Instrumentation Amplifier Explained (with Derivation)

Summary

This video explores the workings and advantages of instrumentation amplifiers, a specialized type of differential amplifier with high gain, high input impedance, and superior common-mode rejection. It explains the limitations of standard differential amplifiers, such as resistor mismatches and low input impedance, which can introduce significant errors in industrial measurements. Through clear examples and circuit analysis, the video demonstrates how instrumentation amplifiers amplify only differential signals while rejecting common-mode noise, making them ideal for applications involving sensors like RTDs, thermocouples, and strain gauges. Viewers gain a thorough understanding of why instrumentation amplifiers are preferred in precise industrial and test measurement setups.

Takeaways

• 😀 Instrumentation amplifiers (IA) are a type of differential amplifier with high gain, high input impedance, and high common-mode rejection ratio (CMRR).
• 😀 IAs are commonly used in industrial applications and test/measurement circuits where precise signal amplification is required.
• 😀 Standard differential amplifiers can face issues with resistor mismatches, which reduce CMRR and introduce errors in output voltage.
• 😀 In industrial environments, noise and interference often affect sensor signals, making differential measurement techniques necessary.
• 😀 Bridge circuits, like those used with RTDs or strain gauges, convert physical parameters into small differential voltages.
• 😀 Even a 1% mismatch in differential amplifier resistors can lead to significant output errors when common-mode voltage is high.
• 😀 Monolithic differential amplifier ICs, such as AD629 and INA106, solve resistor mismatch issues but often have fixed gains.
• 😀 Using buffers before differential amplifier inputs increases input impedance, reducing loading effects from sensors.
• 😀 Instrumentation amplifiers amplify only the differential signal while passing common-mode voltage unchanged, enhancing CMRR.
• 😀 The gain of an instrumentation amplifier can be easily adjusted externally using a single resistor, while internal resistors remain precisely matched.
• 😀 IAs are particularly advantageous when the differential input signal is very small compared to the common-mode signal, ensuring accurate measurements.
• 😀 Overall, instrumentation amplifiers are preferred over standard differential amplifiers in high-precision, noisy, or industrial measurement environments.

Q & A

What is an instrumentation amplifier and how is it different from a standard differential amplifier?

An instrumentation amplifier (IA) is a type of differential amplifier with very high gain, high input impedance, and high common-mode rejection ratio (CMRR). Unlike a standard differential amplifier, it amplifies only the differential input signal while passing common-mode signals unchanged, making it more suitable for precise measurements in noisy environments.

Why is high common-mode rejection ratio (CMRR) important in industrial applications?

High CMRR is crucial because industrial environments often have large common-mode noise or interference. A high CMRR ensures that only the small differential signal from sensors is amplified, while noise is rejected, providing accurate measurement results.

What types of transducers are typically used with differential or instrumentation amplifiers?

Common transducers include RTDs (Resistance Temperature Detectors), strain gauges, and thermocouples. These devices convert physical parameters like temperature, strain, or flow into electrical signals that can be measured and amplified.

How can resistor mismatches in a differential amplifier affect its performance?

Resistor mismatches reduce the differential amplifier's CMRR, allowing common-mode signals to appear at the output. For example, a 1% mismatch can reduce CMRR to around 61 dB, introducing significant errors when the common-mode voltage is large compared to the differential signal.

What is the effect of input impedance on differential amplifier performance?

If the input impedance of the differential amplifier is comparable to the source or bridge resistance, it can load the signal and reduce the differential input voltage available at the amplifier. High input impedance is required to prevent signal attenuation.

How do monolithic differential amplifier ICs like AD629 or INA106 improve performance?

These ICs have precisely matched internal resistors, which increases CMRR and minimizes output errors. They also reduce the impact of resistor mismatches and simplify circuit design.

Why might using buffers or non-inverting amplifiers before a differential amplifier not fully solve common-mode issues?

While buffers can increase input impedance, they also amplify both differential and common-mode signals, so the signal-to-noise ratio does not improve. Additionally, the CMRR is still affected by resistor mismatches in the differential stage.

How does an instrumentation amplifier amplify only the differential signal?

In an IA, the differential input is applied to two op-amp stages with a shared resistor (Rg). The op-amps amplify only the voltage difference between inputs, while any common-mode voltage appears equally at both inputs, causing no current to flow through the resistors. This effectively buffers the common-mode voltage while amplifying only the differential signal.

What is the formula for the output voltage of a typical three-op-amp instrumentation amplifier?

The output voltage is Vout = (R2/R1) × (1 + 2R5/Rg) × (VA - VB), where VA and VB are the input voltages, Rg is the gain-setting resistor, and R1, R2, R5 are internal resistors with matched ratios. This formula shows that the differential signal is amplified while the common-mode signal is rejected.

In what scenarios is an instrumentation amplifier preferred over a standard differential amplifier?

Instrumentation amplifiers are preferred when the differential signal is very small and the common-mode signal is large, such as in industrial measurements, medical sensors, and long-distance signal acquisition. Their high CMRR, high input impedance, and tunable gain make them ideal for accurate signal measurement in noisy environments.

How can the gain of an instrumentation amplifier be adjusted?

The gain of an instrumentation amplifier can be adjusted externally by changing the resistor Rg. The relationship is gain = (1 + 2R5/Rg) × (R2/R1), allowing flexible amplification without affecting the internal resistor matching and CMRR.