Lab: Measurement System Design(April 11,2017): Activities: Cascading op amp: wheatstone Bridge

Date: April 11, 2017

Cascading op Amp Circuit; Wheatstone Bridge Circuit; Temperature Measurement System Design

Activity 1: Cascading op Amp

             In the first activity, we looked at a cascading op amp that essentially is two non-inverting op amps together.

The picture above shows the circuit that we had to analyze.

The picture above shows how we can solve each non-inverting op amp individually and uses the results at Va as the input voltage for the following non-inverting op amp. We still follow ideal op amp rules which mean that since the voltage Va and Vb both connect via the terminal they are equal to each other. Furthermore, in order to find the current we simply use current analysis and since the current moves from high voltage to low voltage the Vo is subtracted by the Vb and as we stated before Vb is equal to Va which we already calculated.

Lab 1: Temperature Measurement system Design

Pre-lab: the pre-lab asks us to understand the wheatstone bridge circuit so that we can implement it into our temperature measurement system.

In the above picture, we work out the relationship between Vab and all the resistors implemented in the design. Note: the R+deltaR will be a nob that will help us change the resistance value of the wheatstone bridge. Initially we want to balance the bridge with the nominal resistance as the source of balance because we want all the resistors to be identical but realistically they are not, we want all our R values to be 10k-ohms. The nominal resistor takes care of this issue. Furthermore, one of the resistors will be a thermistor, which will change depending on its heat. We determined that the thermistor range will be from 10k-ohm at room temperature and 7k-ohm at hot temperature.

In the above picture, we start backwards with the design. Since we want a specific output voltage we work around that idea. The left side of the board shows the difference amplifier. In order to simplify the work of determining the voltages we use the relationship learned for difference op amps, Vo=R2/R1(deltaV). We know Vo and deltaV therefore we can pick R1 to be any resistor from the box and solve for R2; doing this gives us R1=1k-ohmand R2= 5.33k-ohm. Both of these resistors exist in our box, so we use them. Since we already analyzed the wheatstone bridge we know the relationship between Va and Vs, Vb and Vs; therefore, we can determine that Vs should be 5V to get an output, Vo, of 2V.

The picture above shows our entire temperature measurement system design.

The video showing the results of our circuit. We did not get the proper Vo; however, we did make the Vo raise when we added heat to the thermistor.

In Conclusion:

            We were able to make the systems output voltage increase as the temperature increases. Furthermore, we also did not get the proper output voltage. We wanted to get Vo=2V; instead, we got a value of 0.2V. When heat was applied to the thermistor, Vo reached a max of about 0.275V.  We learned how a cascading op amplifier works. We learned that in designing a cascading op amp one of the major advantages is that even thought they result into one circuit, each part can be calculated separately, then the information of one part can help solve for information of the next part.