Coded data generation or conversion – Analog to or from digital conversion – Differential encoder and/or decoder
Reexamination Certificate
1998-08-28
2001-02-06
JeanPierre, Peguy (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Differential encoder and/or decoder
C341S118000, C341S141000
Reexamination Certificate
active
06184810
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates generally to the field of electronic devices and more particularly to method and apparatus for generating analog and digital signals.
BACKGROUND OF THE INVENTION
Mixed signal generation circuits capable of generating both digital and analog signals are useful in a variety of applications, including generating test signals for transmission to various circuit elements. Traditional mixed signal circuit testers generate digital and analog signals using two distinct types of signal sources. Digital signals are typically produced by a digital subsystem and associated digital pin card electronics. Such a system typically comprises a digital vector memory coupled to a one-bit digital-to-analog converter, which essentially serves as an amplifier. Analog signals are generally produced by separate analog instruments, such as arbitrary waveform generators.
A typical arbitrary waveform generator may comprise a bank of memory storing a set of waveform samples. These samples are passed to a multi-bit digital-to-analog converter, which generates an analog signal from the samples received. A problem with this method when employed for on-chip self testing is that implementing the multi-bit digital-to-analog converter requires substantial substrate area. In large testers or on a chip, cost constraints generally prohibit implementing large numbers of these analog instruments, ultimately limiting the volume of mixed test signals produced. The limited analog resources often lead to longer test times, since the inputs must be stimulated one at a time.
Another approach to generating mixed signals is to generate a bitstream with a ring oscillator, and to apply the bitstream to a sigma delta modulator to create a noise-shaped sample. The noise shaped sample is then applied to a one-bit digital-to-analog converter. The output of the digital-to-analog converter is then filtered to form a smooth analog waveform. A problem with this approach is that the resulting analog signal is limited to 20 decibels below full scale. Devices under test typically require full analog stimuli, limiting the value of this approach. Another problem with this method is that the sigma delta modulator hardware is expensive to implement.
SUMMARY OF THE INVENTION
In accordance with the teachings of the present invention, a signal generation circuit is provided that eliminates or substantially reduces problems associated with prior approaches. According to one embodiment of the present invention, a signal generation circuit comprises a memory operable to store a digital signal comprising a stream of one-bit samples that were generated using sigma delta modulation. The signal generation circuit further comprises a signal modification circuit coupled to the memory and operable to receive the digital signal from the memory and to introduce a DC level shift to the digital signal, wherein the output of the signal modification circuit can be filtered to produce an analog signal.
The present invention has several important technical advantages. Because the characteristics of the required analog waveforms are known a-priori, expensive sigma delta modulation hardware may be replaced with a software equivalent. On-chip test circuitry may be simplified by eliminating a multi-bit digital to analog converter. The invention also makes efficient use of existing circuit tester hardware. For example, the existing memory required for digital vector storage can be reused for analog waveform storage. Additionally, circuit elements required to produce DC level shift voltages may be used to program both analog and digital signals.
The invention facilitates production of full scale analog and digital signals from a single channel card. A channel card may refer to a tester resource which supplies stimulus to a device under test and/or measures responses from a device under test. Consequently, a signal tester can test a device with many digital pins and few analog pins, or a device with many analog pins and few digital pins. Additionally, production of full scale analog signals eliminates problems associated with prior approaches that produced analog signals limited to 20 decibels below full scale.
Flexibility in the invention's design facilitates placement of a low pass filter for filtering analog signals on a device-specific test board, on a channel card (thus creating a true per-pin mixed signal test source), or on the device itself. The invention helps to facilitate cross compilation of mixed signal test programs implemented from designer-generated test stimuli. Presently, digital signals can be cross compiled from designer-generated test vectors using commercially available computer aided drafting tools. Although there is currently no mixed signal equivalent to this process, the present invention may be effective in providing a solution in the future.
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patent: 5731772 (1998-03-01), Mikkola et al.
patent: 5781137 (1998-07-01), Knudsen
patent: 5909186 (1999-06-01), Gohringer
patent: 5969653 (1999-10-01), Sachdev
patent: 5974363 (1999-10-01), Gammel et al.
Brady III Wade James
Jean-Pierre Peguy
Stewart Alan K.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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