UC Berkeley EECS 100 Lab B Boser LAB3 Sensor Interface NAME 1 SID NAME 2 SID Sensor Interfaces A very frequent scenario when designing electronic circuits you need some sensory input e g temperature You found a sensor e g a thermistor that converts the actual temperature into an electrical voltage The next step is to interface the sensor to the rest of your system typically a computer microcontroller Figure 1 shows the setup Usually the output of the sensor is a small voltage in the milli or micro Volt range for a full scale signal while the rest of the electronic system e g the computer expects much larger signals typically around a Volt For example the scale we built earlier generated output signals that were only a few milli Volt To overcome this mismatch we need some kind of interface between the sensor and the computer or whatever we would like to connect the sensor to as illustrated in Figure 2 on the next page Sensor interfaces can perform many functions Here we focus on the task of gaining up the signal to appropriate amplitude Specifically we want the interface to perform the function 1 v2 A v v1 where Av is the voltage gain For example if Av 21 and v1 7 2 mV v2 1 pt 0 We will build the sensor interface out of operational amplifiers To test it we need an input and something to verify the output We could use the scale constructed in an earlier lab for the input but this would require us to wire up that circuit again Moreover if we encounter problems we would have to determine if they are due to the interface or the sensor not always a trivial issue A better solution is to synthesize an appropriate input v1 with reliable and well characterized that s why it s expensive laboratory equipment to test our amplifier circuit Once we are satisfied with the result we can combine building blocks and test again Tackling circuits one by one in this fashion significantly simplifies our task and speeds up our work We will use the signal generator to simulate the transducer and the oscilloscope to verify the output from our sensor interface Signal Generator and Oscilloscope Download the manuals for the oscilloscope and signal generator and read the quick start and overview guides Program the signal generator to produce a 1 kHz sinewave with Vs 2 1 V zero to peak amplitude Connect the signal generator to the oscilloscope as shown in Figure 3 on the following page Observe the sinewave on the oscilloscope display What is the zero to peak amplitude predicted measured 1 pt 1 1 pt 2 If you just cannot get this right reread the guide for the function generator Feel free to play in the lab with different settings of the function generator e g higher frequency signals Even test equipment is not ideal when used outside its specifications which the manual explains although in rather technical terms Figure 1 Sensor connected to an electronic system e g a computer 1 February 2 2009 LAB3 v591 Figure 2 Figure 3 Sensor interface Signal generator connected to oscilloscope Operational Amplifier Let s check out the operational amplifier before designing a more complex circuit This way we are sure the part is working and we get all connections right e g supplies without wasting a lot of time debugging a complex setup Download the datasheet of the LMC6482M AM operational amplifier We are using the part in a 8 pin dual in line package Find the following specifications from the datasheet 1 pt Pin number of the positive supply V Maximum supply voltage V V Minimum supply voltage V Maximum input current IB Minimum junction temperature Tj C 2 1 pt 3 1 pt 4 1 pt 5 1 pt 6 1 pt Maximum junction temperature Tj C 7 1 pt Minimum large signal voltage gain Av sourcing into R L 2 k V unitless 8 KNow that we know the pinout of the operational amplifier chip we can connect it to a power supply and an input to see how it reacts in various circumstances The input will come from the function generator and will have values above and below ground In order to amplify negative signals the amplifier will need a negative voltage supply as the output cannot fall below the minimum supplied voltage Also the sum of the supply voltages must not exceed the chip s maximum supply voltage Use 5 volts for Vdd and Vss unless otherwise directed 2 February 2 2009 LAB3 v591 Draw the openloop Vout versus Vin characteristic of the operational amplifier Draw the expected Vout versus Vin characteristics on the plot Turn on the oscilloscope Change the scope to XY mode by pressing the Main Delayed button followed by the XY soft key Set the function generator to sine wave output at 10 Hz with 100mV peak to peak amplitude Label the axes variable units ticks in the graph below and show the measured result in a different color than the expected result 2 pt 8 Most practical opamp circuits use feedback to set the gain to an accurate and reasonable e g 10 value This works very well provided that the feedback is connected correctly Here we compare opamps with positive and negative feedback 3 February 2 2009 LAB3 v591 Which of the two circuits A or B is configured for negative feedback 1 pt 8 Build the negative feedback circuit and generate the XY plot just as you did in WKH ODVW part Copy your measured results to the plot below Repeat all above with a positive feedback circuit Explain what s happening and summarize your result in the graph 2 pt 8 Figure 4 shows the circuit diagram for an inverting amplifier based on the LMC6482 The inputs of the unused opamp OP2 are grounded a good precautionary measure that prevents the part from accidentally turning on or even oscillate and interfere with other devices Signals are often bipolar i e can assume both positive and negative voltage values Since the amplifier output cannot possibly swing below the supply both positive and negative supply voltages Vdd and Vss are needed as indicated in the diagram To avoid clutter the supplies are usually omitted from the circuit diagram Do not forget to connect them in practice as the circuit will obviously not work without Mark the pin numbers for all connections in the diagram and build your circuit on a protoboard Verify all connections before applying power If the part gets hot check if the supply is backwards Choose Vdd Vss 5 V R1 1 k and R2 such that vo vi 10 R2 1 pt 9 Figure 4 Inverting amplifier circuit diagram 4 February2 2009LAB3v591 Program the function generator for a sinusoidal output at 1 kHz and 200 mV zero to peak amplitude Using the oscilloscope verify that vo
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