slyt226 - Texas Instruments Incorporated Amplifiers: Op...

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25 Analog Applications Journal Texas Instruments Incorporated Amplifiers: Op Amps 4Q 2005 www.ti.com/aaj Analog and Mixed-Signal Products Getting the most out of your instrumentation amplifier design Many industrial and medical applications use instrumentation amplifiers (INAs) to condition small signals in the presence of large common-mode voltages and DC potentials. Standard INAs using a unity- gain difference amplifier in the output stage, however, can limit the input common- mode range significantly. Thus, common- mode signals induced by adjacent equipment, as well as large differential DC potentials from differently located signal sources, can increase the input voltage of the INA, causing its input stage to saturate. Satura- tion causes the INA output voltage, although of wrong value, to appear normal to the following processing circuitry. This could lead to disastrous effects with unpredict- able consequences. This article reviews some principles of the classic three-op-amp INA and provides design hints that extend the input common- mode range to avoid saturation while pre- serving overall gain at maximum value. The article also discusses the removal of large differential DC voltages through active filtering, avoiding passive RC filters at the INA input that otherwise would lower its common-mode rejection ratio (CMRR). INA principles Figure 1 shows the block diagram of the classic three-op- amp INA. The inputs, V IN+ and V IN– , are defined through the input polarities of the difference amplifier, A3. By definition, the INA’s input signals are subdivided into a common-mode voltage, V CM , and a differential voltage, V D . While V CM , the voltage common to both inputs, is defined as the average of the sum of V IN+ and V IN– , V D represents the net difference between the two. (1) Solving both equations for V IN+ or V IN– and equating the received terms results in a new set of equations, which, when solved for either input voltage, yields (2) VV V V IN CM D IN CM D +− =+ =− 22 and . V V CM IN IN DI N I N = + 2 and . In the nonsaturated mode, the op amp action of A1 and A2 applies the differential voltage V D across the gain resis- tor, R G , generating the input current, I D : (3) The output voltages of A1 and A2 are therefore Replacing current I D with Equation 3 yields (4) where Equation 4 shows that only the differential component, V D /2, is amplified by the input gain, G 1 , while the common- mode voltage, V CM , passes the input stage with unity gain. The difference amplifier, A3, subtracts V 1 from V 2 and amplifies the difference with the gain G 2 : (5) V G G R R O () = 212 2 2 1 ,. where G R R F G 1 12 . V GV V V G CM D CM D 11 21 and , V IR V V V CM D DF C M D + and . I R V R D IN IN G D G = + = . By Thomas Kugelstadt (Email: tk@ti.com) Senior Systems Engineer, Industrial Systems A3 + + R 1 R 2 R 1 R F R G I D V/ 2 D V CM R 2 V O V =V –V /2 IN CM D V = V – G V /2) 1C M 1 D ( A2 G =1+2R /R 1F G G=R/ R 1 V REF + + + R F 2 D A1 V= V+ V / 2 IN+ CM D V = V + G V /2) 2C D ( Figure 1. Classic three-op-amp INA and its voltage nodes
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Texas Instruments Incorporated
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This note was uploaded on 11/14/2011 for the course ECE 3820 taught by Professor Wang during the Fall '11 term at GWU.

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slyt226 - Texas Instruments Incorporated Amplifiers: Op...

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