4/11 Temperature Measurement System Design & Wheatstone Bridge Circuits

We talked about cascaded op amp circuits.

A cascade connection is a head-to-tail arrangement of two or more op amp circuits such that the output of one is the input of the next.

(From Day 14.5 Cascaded Op Amps2.pdf)

We did a example of cascaded op amp circuit below:

We could see the circuit as two separate circuit and solve it by cutting it half.

 After this example, we did the lab.

Temperature Measurement System Design

In order to have a voltage output greater than 2, we need to find R1 and R2 that satisfy this situation, so we pick R1 to be 10k ohms and R2 to be 56k ohms.

In this lab, there were two parts. One is Wheatstone bridge design, and another is difference amplifier design.

The circuit below was our design of this lab.

The actual values of resistance are next to the resistor symbols.

The picture below is the basic set up for Wheatstone bridge design.

By balancing the bridge, we got the voltage across the thermistor to be 0V at room temperature like the picture shown below.

And this is the video that show the bridge working:

At room temperature, the voltage across the thermistor is 0V, and at body temperature, the voltage across the thermistor is 1.05V.

Next step, we connected Wheatstone bridge to a difference amplifier to produce a voltage output greater than 2V.

The video below shows that the entire system working:

From the video, we could see that the range of voltage output is from 0V to 4.12V, which satisfies the result what we expected the circuit to have.

By designing this circuit, we can make the output voltage whatever we want by easily changing R1 and R2.

After the lab, we leaned Instrumentation Amplifiers, which is the most useful and versatile op amp circuits for precision measurement and process control.

The voltage output follows the equation below:

 

The instrumentation amplifier is an extension of the difference amplifier in that it amplifies the difference between its input signals.

(From Day 14.5 Cascaded Op Amps2.pdf)

Summary

We leaned how to set up a temperature measurement system and how to balance Wheatstone bridge. Also, we learned a new amplifier, called Instrumentation Amplifiers. By learning these amplifiers, we can easily change the original voltage input to any output voltage we want.