Instrumentation Amplifier Explained (with Derivation)

Introduction to Instrumentation Amplifier

In this section, the video introduces the concept of an instrumentation amplifier, highlighting its high gain, common mode rejection ratio, and input impedance.

Differential Amplifier vs. Instrumentation Amplifier

• Differential amplifiers are commonly used in industrial applications for measuring parameters like temperature and humidity.

• Transducers such as RTD, Strain Gauge, and Thermocouples convert physical parameters into electrical signals for measurement.

• Bridge circuits are utilized with transducers like RTD to convert measured parameters into electrical signals based on resistance changes.

Importance of Common Mode Rejection

• In industrial settings, differential amplifiers help mitigate noise and interference in measurements taken over long distances.

• Differential signals from bridge circuits can be amplified by instrumentation amplifiers to enhance signal quality.

Functionality of Differential Amplifiers

• Detailed explanation of how a differential amplifier operates in rejecting common mode noise.

• Formulas provided for calculating output voltages based on resistor configurations in the circuit.

Handling Common Mode Voltage

• Discussion on handling common mode voltage along with differential input voltage in differential amplifiers.

New Section

In this section, the transcript discusses the output voltage of a differential amplifier and how it is affected by common mode voltage, differential input voltage, resistor mismatches, and common mode rejection ratio.

Output Voltage Calculation

• The output voltage of the differential amplifier is calculated using specific resistor values in an expression.

• The output voltage (Vout) is determined as 0.009 times the common mode voltage plus 10.0045 times the differential input voltage.

• Mismatched resistors can reduce the common mode rejection ratio to around 61 dB, impacting the output voltage accuracy.

Impact of Common Mode Voltage and Differential Input

• A common mode signal of 1V results in a 9mV output while a 10mV differential input leads to almost 100mV at the output.

• Resistor mismatches can introduce errors in the output voltage, especially with higher common mode signals or multiple mismatched resistors.

New Section

This part delves into solutions for reducing errors in differential amplifiers caused by resistor mismatches and low input impedance.

Using Monolithic ICs

• Monolithic ICs like AD 629 and INA 106 offer matched resistors internally, enhancing common mode rejection ratio and minimizing error in output voltage.

• However, these ICs have fixed gains limiting flexibility in adjusting gain levels for different applications.

Addressing Input Impedance Issues

• Buffers can be employed before inputs to increase input impedance of the differential amplifier, improving performance.

• Buffer circuits combined with monolithic ICs help mitigate mismatch issues between resistor pairs but may not allow gain adjustments as needed for specific applications.

New Section

This segment explores alternatives to buffer circuits for addressing gain adjustment limitations in monolithic IC-based differential amplifiers.

Non-Inverting Op-Amp Solution

• Non-inverting op-amps can replace buffer circuits to provide initial gain but may amplify both differential and common-mode signals simultaneously affecting signal-to-noise ratio.

Differential Amplifier and Instrumentation Amplifier

In this section, the speaker explains the functioning of differential amplifiers and instrumentation amplifiers, focusing on their applications in circuits where precise signal amplification is crucial.

Understanding Ideal Op-Amps

• The assumption of ideal op-amps implies that voltages at corresponding terminals are equal.

• Current flow through resistors Rg, R5, and R6 is determined by voltage differentials due to ideal op-amp properties.

Differential Amplification

• Voltage difference between input terminals (VB' - VA') is calculated based on current Ig flowing through resistors.

• Output voltage Vout of the differential amplifier amplifies the differential input signal using resistor values.

Common Mode Rejection Ratio in Instrumentation Amplifiers

This part delves into how instrumentation amplifiers maintain common-mode voltages while enhancing differential signals for improved circuit performance.

Common Mode Voltage Handling

• Instrumentation amplifiers pass common-mode voltages unchanged while amplifying only differential inputs.

• Common mode rejection ratio is increased by treating common-mode signals as buffers or unity followers.

Application and Benefits

• Instrumentation amplifiers excel in scenarios with small differential input signals overshadowed by larger common-mode inputs.