Temperature Measurement System Design

Today we have a concept of combination of different operating amplifiers. A group of different types of operating amplifiers connected together is called cascaded operational amplifiers. In this lab, we use a Wheatstone bridge and a difference amplifier to measure temperature. The change in temperature results in the change in resistance. Wheatstone bridge converts the change in resistance to the change in voltage. The difference amplifier will amplify the voltage change to a specific output voltage.

We designed a circuit as following:

First we setup the Wheatstone Bridge circuit without operational amplifier portion. In order to balance the bridge, we add a potentiometer in series with one of the 10 KΩ resistor. We tune it to a specific point when the voltage between the bridge is zero in room temperature.

	Experimental	Theoretical	Percent Error
R (kΩ)	9.8	10	2.00%
Rth low (kΩ)	7.15	7	2.14%
Rth high (kΩ)	11.74	10	17.40%
ΔR (kΩ)	4.59	3	53.00%
Vin Low (V)	0	0	N/A
Vin High (V)	0.557	0.375	48.53%

Here is a video of the Wheatstone Bridge

Then we add the difference amplifier into the circuit and form our final configuration

	Experimental	Theoretical	Percent Error
R1 (kΩ)	9.8	10	2.00%
R2 (kΩ)	56.2	56	0.36%
R3 (kΩ)	9.8	10	2.00%
R4 (kΩ)	56.2	56	0.36%
Vin Low (V)	0	0	N/A
Vin High (V)	0.557	0.375	48.53%
Vout Low (V)	0	0	N/A
Vout High (V)	2.1	2.1	0.00%

 Here is a video of the entire system

It performs as we expected. The voltage change due to temperature change is successfully amplified by the difference amplifier.

Summary: Wheatstone Bridge is a convenient mechanism that converts a resistance change to a voltage change. The drawback of Wheatstone Bridge design is that it requires three resistors with the same resistance as the changing resistor. We also have to balance the bridge before we take any measurement.

Cascaded Operational Amplifier shows how we can do composite mathematical operations. The basic building blocks are inverting amplifier, non-inverting amplifier, voltage follower, summer and difference amplifier. By applying different kinds of operational amplifiers, we can combine different operations in a expression and use a circuit to represent it. It shows the potential of using analog circuit to make a calculator. Instrumentation Amplifier is a commonly used cascaded operational amplifier. The instrumentation amplifier is an extension of the difference amplifier in that it amplifies the difference between its input signals.