A Low-Cost Virtual Instrumentation Tester for Teaching Mixed Signal Characterization Concepts

Author(s):
J.R. Porter - Texas A&M University
M.R. Warren - Texas A&M University

Industry:
University/Education

Products:
Data Acquisition, Digital I/O, High-Speed Digital I/O, LabVIEW, Instrument Connectivity

The Challenge:
Creating a low-cost virtual instrumentation tool for teaching mixed-signal integrated circuit characterization concepts. The tester must have the functionality needed for students to properly learn mixed-signal testing techniques, but must be affordable enough for use in academic institutions.

The Solution:
Using LabVIEW, in conjunction with a high-speed digital I/O card, a multifunction DAQ card, and GPIB-interfaced test and measurement equipment, a powerful yet low-cost virtual instrumentation tester for teaching mixed signal characterization can be created.

Abstract
Characterizing mixed-signal integrated circuits typically requires extremely high-end test equipment, ranging in cost from fifty thousand to several million dollars. While such equipment is available in industry, it is too expensive for the typical academic laboratory. Using standard academic-quality electronic lab equipment, National Instruments data acquisition (DAQ) cards and LabVIEW, we have implemented a low-cost tester capable of demonstrating analog-to-digital (A/D) and digital-to-analog (D/A) converter testing in an academic environment. The tester is designed to be modular, so that students can "mix and match"virtual instrumentation components and design their own test set-ups.

Low Cost - High Functionality
Presently, the Electronics Engineering Technology program at Texas A&M University offers two courses in mixed signal testing. While these courses are currently being taught using a Teradyne A567 production tester, the cost of such a device makes it prohibitively expensive for many universities. For this reason, a smaller, more cost-effective station is necessary to teach a mixed signal curriculum. Using National Instruments LabVIEW virtual instrumentation development environment, a multifunction data acquisition PCI card and a high-speed digital I/O PCI card, a low cost virtual instrument (VI) mixed signal characterization tester has been created as shown in Figure 1. As implemented, this tester currently allows students to test eight bit-mixed signal devices and demonstrates concepts such as:

• D/A converter characterization
• A/D converter characterization
• Appropriate equipment selection
• Focused calibration concepts
• Noise issues
• Sampling rate and coherency issues

By using a National Instruments PCI-DIO-32HS digital I/O card, a digital pattern generator and analyzer has been developed for creating, capturing, and analyzing digital signals sent to and read from a device under test (DUT). The card also allows the virtual instrumentation software to control registers and produce clock signals needed by a DUT. The card can also be configured for either serial or parallel communication between the software and the DUT. In addition, the four digital ports give room for later expansion. For example, a high precision D/A is currently being added to give the VI the ability to test A/D converters with more than eight bits of resolution.
Next, by using a PCI MIO-16XE multifunctional DAQ card, the system gains the ability to accurately digitize incoming analog signals for D/A testing and to produce arbitrary output waveforms for A/D testing. Finally, by adding a general-purpose interface bus (GPIB) card, the standard bench equipment found in most academic electronics labs can be linked to allow even more flexibility over the control of the DUT’s environment. Such equipment might include digital multimeters (DMM), power supplies, and arbitrary waveform generators (AWG). For example, having remote control over the power supply, one can perform power supply sensitivity tests.

Modular by Design
To ensure that the tester is flexible, both the software and hardware have been designed to be as modular as possible. Through the use of modular sub-VIs, students can easily "mix and match" subroutines creating the ability to quickly design and run any test. For example, the sub-VI’s are organized in such a way that it is very easy to convert a parallel I/O DUT test to a serial I/O test. Also, all of the data manipulation VI’s have been designed so that students can replace any portion of a test in order to validate their own routines. By allowing a saved data set to be used as the input into the student’s routine, the routine can be validated without having to have run a physical test. This type of error checking of a test VI allows one to modularly troubleshoot a test that is not performing up to expectations.
The hardware aspect of the tester gives the student the ability to perform a single test with different types of instrumentation in order to teach appropriate equipment selection. As an example, to perform a simple ramp test of a D/A, the test VI is required to send the digital code to the DUT and measure the output. The end measurement can be taken with the DAQ card, the precision multimeter, or the low-cost digital multimeter. By running the test with three different types of instrumentation the student learns the necessity of choosing the most effective piece of equipment to run the test with. By keeping the hardware modular, more flexibility is given to the students in deciding how a test should be performed and thus learning from trial and error.

Focused Calibrations for Accuracy
One of the drawbacks to the virtual instrumentation tester is the inaccuracy inherent in low cost measurement equipment. Fortunately, this demonstrates a problem that one also sees on a high-end tester and can be corrected. Through the use of the GPIB bus, one can integrate more precise measurement equipment into the tester and use focused calibrations to correct measurement equipment errors. For example, one can take the calibration of a high precision multimeter and transfer it to the VI’s DAQ card. By calibrating the analog section of the system to the precision multimeter, errors in offset and gain of the measurement equipment can be calibrated out, creating reliable input and output analog voltages. By giving the students the ability to either perform focused calibrations or not perform calibrations on a particular test, they can see how instrument error effects the end result and how a focused calibration can increase the accuracy of their test.

Conclusions
By keeping the functionality high and the price relatively low, a low-cost virtual instrumentation tester is ideal as the centerpiece for a lab-based mixed signal curriculum. Where a high-production tester would allow for faster and more accurate testing, the low-cost virtual instrument tester gives the student a hands-on experience of the basics needed to use the high-end testers. Through modular sub-VI’s and affordable PCI cards, the virtual instrumentation tester gives the student the experience needed to excel in the area of mixed signal test.

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