The document discusses the instrumentation amplifier (IA). It begins by introducing the IA, noting its high input impedance, precisely adjustable gain using a single resistor, and high common mode rejection. It then describes the two stages of an IA: the first offers high input impedance and sets the gain, while the second is a differential amplifier with feedback and grounding that offers very high input impedance. Applications discussed include using a thermistor in a bridge circuit with an IA to indicate temperature.

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

2. OVERVIEW

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.

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.

5.  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. The output voltage between the outputs of two OPAMPS is given as:
vo = (v2-v1)(1 +
2𝑅
𝑅𝑔
)
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 =
𝑣2 −𝑣1
𝑅𝑔
------------------ (1)
Since none of the current can flow into OPAMPs input ,the output voltage
vo = Ig(2R + Rg)
=
𝑣2 −𝑣1
𝑅𝑔
(2𝑅 + 𝑅𝑔) ( from (1) )
vo = (v2-v1)(1 +
2𝑅
𝑅𝑔
)

7. UNITY GAIN DIFFERENTIAL AMPLIFIER
 Three terminals are brought out at the output, that are
TERMINALS
AT THE OUPUT
SENSE
ACCESS TO FEEDBACK LOOP
REFERENCE
D.C REF., GROUNDED
OUTPUT
SENSE FEEDBACK IS CONNECTED

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.

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 =
𝑽𝟐+𝑽𝒄𝒎 −(𝑽𝟏−𝑽𝒄𝒎)
𝑹𝒈
 Ig =
𝑽𝟐 −𝑽𝟏
𝑹𝒈
,,,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+
𝟐𝑹𝟏
𝑹𝒈
)
 and
 Vo = V4 - V3
 Therefore , the output voltage is:
 Vo = (1+
𝟐𝑹𝟏
𝑹𝒈
)(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+
𝟐𝑹𝒈
𝑹𝟏
) for the differential inputs v1 and v2.
 Gain = Ao= Vo/Vd = (1+
𝟐𝑹𝒈
𝑹𝟏
)
 The equation : (v2-v1)(1 +
𝟐𝑹
𝑹𝒈
) , 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. 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 ΔR.

14.  AK Sawhney,”Signal Conditioning”,in A Course in Electrical and Electronic
Measurements and Instrumentation,19th 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,3rd Revised Edition
,New Delhi ,India: McGraw Hill Education(India)Pvt. Ltd. 2013,Chapter
14,Section14.4,Page 518-521
REFERENCE