Instrumentation Amplifier (In-Amp) forms the basic component of every measuring instrument and testing equipment. Instrumentation Amplifier is a type of Differential Amplifier which offers high Common-Mode Rejection. Instrumentation Amplifier is available in integrated circuit form and can also be built using Op-amps and Resistors which have very low tolerance value called as Precision Resistors. This post will provide you a better understanding about what is Instrumentation Amplifier, its Working Principle, Applications, Advantages and Disadvantages.
What is Instrumentation Amplifier
Instrumentation Amplifiers are basically used to amplify small differential signals. Instrumentation Amplifier provides the most important function of Common-Mode Rejection (CMR). It cancels out any signals that have the same potential on both the inputs. The signals that have a potential difference between the inputs get amplified.
An Instrumentation Amplifier (In-Amp) is used for low-frequency signals (≪1 MHz) to provide a large amount of Gain. It amplifies the input signal rejecting Common-Mode Noise that is present in the input signal.
Fig. 1 – Introduction to Instrumentation Amplifier
Basically, a typical Instrumentation Amplifier configuration consists of three Op-amps and several resistors. To achieve the highest CMRR (Common Mode Rejection Ratio), high-precision resistors are used (0.1 % tolerance or better).
Fig. 2 below shows the Pin configuration and Physical view of IC, AD620 In-Amp (Instrumentation Amplifier). This has been the industry standard, high performance, low cost amplifier. It is completely monolithic available in both 8-lead DIP and SOIC packages. The user can obtain any desired gain from 1 to 1000 using a single external resistor. By design, the fixed resistor values for gains of 10 and 100 are standard 1% metal film resistor values.
Fig. 2 – (a) Pin Configuration (b) AD620 Instrumentation Amplifier
Working Principle of Instrumentation Amplifier
Figure 3 below represents the configuration of the Instrumentation Amplifier using two Op-amps where V1 and V2 are the input voltages and V01, Vo2 are the outputs of the Op-amp 1 and Op-amp 2 respectively. R1, R2, R3 are the resistors and the output stage of the Instrumentation Amplifier is a difference amplifier, whose output Vout is the amplified difference of the input signals.
The inputs of the two buffer Op-amps draw no current and hence the voltage drop across Rg is proportional to the differential voltage V1 and V2. This produces a current that runs entirely through the resistors R and the voltage produced acts as the input to the differential amplifier or Subtractor circuit.
All the Resistors except Rg are equal. Rg may be an external resistor connected across two pins of the IC. If the pins are not connected, then the gain of the amplifier is 1 but preferably different gains may be obtained by connecting a resistor of relevant value. Alternatively, a number of resistors may be fabricated on the chip to give Gains of 1, 10, 100 and 1000.
Fig. 3 – Instrumentation Amplifier Configuration
Similar to the Op-amp circuit, the input buffer amplifiers (Op-amp 1 and Op-amp 2) of the Instrumentation Amplifier pass the common-mode signal through at unity gain. The signal gets amplified by both buffers. The output signals from the two buffers connect to the subtractor section of the Instrumentation amplifier. The differential signal is amplified at low gain or unity and the common-mode voltage is attenuated.
The potential at node A is the inverting input voltage V1. From the virtual short concept the potential at node B and G is also V1. The potential at node D is the non-inverting input voltage V2. Hence the potential at node C and H is also V2 .
The current I through the resistors R1, Rgain and R1 remains the same as ideally the current to the input stage Op-amps is zero.
Applying Ohm’s law between the nodes E and F
I = (Vo1-Vo2)/(R1+Rgain+R1)
I = (Vo1-Vo2)/(2R1+Rgain)
Since there is no current flow to the input of the op-amps 1 & 2, the current I between the nodes G and H can be given as,
I = (VG-VH)/Rgain = (V1-V2)/Rgain
The output of the difference amplifier is given by: –
Vo = (R3/R2)(Vo1-Vo2)
Theoretically, this means that the end user may obtain Gain in the front end as desired without increasing the common-mode gain and error. That is, the differential signal will be increased by gain and thus CMRR is directly proportional to gain.
Applications of Instrumentation Amplifier
The applications of Instrumentation Amplifier are:
- They are used extensively in Bio-medical applications like ECG’s and EEG’s.
- Instrumentation Amplifiers are used where long-term stability is essential like Industrial applications that includes automation.
- Instrumentation amplifiers are incorporated with pressure transducers in Weighing Systems to monitor various physical quantities such as weight, force, pressure, displacement and torque.
- They are used in Gaming industry.
- Instrumentation Amplifiers are also used in hand held batteries.
Advantages of Instrumentation Amplifier
The advantages of Instrumentation Amplifier are:
- Offset voltage is minimized.
- Voltage Gain is high as the configuration uses high precision resistors.
- The Gain of the circuit can be varied by using specific value of resistor.
- Non-linearity is very low. It is an inherent performance limitation of the device and cannot be removed by external adjustment but can only be designed by the manufacturer.
- Input impedance is very high to avoid loading down the input signal source and Output impedance is very low.
- Common-mode rejection is very high.
Disadvantage of Instrumentation Amplifier
The biggest disadvantage of Instrumentation Amplifier is the occurrence of noise when used for long range transmission purpose
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