07-Apr-2016: Day 14

Exercise 1:

Cascading operational amplifiers may look frighting until we realize that each piece is to be attacked as a small individual block. The total gain is then just the product of the individual gains. Below is an example.

Exercise 2:

A strain gauge is a device used for indicating the strain of a material or structure at some point of attachment. Students have probably worked with these in the form of a force sensor. As the material is deformed, its resistivity changes and thus resulting is a correlation between strain and resistance.

Exercise 3:

Building our first multi-component circuit...

Temperature Measurement System Design Lab:
(Long Name = Long/Tedious Lab)

In this lab we were to design and implement a measurement system that outputs a voltage which is related to the temperature. A thermistor, resistor that decreases as temperature increases, will be used in a Wheatstone bridge and then connected to a difference amplifier to give us some voltage response on the output as a function of the temperature.

The Wheatstone is depicted below.

The voltage drops at point 'a' and 'b' are depicted below.

With the above being true, then the difference in potential energy between points 'a' and 'b' may be written as follows.

Notice above that if the dR << R then it can as shown below.

If R >> dR then 4R >>>> dR, therefore dR may be ignored in the denominator but not in the numerator and the above is true.

Next, we build and become frustrated... ideally because the resistors in the Wheatstone bridge are all "equivalent" to the thermistor at room temperature then we should be getting a voltage reading of zero. Unfortunately in the real world we must work with what we have and what we have is a variance of almost 1 kilo-Ohm between the room temperature thermistor and the other resistors in the bridge.

We fought and scratched our heads before realizing that if we had simply ignored the fact that we could not start at zero then when we run the experiment the voltage decreases from some small positive voltage, reaches zero, then continues to increase in the negative direction. When the voltage is zero, here is where the thermistor is "equivalent" to the other resistors in the bridge. Still not convinced? Well by now implementing a potentiometer into the bridge connected in line with the thermistor across 'b' then we could dial our resistor value to match that of the thermistor at room temperature and reach a value of "zero" because "zero" is good enough, as seen below.

Below is a video of what happens.

It also turns out that the voltage you put in plays a large role in the interference that is picked up and below you see we used only 20 mV to reach a "pretty" result.

Below is a sketch of the circuit we built and some of the chicken scratch we jotted down.

As it turns out, the issue we had of the mysterious voltage readings even when voltage input was set to zero could be simply solved with an instrumentation amplifier.

Exercise 4:

Our next brief topic of discussion was about digital to analog converters, DACs. They are in many of our pockets without many of us even knowing about them. Video and audio transmissions are often transferred using analog signals while high resolution video is an example of digital signals. Most communication between integrated circuits is in the form of a digital signal so the DACs act as mediums between the integrated circuits and input/output "hardware".

Up next we will be discussing capacitors and inductors... stay tuned.