10. Computer-Assisted Data Acquisition and Analysis

Transcription

1 10. Computer-Assisted Data Acquisition and Analysis Objective The purpose of this experiment is to practice computer-assisted data acquisition and analysis. Students use LabVIEW programs to control the computer interface board. The board generates voltages to drive external circuits and performs measurements. Acquired data are analyzed by the program to generate device characteristics. Results can be presented graphically on the computer screen. Background Electronic circuits have broad applications in computers, communications, and consumer electronics. Benefited from progresses of electronic integrated circuits, the performance of computers has increased substantially. Computer automation provides high accuracy and reproducibility. It can also be user-friendly. For example, the modern curve tracer in the electronic circuits laboratory is a special purpose computer. By pushing few keys, it can be set up to characterize a wide variety of devices. In this experiment, students learn how to use a computer to acquire experimental data and to perform data analysis. Student Preparation The following parts are needed: 1. One 2N3904 BJT and one MOSFET 2. A collection of resistors Students should review Introduction to Data Acquisition and Exps. 3, 4, and 5 before attending the laboratory. Instruction The brain of a computer automation system is the central processing unit (CPU). CPU operates with digital signals. To interface with external analog signals, both the analog-to-digital (A/D) and the digital-to-analog (D/A) converters are needed. The A/D interface converts an analog voltage into two bytes of digital data. It has buffer amplifier,

2 sample-and-hold, and multiplexer circuits. The buffer amplifier amplifies the input signal. The sample-and-hold circuit samples the analog voltage and holds the reading for few microseconds using a capacitor. The multiplexer enables several analog signals converted in sequence. The D/A interface converts the digital signal into analog voltage to drive external circuits. The D/A circuit has a reference voltage, a resistor network for voltage dividing, and a buffer amplifier. As commanded by cpu, switches are set up to derive the desired voltage by dividing the reference voltage. The voltage is amplified by an inverting operational amplifier to become the output. The A/D interface uses 12 bits to represent an input voltage. The range of voltages which can be digitized is -10 to 10 V. In the digital form, -10 V is represented by 0. 0 V is presented by 2047 and 10 V is represented by The resolution is 20 V / 2 12 or 5 mv. The D/A circuit also has a comparable resolution. Software is needed for computer automation. The software used in the laboratory is LabVIEW. LabVIEW is essentially a computer language developed specifically for scientific data acquisition and analysis using graphics. Programs have been written to perform device characterization. Students may simply use existing programs. However, students are also encouraged to modify them for additional features and improved performance. In order to modify programs, students must first learn what existing programs do in details. Devices to be characterized include n-channel MOSFET and 2N3904 BJT. Based on measurements done in Exps. 4 and 5, a voltage scan of at least 10 V is needed to thoroughly characterize such devices. The D/A output is indeed 10 to 10 V. The A/D can also cover -10 to 10 V. There are four programs written for this experiment. They are in the c:\353 directory. Programs with.vi extension are LabVIEW programs. The program daad.vi performs D/A and A/D on two channels. Users may type in a desired D/A voltage for channel 0. The output of D/A channel 0 is measured by A/D channel 0. The reading is displayed on the computer screen. Similarly, users may also control and read channel 1. The D/A output can also be measured by using a multimeter. If the multimeter reading is slightly different from the A/D data, there is an offset either in A/D or in multimeter. The offset can be eliminated by calibration using voltage standard. However, a small offset in

3 the mv range can be tolerated in this experiment. Likewise, there may also be a slight difference between the D/A voltage desired and the actual voltage measured. Using this program, students may check the operation of the interface board. Examining the diagram of this simple program can help users to learn how to program in LabVIEW The program mosfet.vi can be used to measure I D versus V DS curves for different V GS values. V GS is a user input. The program bjt1.vi can be used to measure the output characteristics of a BJT while bjt2.vi measures the input characteristics. The program waveform.vi can be used to digitize waveforms. Only moderate frequency waveforms can be digitized because the A/D conversion rate is limited to approximately 200,000 samples/sec. The fastest rate is achieved by digitizing data directly into memory. However, because the memory is limited, if many data points are needed, they should be digitized and recorded on the hard drive. The digitization rate, of course, is substantially reduced. In order to generate plots from experimentally measured data, data analysis and graphics have been incorporated into existing LabVIEW programs. The raw data are converted into physical parameters. Results are displayed on the computer screen. The programs also record data as text files. The data acquisition board is recognized by LabVIEW as device 1. Both D/A channels are used in all measurements, therefore, fill D/A Channel with 0:1. A/D Channel should be filled either as 0 or 0:1 depending how many channels are used in measurements. In addition, fill resistance values used in the circuit so that current can be calculated. Procedures 1. Run daad.vi program using the following steps. a. Connect D/A channel 0 and A/D channel 0 to the same point on the prototype board. Do the same for D/A channel 1 and A/D channel 1. Connect all ground leads together. b. Start LabVIEW. c. Open daad.vi in c:\353.

4 d. Key in several voltages between -10 to 10 V for both D/A channels. Run the program. Measure the D/A output with a multimeter. Compare measurements to results displayed on the computer screen. Fig The test circuit for characterizing the n-channel MOSFET. 2. Connect the MOSFET test circuit shown in Fig The D/A channel 1 is connected to the gate which requires a positive V GS as bias. The D/A channel 0 is connected as V CC for driving the MOSFET. A/D channel 0 is connected to the drain for V DS measurement. 3. Follow the same procedure of running daad.vi, load mosfet.vi. Enter a gate voltage and run the program. Enter the file name to record data. Repeat the procedure for different gate voltages. Use a different file name each time. Obtain at least six curves corresponding to gate voltages from 0 to 10 V. Copy data files onto a floppy disk. Students may generate graphs by using spreadsheet or other programs from recorded data. Fig The test circuit for characterizing the output of BJT.

5 4. Connect the BJT test circuit shown in Fig The D/A channel 1 is connected to the 100-kΩ resistor. The D/A channel 0 is connected as V CC for driving the BJT. A/D channel 0 is connected to the collector for V CE measurement. A/D channel 1 is connected to the base for V BE measurement. 5. Load bjt1.vi. Enter a voltage for D/A channel 1 and run the program. Enter a file name to record data. Repeat the process for several voltages on D/A channel 1. Obtain at least six curves corresponding to base current from 0 to approximately 100 µa. Copy data files onto a floppy disk. Students may generate graphs from data recorded on the floppy disk. 6. Build the BJT test circuit shown in Fig D/A channel 0 is connected as V CE. D/A channel 1 is connected to the base resistor to provide a base bias. A/D channel 0 is connected to the base for V BE measurement. 5. Load bjt2.vi. Enter several voltages for V CE. Record data on the floppy disk. 8. To digitize a low to moderate frequency waveform, connect a 1000 Hz, 2-V peak-topeak sinusoidal waveform to the oscilloscope and to A/D channel 0. Ground the ground lead. Fig. 3. The test circuit for characterizing the input of BJT. 9. Load and run waveform.vi. Students may start with a sampling rate of 100,000 samples/sec. Compare waveforms displayed on the oscilloscope and on the computer screen. Reduce the sampling rate. Eventually the sampling rate becomes too low to capture the waveform faithfully. The computer screen doesn't show a nice sinusoidal waveform. Broken waveforms or random dots within the 2-V peak-to-peak range may appear. Record the minimum sampling rate for which the sinusoidal waveform can be

6 digitized accurately. The data of the digitized waveform are recorded in a data file. Use a spread sheet program to read in the data file and to plot the data. Data Analysis and Report 1. From measurements report the offset or accuracy of D/A and A/D converters. 2. Plot the characteristics of MOSFET as measured by the computer. 3. Plot both the input and output characteristics of BJT. 4. Report the minimum sampling rate above which a 1000-Hz sinusoidal wave can be digitized accurately. Plot the waveform.

7 Scoring Sheet 10. Computer-Assisted Data Acquisition and Analysis Item Credit Score Basic A/D D/A and Offset Measurement 2 Output Characteristics of MOSFET 2 Output Characteristics of BJT 2 Input Characteristics of BJT 2 Transient Waveform 2 Total 10 TA Signature: Date: