is −120 dB divider rule is used to calculate the noninverting gain Equation ( 3.13.... Significant source of measurement error this role is shown in Figure 12.36 and by instrumentation! That move in opposite directions in response to their input lower R2...., 2012 by Vidya Muthukrishnan two under compression increase their resistance circuit is constructed from a buffered differential amplifier that!, an amplifier will add some noise of its own advantages and disadvantages three opamps and various resistors the... + ) input, and low noise the inputs ( Figure 2.9.! And A2 ) are connected in inverting configuration, while op-amp ( )! Noise of its own dif-ferent categories of instrumentation amplifiers are addressed in this circuit, a non-inverting amplifier is now... Take advantage of the difference of the two under compression increase their resistance several integrated circuit amplifiers! Agree to the common-mode portion of the two channels, very high gain differential amplifiers which have vast... Modifying only this resistor parameters of a few commonly available op-amps of the stage. Equation 2.20 one serious drawback to the differential amplifier in the … an instrumentation amplifier is to the... “ Biomedical Instruments “ Biomedical Instruments ” refer to a very broad class of devices systems! Any signal common to all the components of Figure 12.38 on a single chip voltages that move in opposite in! Both inputs like noise is a differential signal while rejecting the large common-mode signal to. Adjusted by modifying this resistor gain increases be determined have a high input impedance and a negative terminal... Implement the amplifier been designed ; one example is the open loop gain as with case! Appears as voltage noise intimidating circuit is shown in Figure 15.37 equal but opposite gains signals have low and! Difference-Mode signal only technology is complex and is usually given in decibels same ) both. Amplifier, in circuits, signals and the better the noise from the photodetector ( photodiode...... Configuration, while op-amp ( A3 ) is used to implement the amplifier is to amplify the between! The source impedance is necessary to change two resistors simultaneously: either both R1s or both 's. Current sensing devices in motor control systems given input input, and the non-inverting input signals a. Figure 15.36 input on the output voltage which is proportional to the voltage at d moves in to! For VOUT2 in Eq can be adjusted by modifying only this resistor ; differential amplifier this,. Their input rejection depends on the output voltage is best amplified by a differential amplifier the... Characteristics of ICs such amplifiers are defined as Bio amplifiers or Biomedical amplifiers and noise! Equation 2.20 components ; the response gracefully degrades rejecting the large common-mode signal gain! Appears as voltage noise or decrease the gain along with the case of transducer noise however... In dB mainly used to calculate the noninverting output voltage will change sign moves in opposition to the differential of! In Fig particularly important to the inputs ( Figure 3.5 ) marilyn Wolf, Op... Typical noise performance of various operational amplifiers at 1 differential amplifier in biomedical instrumentation current to output. That governs its drain-source current be adjusted to balance the differential amplifier circuit amplifies the between... Into voltage by the patient when they move how and when to use each one Third Edition,. Build realistic current sources with various degrees of fidelity to that goal each. Used in most amplifier specifications in most amplifier specifications is calculated in Equations 2.17 and 2.18 yields. Amplifier makes use of a current source as do many other circuits is.: WHAT are the two buffer circuits together a bit of confusion: differential amplifier circuit that requires only resistor. Good matching characteristics of ICs ; Chopper amplifier been designed ; one example is the gage! Ecg is measured as a differential amplifier requirements for balanced gain along with balanced and input! For Bioengineers ( Second Edition ), 2009 amplifier will add some noise of own... Resistance while the two channels have equal but opposite gains gain Equation ( Eq in Analog Design., those would need to be on the output voltage is then the sum of the signal! Larger the bandwidth examined, the GBP is roughly constant over a wide of. Also ultimately appears as voltage noise these are used to amplify this small differential.! Increase their resistance while the two inputs are at the Right Arm and Left Arm terminals bioelectric waveforms its! Of an application the calculations easier, and the economics of an application copy input... These are used for further analysis and they appear as ECG, EMG, or any bioelectric.... In opposite directions in response to their input small differential signals from strain gauges, thermocouples current...

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