JNTU B.Tech III SIGNAL CONDITIONING CIRCUITS Previous Question Paper (October - 2020 - R13)

A 2-op amp instrumentation amplifier is designed to provide high input impedance and high common-mode rejection ratio (CMRR) using fewer components than the standard 3-op amp configuration. It consists of two operational amplifiers where the inputs are applied to the non-inverting terminals of the two op-amps, and a resistor network is used to set the gain.

The circuit configuration typically involves two op-amps connected in a non-inverting buffer-like arrangement with a common gain-setting resistor connected between the inverting terminals. The output is taken from the second op-amp, which acts as a differential amplifier stage. The gain of this configuration is given by:

Key features include:

• High input impedance due to the non-inverting input configuration.
• Adjustable gain through a single external resistor R_g.
• Lower component count compared to the 3-op amp design.
• However, it is more sensitive to resistor matching and has limited common-mode range compared to the 3-op amp version.

Calibration and balancing of a Wheatstone bridge are essential to ensure accurate measurements of unknown resistance. Balancing is achieved by adjusting the variable arm of the bridge until the potential difference across the galvanometer or detector becomes zero, indicating that the bridge is in a null state. At this point, the ratio of the resistances in the opposite arms is equal.

Calibration involves the following steps:

• Zero Balance: With the unknown resistance disconnected or shorted, the bridge is adjusted using a potentiometer or variable resistor until the output voltage is zero. This compensates for any initial offset or lead resistance.
• Sensitivity Calibration: A known standard resistance is inserted in place of the unknown resistance. The bridge output is measured, and the excitation voltage or gain of the following amplifier is adjusted so that the output corresponds to the known value.
• Linearity Check: By using multiple known resistance values, the output is verified to ensure it follows a linear relationship, confirming that the bridge is correctly calibrated for the expected measurement range.