Week 7 day 1- Temperature Measurement system Design

Lecture:

Our first task is to calculate current flowing through  a cascading op amp, in this problem we have 2 non-inverting ops connected together

 Using The Formula of non-inverting amp we calculated the voltage out of the op amp. we then use nodal analysis to calculate the current flowing through the 10 K Ω Resistor to find i:

Temperature Measurement System Lab part II 

In our first part of Temperature Measurement system Lab, we tried to design a voltage divider that Fluctuates 0.5 V between 25 and 37-degree celsius using a thermistor that has 11 kΩ

 resistance at room temperature and 7 kΩ resistance at 37 degrees. Today our lab objective is to design a circuit that increases by 2 V using the same thermistor.

 Our First step is to set up a Wheatstone bridge Circuits and measured the changes of potential difference when the resistance of thermistor went down. For our pre-lab, we calculated the theoretical Potential difference between Wheatstone bridge, consisted of 3 identical resistors and a thermistor, related to the changes of resistance in the thermistor:

Using the formula derived above, we plugged in our measured value to calculate the potential difference when the thermistor is at 37-degree celsius. our theoretical change of Wheatstone bridge is .375 V. Using this value as our Voltage input in our op amp, and assuming we wanted a 2V difference across our voltage output we calculate our desired op Amp gain to be 5.333. Using this gain we started our circuit design using 56 K and 10 K ohm resistance to obtain this gain.  Below is the sketch of our circuit: 

The first thing that we realised with our theoretical Wheatstone bridge is that our initial potential difference is not zero, like what it was supposed to be. To fix this problem we substituted our resistor that is in series to the thermistor with a potentiometer so we can change our resistance to set our initial potential difference to zero.  Below is the picture of our build:

We measured our output voltage to be 4 V instead of 2 volts, this means that we saturated our op amp, and we were doing something wrong in designing our circuit. 

We found out that we made 2 mistakes:

1. One is we place our potentiometer and thermistor backwards. 
2. When we recheck our design, we found out that we are connecting the 56 KΩ resistor connected to the non-inverting input, instead of connecting it to the ground. Below is the fixed design of our circuit:

Picture of the thermistor at 25-degree celsius and 37-degree celsius:

Summary :
Today in Lecture we learn to calculate gain when we have multiple op-amps in a circuit. a gain in op in a cascading op amp is equal to the product of each op amp cascaded together in a head to tail. We also Learn on two application of op Amp, DAC converter who converts digital signal to analog signal and instrumentational amplifier. In our Lab, we created more precise temperature measurement system by using difference amplifier to amplify voltage difference across Wheatstone bridge with a thermistor and potentiometer. Instead of only 0.5 V difference between 25-degree celsius and 37-degree celsius, we obtain a potential difference of 2 V by using opAmp with a gain of 6.