Day Twelve - Temperature Measurement System Design with Wheatstone Bridge Circuits 4/11/2017

Today, we talked more about on op-amps. After introducing almost all types of op-amp, we started to analyse a circuit with multiple op-amps (cascaded op-amps).

One simple rule for the cascaded op-amps is the total gain of the cascaded op-amps is the multiply of the gains of all independent op-amps.
eg. first one has A1, second one has A2, third one has A3, then the total gain of the cascaded is A(total) = A1*A2*A3.

It is critical to remember that nodal analysis is a useful tool on op-amps analysis.

Let's review the equations for summing and differential op-amps.

This is for summing, note that there is a negative because it is a variable of inverting op-amps.

This is for differential, note that the second equation must be satisfied for difference op-amps. 

V2 is the voltage supply where supplying the (+) terminal of the input.

V1 is the voltage supply where supply the (-) terminal of the input.

Then we talked about Digital to Analog Converter, which has a intuitive sense on how computer software sending 0 or 1 to operate.

Digital is 0 or 1, and we want to transfer them into a range values.

After that, we talked about instrumentation amplifier.

These are the schematic and equation for instrumentation amplifier.

Note that this amplifier is just a combination of three amplifiers. It seems like a combination of two non-inverting and one differential amplifiers.

The purpose of the instrumentation amplifier is to simplify the changing of the gain. For example, R4 is the Rgain, R3 = R1 = R2, and by varying the values of R4, we can easily change the overall gain. 

Lab:
Part One - Balancing the Wheatstone Bridge
In order to design our temperature measurement system, we have to build a circuit which has zero voltage output at normal condition and has non-zero voltage output at abnormal condition(changing resistance). This circuit comes out as the Wheatstone Bridge.

Here is the sketch of the circuit to convert resistance variation to voltage variation.
Note R1 is the potentiometer that will change to get the balance, and Rnom is the variable resistance sensor.

During room temperature, the output voltage is 0V, and the output voltage is 1.05V with the temperature of human.
                                    
Part Two - Applying to Difference Amplifier Design

This is the schematic we draw for the whole circuit, and we want a gain of 5.333
Actual values: R2 = 56.1k, R1 = 8.25k, R2 = 8.75k and R1 = 55.8k ohms.
From our measurement, the range of the amplified voltage output is from 0V with room temperature to 4.12V with human temperature.

This is a video of the entire system working.

Summary and Discussion:
From this lab, we learned that the op-amps can be used to enlarge the output voltage of a small device which only has a small voltage output, for example, the temperature measurement system without using op-amps. Using op-amps in real life can give the system a more obvious increase of voltage, and thus a more accurate result for any measurement system. However, from our experiment, the actual voltage gain is not necessary the same as the calculation because there are always +VCC,         -VCC, and offset voltage of the device affecting the voltage output.