4/07/16 - Wheatstone Bridges and DACs

Today, we continue the discussion of all the different op amp circuits we can create, and if we were to connect multiple op amps up in a circuit, Here are some calculations showing what would the output be based on different configurations.

Here we have an example of the output of one non-inverting amplifier being the input to another non-imputing amplifier. We solve for each circuits' gains and then multiply them to find our final output voltage.

Next up, we have an application for the difference amplifier. We are told that a strain gauge is designed with a varying reisstance based on how far the item has changed in shape, since it won't have the same resistance.

Here we hook up that small difference in resistance to a circuit known as the Wheatstone bridge, we have the same resistor ratio to be out positive input terminal and hook up a varying resistor and the thermistor to the negative input terminal.

Here are all the recorded values of the elements used in our circuit, we also measured the difference in voltage when we touched the thermistor and found that we had a desired gain of 4 to give us the 0-2V range that Professor Mason wanted.

We set up the circuit with the Wheatstone Bridge on the right side and the difference amplifier on the right side. We notice that the potentiometer does not have the resistance range to match the thermistor at room temperature, so we hooked up a resistor in series to give it an offset.

Please ignore the mini-inductor in the circuit.

We begin by calibrating the Wheatstone bridge to give us an initial difference of 0V by turning the potentiometer.

Then we connected the two terminals of the op amp top find that we were not getting what we expected as our output, we were even getting a voltage though our difference should be 0V.

After double checking everything, we notice that we were producing a non-zero output voltage even though our Wheatstone Bridge was inputting a 0V difference.

We could not produce results that confirmed our discussion of the strain gauge and instead, blamed it on the lousy op amp.

Now we move on to the topic of Digital to Analog Converters, DAC for short. We are asked how would we produce a analog output given a digital signal. Based on the most and least significant digit, we can convert this 4bit digital number into a analog siganl with a summing amplifier and different resistors for the inputs.

An example problem dealing with the DAC and asked to turn the 4bit 1001 into its analog equivalent given that our range is 0-1V and our feedback resistance is 500 Ohms.