Coded data generation or conversion – Analog to or from digital conversion – Analog to digital conversion
Reexamination Certificate
2001-07-14
2002-08-06
Tokar, Michael (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Analog to digital conversion
C341S156000, C341S158000, C341S159000, C341S160000, C341S152000, C341S118000, C341S122000, C341S161000
Reexamination Certificate
active
06429799
ABSTRACT:
TECHNICAL FIELD
The invention relates to processing of analog signals. In particular, the invention relates to analog to digital conversion of analog signals using timestamps of characteristic signal events.
BACKGROUND ART
Analog signals are generated by and/or used in a wide variety of devices and systems. In many of these systems, the analog signals serve as a means of transferring information from one portion of the system to another. Devices that make up systems employing analog signals function to generate, modify, receive and/or detect the analog signals. Examples of systems or devices that make use of analog signals include sensors for monitoring environmental or other system conditions and a wide variety of different communications systems.
In many practical situations encountered in the real world, it is necessary or at least desirable to transform analog signals into a digital representation. This is especially true in cases where digital methodologies are used largely to process and analyze the analog signals. For example, most manufacturers of integrated circuits (ICs) employ some form of automated test equipment (ATE) to test the IC products being manufactured. While ATEs are overwhelmingly implemented based on digital technologies, many of the modem ICs that are being manufactured and tested, produce or use analog output signals. This has become particularly true as modem system-on-a-chip devices are transitioned from the concept to the product phase. The problem for the designers and users of ATEs is how to transform analog signals into a format that can be utilized by the digital ATE. A related problem is the reconstruction of the analog signal from the digital representation.
The conventional approach to converting an analog signal into a digital representation is to use an analog to digital converter (ADC). Conventional ADCs sample the amplitude of the analog signal or waveform at successive, regularly spaced, points in time. The sampled amplitude values are converted to a digital format (i.e., digitized) by one of several approaches well known in the art. Once digitized, the analog signal is represented by a sequence of digital values representing the amplitudes sampled by the ADC. Normally, timing of the digital values in the amplitude sequence is known implicitly from the conversion scheme being used. Among the commonly employed ADC approaches known in the art are the over-sampling converters, such as the delta-sigma modulator-based ADCs, the successive approximation ADCs, and the so-called flash ADCs. Each of these technologies ultimately produces a string of digital words, each word representing a sampled amplitude value in digital form, in a time sequence at regularly spaced time intervals.
The analog signal can be reconstructed from the digital words produced by the conventional ADC using a digital to analog converter (DAC). The DAC ‘reads’ or processes each successive digital word in the time-sequence and produces an analog voltage level at a DAC output port that corresponds to each of the words. By reading the digital words in a manner that is consistent with the order and timing of the original analog to digital conversion, the DAC can accurately reconstruct the analog signal.
While conventional amplitude sampled analog to digital conversion or amplitude sequence analog to digital conversion can provide high fidelity conversion of analog signals to a digital form, the conventional ADCs can be costly to implement in some instances. In particular, many of the conventional ADC technologies are not well suited for simple, accurate on-chip implementations. This is especially true when considering on-chip conversion of analog signals for built-in-self-test (BIST) purposes or in design for test (DFT) instances used in conjunction with an external digital ATE. Similarly, the use of conventional ADC approaches as an interface between an analog device and an ATE can pose many problems, not the least of which is the need for extra dedicated resources in the ATE to accommodate the often high data rate digital signals generated by a conventional time-sampling ADC. Finally, the bandwidth of many conventional ADCs is severely limited by the circuitry necessary to affect the analog to digital conversion, especially when many bits of amplitude accuracy are desired.
Accordingly, it would be advantageous to have a method and apparatus for transforming an analog signal into a digital representation that preserved key characteristics of the analog signal and that could optionally provide for accurate signal reconstruction from the digital representation. In addition, it would be beneficial if such a method and apparatus could be applied to any analog signal, had high bandwidth capability, and could be implemented efficiently either on-chip or off-chip. Such a method and apparatus would solve a long-standing need in the area of analog to digital signal conversion, especially as the conversion relates to processing and testing of analog signals by digital systems such as ATEs.
SUMMARY OF THE INVENTION
The present invention is a novel method and apparatus for converting analog signals into a digital representation consisting of timestamps. The digital representation of the analog signal produced by the method and apparatus of the present invention is based on a time sequence and not on a conventional amplitude sequence. Instead, the method and apparatus of the present invention produces a sequence of digitized time samples at or corresponding to the occurrence of a set of predetermined amplitude events within the analog signal. In other words, the present invention maps the analog signal to a series of events and records the time of occurrence of these events. The events according to the present invention are defined by an amplitude relationship between an analog signal or signal under test (SUT) and one or more time-varying reference signals. The time record of the occurrence of the events produced by the method and apparatus of the present invention is, or at least can be thought of as, a sequence of timestamps. The timestamp sequence generated for an analog signal by the method and apparatus of the present invention combined with knowledge of the events associated with the timestamp sequence can provide enough information to allow the reconstruction of the signal from the timestamps.
In one aspect of the present invention, a method of converting an analog signal into a timestamp representation is provided. In particular, the analog signal may be either a signal produced at an output of a device under test, a signal internal to the device under test, or a signal received from an unspecified analog signal source. The method comprises the step of generating a quantity N of timing-varying reference signals. The quantity N is an integer equal to or greater than one. The method further comprises the step of comparing the analog signal to the quantity N of time-varying reference signals. During the step of comparing a determination is made as to whether an amplitude of the analog signal is greater than, equal to, or less than one or more of amplitudes of the N time-varying reference signals. The method further comprises the step of producing a timestamp each time the analog signal amplitude is determined to be equal to the amplitude of one of the time-varying reference signals. The method of converting further comprises an optional step of storing the timestamps and an optional step of reconstructing the analog signal from the timestamps. Furthermore, the step of comparing comprises either simultaneously comparing the analog signal to the quantity N of reference signals in parallel or sequentially comparing the analog signal to each one of the quantity N of reference signals until all of the reference signals have been compared.
In a preferred embodiment, the step of producing a timestamp comprises the step of establishing logic levels in a quantity N of digital signals, one digital signal corresponding to each different one of the N reference signals. A first logic level is
Kamas Linda Argon
Rivoir Jochen
Mai Lam T.
Tokar Michael
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