Electronic Scale Laboratory ES-1 San José State University Department of Mechanical and Aerospace Engineering Fall 2000 23OCT00 The Electronic Scale Pre lab Questions 1. How many strain gages are there in the Wheatstone bridge circuit? 2. What is the purpose of the trim pot used in the Wheatstone bridge? 3. When adjusting the offset of the amplifier for the electronic scale, where should the offset be measured? Purpose • To introduce operational amplifiers and their use in amplifying signal sources • To configure, build, and test several common amplifier types. • To learn how to interface an analog signal source to an A/D converter • To build a digital electronic scale using strain gages, an amplifier, an A/D converter, and a microcontroller Components Qty. Item 1 LF353 dual operational amplifier 1 ADC0831CCN 8 bit A/D converter 1 cantilever beam assembly with strain-gauges mounted on the top and bottom surfaces near the base 1 1 k resistor 1 4.7 k resistor 2 10 kΩ resistors 2 200 k resistors 1 1 kΩ trim pot 1 100 k trim pot 1 0.1 µF capacitor 1 10 µF capacitor Introduction The Devices Figure 1 shows two common packages for operational amplifiers. The single package on the left contains one operational amplifier. The LF741 was one of the earliest IC op-amps and is still widely used. There are many varieties of op-amps available today. In this lab we will be dealing with op-amps designed for small signal amplification. There are also power op-amps that are designed to source or sink relatively large currents at relatively high voltages. Small –signal single op-amp IC’s areElectronic Scale Laboratory ES-2 San José State University Department of Mechanical and Aerospace Engineering Fall 2000 23OCT00 usually pin-compatible with the ‘741, but one should always check the data sheets to be sure. We will use the LF353 dual op-amp IC in this lab experiment. It contains two independent op-amps as shown. There are also quad-packages that contain 4 independent op-amps. Op-amps are active devices, which means they must be powered in order to function. The typical configuration will use symmetrical positive and negative supply voltages (VS) in the range between ±5 V and ±15 V. There are some op-amps that are designed to work with single-sided supplies, i.e., where –VS is ground, and +VS is +5 V to +15 V. 7 1 2 3 4 5 6 8 - + 7 1 2 3 4 5 6 8 ‘741 type, single LF353, dual Balance Balance - input + input - VS + VS Output N. C. Output A Output B - VS + VS - input A + input A + input B - input B A B - - + + Figure 1 Examples of single and dual op-amp IC packages. We will use the LF353 dual op-amp in this experiment. Procedure 1. Design and construct an inverting amplifier (see Figure 2) with a gain of -20. Use a 10k resistor for R1. This will save you time later). Note the minus sign on the gain. The output polarity is inverted from the input. This is why it is called an “inverting amplifier.” What must the resistance of R2 be? Set up the power supply for voltages of ±12 V before connecting the power supply to the op-amp. Turn off the power supply before you wire it to the op-amp. It is always a good idea to do whatever wiring you need with all power to the circuit turned off. R2 R1 Vout Vin V+ V- COM Figure 2 An inverting amplifier. The gain of the amplifier is -R2/R1. It is always a good idea to keep the leads to the inputs of the op-amp and the feedback loop short to minimize noise pickup. To test your circuit, first, set up the function generator with high Z termination to output a 2 kHz sine wave with an amplitude of 100 mV peak-to-peak (p-p). Check the output of the function generator with the ‘scope first before you connect it to the amplifier. Then, connect the ‘scope probe from Channel 1 to the input of your amplifier circuit, and the ‘scope probe from Channel 2Electronic Scale Laboratory ES-3 San José State University Department of Mechanical and Aerospace Engineering Fall 2000 23OCT00 to the output of the amplifier. Where should the ground clips of the scope probe be clipped? Before you attach the clips, turn to the last page to check your answer. Now apply power to the circuit, and connect the function generator to the input of the amplifier. Compare the signals from Channel 1 and 2 displayed on the screen. Do the voltage peaks appear to be opposite each other? Does the amplitude of the output signal agree with your gain calculation? Increase the function generator amplitude until the op amp output appears to be chopped off at the peaks. At what input amplitude is the output clipped? Explain why this occurs. 2. Using the other op-amp on the LF353, construct a non-inverting amplifier with a gain of 50 as shown in Figure 3. Let R1=1k, and use a 100 k trim pot for R2. The gain of this circuit is (R1+R2)/R1. R2R1VoutVinCOM Figure 3 A non-inverting amplifier. The gain of the amplifier is 1+R2/R1. Test this circuit in the same manner as before, then input a 10kHz square-wave with an amplitude of 0.5V p-p. Compare the two signals displayed on the screen. Are the voltage peaks synchronized? With a square-wave, how would you tell if the voltage peaks were being chopped? Most importantly, what is the gain of this circuit? The Electronic Scale 3. Obtain a board with a cantilevered strain-gauge bracket from the instructor. These strain gauges convert mechanical strain into a proportional change in resistance. Here, we’re going to incorporate the strain gauges into a circuit called a Wheatstone bridge as shown in Figure 4. Basically, a Wheatstone bridge uses four resistors, which are grouped into two pairs and connected to a voltage supply. The resistors in each pair are identical in value to each other, but not necessarily equal to the resistors in the other pair. For example, one pair might consist of two 10k resistors, and the other two might be 20k resistors. Even though the resistance of one pair is double in value of that of the other, the voltage, Vout at the center nodes of each pair is the same. Can you see why? In fact, the only way Vout will change is if one of the resistances changes, and this is where the strain gauge comes in to play. By replacing one of the pairs of resistors in the Wheatstone bridge circuit with the strain gauges that are mounted on the beam, the voltage at their common node will vary according to the strain in the beam. Since one strain gauge is on top, and the