Instrumentation amplifier

b.tech electrical book ak sawhney ,,instrumentation

1. INSTRUMENTATION AMPLIFIER (I.A.) IFHAM H.MALICK MOHD. SAMAR ANSARI IMRAN PERVEZ SYED MOHD MUSTAFA HASAN MOHD ABUZAR

2. OVERVIEW INSTRUMENTATION AMPLIFIERS Unity Gain Diffrential Amplifier Input Stage Preface Three Amplifier Configuration Applications

3. INTRODUCTION The instrumentation amplifier is a dedicated differential amplifier with extremely High Input Impedance. Its gain can be precisely adjusted by single internal or external resistor. The High Common Mode Rejection makes it useful in recovering small signal in large common-mode offsets and noise. INSTRUMENTATION AMPLIFIERS

4. INTRODUCTION Instrumentation Amplifier is a closed loop device with carefully set gain unlike a OPAMP which is an open loop device and variable gain. This allows I.A. to be optimized for its role as signal conditioner of low level signal in large noise. It has two states: First Stage: Offers high Input Impedance to both input signals and allows to set gain using one resistor. Second Stage: It is a differential amplifier with output, negative feedback and grounded. INSTRUMENTATION AMPLIFIERS

5. INSTRUMENTATION AMPLIFIERS This stage offers very high input impedence to both input signals Consists of two carefully matched OPAMPS. Each input v1 and v2 is applied to the non-inverting input terminal of the OPAMP. Outputs are connected together through a string of resistors. Two resistors designated by R are internal to the integrated circuit. Rg is the gain setting resistor which may be connected internally or externally. INPUT STAGE

6. INSTRUMENTATION AMPLIFIERS The output voltage between the outputs of two OPAMPS is given as: vo = (v2-v1) The equation can be derived as follows : Since , the voltage at the top of Rg is v1 and at the bottom is v2 Vrg = v2 – v1 = IgRg Ig is the current across the resistance Rg => Ig = ------------------ (1) Since none of the current can flow into OPAMPs input ,the output voltage vo = Ig(2R + Rg ) = ( from (1) ) vo = (v2-v1)  

7. UNITY GAIN DIFFERENTIAL AMPLIFIER Three terminals are brought out at the output, that are INSTRUMENTATION AMPLIFIERS

8. UNITY GAIN DIFFERENTIAL AMPLIFIER The sense terminal gives feedback access to the output loop. The reference terminals allow to establish d.c reference i.e., grounded potential of the output. The sense terminal is directly connected to output and tie reference to the output. INSTRUMENTATION AMPLIFIERS

9. Three Amplifier Configuration

10. This circuit is being described to show the elimination of a common mode signal. The circuit of shown figure is used for this purpose. This circuit uses three operational amplifiers and hence it is called “three amplifier configuration”. v1 and v2 are the desired signals and Vcm is the common mode signal. For simplicity , it is assumed R2=R3=R4=R5 Therefore ,we can write, Ig = Ig = ,,,1. the same current Ig flows through R1 , Rg and R1 . Therefore , V4 – V3 = Ig * (R1 + Rg +R1 ) ,,,,,,,2. from 1. and 2. we get , V4 – V3 = (V2 – V1)(1+ ) and Vo = V4 - V3 Therefore , the output voltage is: Vo = (1+ )(V2-V1)  

11. OPAMP 1 and OPAMP 2 act as buffers with unity gain for the common mode signal and with a gain of (1+ ) for the differential inputs v1 and v2 . Gain = Ao = Vo/ Vd = ( 1+ ) The equation : (v2-v1)(1 + ) , shows that there is no output corresponding to the common mode input signal , i.e , V2 becomes equal to V1 . This is because OPAMP 3 acts as a difference amplifier with a high value of CMRR . The gain can be changed by changing resistance Rg .  

12. INSTRUMENTATION AMPLIFIERS We shall now consider some important applications of instrumentation amplifiers using resistance type transducers. Temperature Indicators Using Thermistor: Thermistor is a relative passive type of temperature resistance transducer. It is usually a two terminal device. It has resistance as its fundamental property. In the bridge circuit of fig. the output meter is calibrated in °C or ° F. The bridge is balanced initially . Variation in temperature causes the variation in resistance causing the bridge to become unbalanced which in turn causes the deflection at the output. APPLICATIONS

13. Hence by selecting a suitable gain for the differential instrumentation amplifier the meter can be calibrated to read a desired temperature. The meter movement depends on the amount of unbalance in the bridge due to the change in the value of thermistor resistance Δ R. INSTRUMENTATION AMPLIFIERS

14. AK Sawhney ,”Signal Conditioning”,in A Course in Electrical and Electronic Measurements and Instrumentation,19 th Revised Edition,New Delhi,India:Dhanpat Rai and Co.2011,Chapter 26,Section 26.7,Page 900-902 HS Kalsi ,”Signal Conditioning ”,in Electronic Instrumentation,3 rd Revised Edition ,New Delhi ,India: McGraw Hill Education(India) Pvt. Ltd. 2013,Chapter 14,Section14.4,Page 518-521 INSTRUMENTATION AMPLIFIERS REFERENCE