Steve Professor Errede Physics 398EMI & 343 University of Illinois at Urbana-Champaign
Use of A PC-Based Data Acquisition System For Measurement of 2-Lead Electronic Device DC Current-Voltage Relations
We have developed a simple, PC-based data acquisition system for the purpose of measuring the static, DC current-voltage (I-V) relation associated with an arbitrary 2-lead electronic device e.g. a resistor or any type of non-linear device such as silicon and/or germanium rectifier and signal diodes, Zener diodes, Light Emitting Diodes (LEDs), varistors, etc. The host PC used for this system interacts with a National Instruments LabPC+ general-purpose, PCI-based DAQ board, which consists of two 12-bit Digital-to-Analog Converters (DACs) , eight 12-bit Analog-to-Digital Converters (ADCs), two CounterTimers, and 24 channels of read/write programmable Transistor-Transistor Logic (TTL) Digital Input/Output (I/O) Lines. We used the National Instruments LabWindows/CVI and NI-DAQ Software to write a C-based DAQ program to carry out the DC I-V measurements, which we called IV2 (2 = 2nd version). As shown in the figure below, we use DAC0 of the LabPC+ card to apply a voltage between 5.0 volts and +5.0 volts across the 2 leads of the electronic device we want to measure the I-V relation. A 1.0 Ohm shunt resistor is inserted in series with the 2-lead device, for the purpose of measuring the current. We use ADC0 to measure the voltage drop across the 1.0 Ohm resistor, and then use Ohms Law (I = V/R) to obtain the current flowing through the 1.0 Ohm resistor, which is also the current flowing through the device, since they are in series with each other. As mentioned previously, the LabPC+ DACs have 12 bit resolution, and a dynamic range of 10.0 volts. Thus since 212 = 4096 DAC counts, the Least Significant Bit (LSB) or least count corresponds to 10 volts/4096 DAC counts = 2.44 mV/DAC count. Now it turns out that the LabPC+ DACs are only able to source/sink current up to a maximum of ~ 14 mA, thus the maximum voltage drop across the 1.0 Ohm resistor we anticipate would be ~ 14 mV. The LabPC+ 12-bit ADCs have programmable gain, so that the maximum dynamic range of +_5.0 Volts (10.0 Volt span) would have a LSB again of 2.44 mV/ADC count. This ADC resolution is not very useful for measuring small, ~ mV potential differences across the 1.0 Ohm resistor. However, the maximum programmable gain of 100 corresponds to a dynamic range of +_50 mV (100 mV span) which thus has a LSB of 24.4 V/ADC count, which is much more sensitive for our needs. Hence we digitize the voltage across the 1.0 Ohm resistor with ADC0, using a programmable gain of 100 to maximize the resolution on V, and hence the current, I flowing through the circuit.
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V_DAC0
Host PC
LabPC+ DAQ Card V_ADC0
2-Lead Device To Be Measured
1-Ohm Resistor Ground Figure 1: Hardware Setup for IV2 DAQ Experiment We also employ the use signal-averaging techniques in the IV2 DAQ program, to minimize the effects of electrical noise fluctuations and noise pickup associated with the V_ADC0 measurement. For a given DAC0 voltage value, V = V_DAC0 we take 4000 ADC0 measurements, and obtain the average value <V_ADC0>, from which we obtain a mean value of current at this DAC0 voltage, <I(V)>. It turns out that when we set DAC0 voltage to V_DAC0 = 0.000 volts, that we find that <V_ADC0> is NOT precisely equal to 0.000 volts, but something slightly different from zero. This slight voltage offset occurs for multiple reasons the DACs have a slight voltage offset themselves; there also exist small contact potential differences between metals and also the ADCs will have small voltage offsets from zero potential difference. Thus, the first thing the IV2 program does before taking actual I-V data is to take 100 measurements of 4000 ADC0 samples each, for V_DAC0 = 0.000 volts. We call this the so called pedestal, <V_PED0>, or zero-voltage offset for ADC0. We then subtract this from all subsequent measurements of V_ADC0. Thus: <I> = (<V_ADC0> <V_PED0>)/R Now it also turns out that the R = 1.0 Ohm metal-oxide resistor we are using is not precisely R = 1.0 Ohm, but close to it. It is not trivial to accurately measure the resistance of a 1 Ohm resistor with just using e.g. a DVM. There are several ways to do this; we chose to simply calibrate our apparatus by measuring the resistance of knownvalue resistors, then correcting the IV2-obtained value of the 1.0 resistor in the IV2 software until the IV2 measured resistance e.g. of a 330 Ohm resistor, which we measured with a Fluke 77 DVM to be 346.1 Ohms, after subtracting the 0.1 Ohm DVM lead resistance, agreed with each other. The correction calibration factor we needed to apply to the 1.0 Ohm resistor was less than ~ 10%.
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The user needs to log onto the host computer, where the IV2 DAQ program resides with the usual user name, password and domain name (consult with the Lab TA for this information if you dont yet know it). If the PC was in fact rebooted from a cold start power up, then before running the IV2 program, the user will first need to run the NI-DAQ Configuration Utility to fully and properly initialize the LabPC+ interface card. Use the mouse to go to the START menu on the PC, go to Programs, then to National Instruments DAQ, then to the National Instruments DAQ Configuration Utility double click on this. The NI-DAQ Configuration Utility panel will appear. Double-click on the right-most square button in the row of buttons just under the header bar. This is the Run Test Panel button. Double-click on this button and the LabPC+ Test Panel will then appear. Double-click on the Run button with the mouse. If everything is working properly with the LabPC+ DAQ card, you will see some kind of a green trace appear on the Oscilloscope display you should get NO error messages. If you get no error messages, then stop running the LabPC+ Test Panel program and exit the NI-DAQ Configuration Utility. Everything is now set to run the IV2 DAQ program. If you get any kind of error message(s), exit the NI-DAQ Configuration Utility program, shut down the PC (properly/gracefully), then reboot the PC from a cold-start, and then after logging in again, re-run the NI-DAQ LabPC+ Configuration Utility/LabPC+ Test Panel. Everything should work fine this time. If not, please bring this problem promptly to the attention of the Lab TA! Before running the IV2 program, one also needs to check to make certain that the green 50-pin terminal block for the IV2 experiment is in fact connected to the 50-pin ribbon cable, which connects the National Instruments LabPC+ DAQ card to the green 50-pin terminal block. It is very important to note that the 50 pin ribbon cable has a definite polarity associated with it pin 1 of the ribbon cable is denoted by a slightly raised triangle on the light gray 50-pin connectors at both ends of this cable. We have used a red magic marker to highlight this triangle the on light gray 50-pin connectors and their mating connectors at the LabPC+ card and the 50-pin green terminal block in order to make the cable polarity more obvious to the user. The green terminal block has two alligator clip leads connected to it. (This PC is also used for DAQ work for the Physics 303 Advanced/Modern Physics Lab Chaotic Water Drop Experiment, which has its own DAQ electronics, connected to the host PC via the same 50-pin ribbon cable) The black alligator clip is the negative lead, which is connected to analog ground via the 1.0 Ohm shunt resistor. The red alligator clip is the positive lead, which is connected to the output of DAC0. Connect the 2-lead electronic device for which you want to measure the DC I-V relation of, to the red & black alligator leads. Note that for a diode, to forward bias the diode, connect the band end of the diode to the black (negative) lead. To run the IV2 program, double-click on the IV2 icon on the desktop. A LabWindows/CVI project window will appear. Use the mouse to pull down on the Run menu, and click on Run Project. The IV2 main display panel will then appear. Doubleclick on the blue task bar at the top to expand the IV2 display panel to the full screen size. Use the mouse to click on the Initialize DAQ button to initialize the LabPC+ DAQ cards electronics. Then click on the Start button to start the IV2 programs data acquisition. 3
As mentioned previously, the program will first obtain the ADC0 pedestal, <V_PED0> associated with V_DAC0 = 0.000 volts. Then V_DAC0 is stepped in increments of 9.765625 mV (exactly 4 DAC counts to avoid software V & I aliasing problems) for positive increasing DAC0 voltage, until the current exceeds +12.0 mA, or V_DAC0 exceeds +4.95 Volts, whichever comes first. As mentioned above, at each DAC0 setting, 4000 ADC0 samples are taken, and <ADC0> is computed. Then for each V_DAC0 value, we compute <I(V)> (with R = 1/0.932100 Ohms = 1.072846 Ohms) from: <I(V)> = (<V_ADC0> <V_PED0>)/R All of this data is stored in arrays in the IV2 DAQ program. When the upper limit of +12 mA or VDAC0 = +4.95 is reached, the IV2 DAQ program sets V_DAC0 = 0.000 Volts and then begins taking data in the negative V_DAC0 direction, again in increments of 9.765625 mV (exactly 4 DAC counts) mV, until the current exceeds 12.0 mA or V_DAC0 becomes more negative than 4.95 volts, whichever comes first. The program then automatically terminates the data-taking Note that the user can also abort the data-taking at any time simply by using the mouse to click on the STOP button of the IV2 main panel display, but then data-arrays are not properly filled. The user will have to START the DAQ again and allow it to complete normally in order for the data arrays in the program to be properly filled. Once the IV2 data-taking is complete, the user can then use the mouse to pull down on the on-line plot menus there are a total of nine on-line plots that can be viewed, either as linear x-y plots or as semi-log plots: 1.) I vs. V 2.) Static Resistance, R = V/I vs. V 3.) Static Conductance, G = 1/R vs. V 4.) Static Resistance, R = V/I vs. I 5.) Static Conductance, G = 1/R vs. I 6.) Dynamic Resistance, Rd = dV/dI vs. V 7.) Dynamic Conductance/Transconductance, gm = dI/dV vs. V 8.) Dynamic Resistance, Rd = dV/dI vs. I 9.) Dynamic Conductance/Transconductance, gm = dI/dV vs. V Note also that for each of the on-line plots, one can use the mouse to click on either or both of the two (yellow & magenta) snap-to-data cursors. The cursor X-Y display ports for each cursor will tell you the X-Y data values of each cursor for the plot you are currently looking at. Note further that for each new on-line plot drawn, the cursor data ports update only when the user activates these cursors, each time for each plot. The user can also move the cursors along the data curves using the up/down and/or left/right arrow keys on the keyboard . Hard-copies of the on-line plots can be made by using the mouse to click on the PRINT button, just underneath the plot display on the IV2 main panel display.
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The IV2 DAQ program is not without some physical limitations. As you wi...