JNTU B.Tech III SIGNAL CONDITIONING CIRCUITS Previous Question Paper (February - 2022 - R13)

A resistive potentiometer consists of a resistive element and a sliding contact. The output voltage is proportional to the wiper position, represented by the voltage divider rule: Vout = Vin * (R2 / (R1 + R2)). In signal conditioning, a buffer amplifier is typically used at the output to prevent loading effects, ensuring the measurement does not alter the voltage division ratio.

For thermistors, which are temperature-sensitive resistors, the resistance change is non-linear. To condition this signal, a Wheatstone bridge circuit is commonly employed. The thermistor is placed in one arm of the bridge. As temperature changes, the resistance of the thermistor changes, causing an imbalance in the bridge and producing a differential output voltage. This voltage is proportional to the temperature change and is subsequently amplified by a differential amplifier to provide a usable signal range.

The three op-amp instrumentation amplifier consists of two input buffer stages followed by a differential amplifier stage. The first stage uses two op-amps in a non-inverting configuration to provide high input impedance and high common-mode rejection. The second stage is a standard differential amplifier that subtracts the two buffered signals to produce a single-ended output proportional to the difference between the inputs.

The output voltage is given by: Vout = (V2 - V1) * (1 + 2R1 / Rg) * (R3 / R2), where Rg is the gain-setting resistor. The advantages of this configuration include: 1) Very high input impedance, which prevents loading of the signal source; 2) Excellent Common-Mode Rejection Ratio (CMRR), allowing it to reject noise common to both input lines; 3) The gain can be easily adjusted by changing a single external resistor (Rg) without affecting the CMRR.