Coded data generation or conversion – Analog to or from digital conversion – Digital to analog conversion
Reexamination Certificate
1999-09-29
2001-09-18
JeanPierre, Peguy (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Digital to analog conversion
C341S163000
Reexamination Certificate
active
06292125
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to digital-to-analog converters (DACs). More specifically, a DAC using weights stored in a look up table is disclosed.
2. Description of Related Art
In a DAC, a binary input is converted to an analog voltage or current output that is proportional to the value of the binary input. For an n-bit binary input, the converter output is one of 2
n
possible analog outputs. For a DAC having voltage output, the voltages representing the possible outputs may be obtained with a network of capacitors, resistors, or current sources and current switches connected to the binary input.
Digital-to-analog conversion can be achieved using a number of different methods. One example is a ladder network using a network of resistors. A ladder network typically receives binary input voltages, each input voltage being 0 V or V
reference
, and provides an analog output voltage proportional to the value of the binary input. For example, a ladder network may have four stages for accepting four input voltages, representing four bits of digital data or sixteen possible input values, and an output voltage at one of sixteen possible output levels proportional to the value of the digital data input. Providing more stages in a ladder network allows more bits of binary input and requires greater voltage resolution. In general, the voltage resolution of a ladder network with n ladder stages is V
reference
/2
n
.
A DAC typically contains a set of weights generated by an analog circuit typically scaled in a binary fashion. The DAC receives a binary input which selects certain weights for the output. One problem associated with DACs is that the DAC presumes that the analog voltage or current circuit generates ideal output steps while the analog circuit generates non-ideal output steps. Thus, the DAC output voltage varies from the voltage selected by the binary input. The non-ideal characteristics of the output may be due to such factors as process variations and/or temperature.
What is needed is a DAC that eliminates the non-ideal characteristics of the output and/or compensates for irregularities in order to achieve a more accurate and more reliable digital-to-analog conversion. Ideally, the output of the DAC should be independent of variations in factors that are difficult to control such as the temperature and device characteristics that vary as a result of the manufacturing process. It is also desirable to provide a DAC that can be calibrated and recalibrated over time to take into account non-ideal characteristics of the output steps.
SUMMARY OF THE INVENTION
A system and method for digital-to-analog conversion is disclosed that provides an accurate and reliable digital-to-analog conversion. The DAC is calibrated initially to take into account non-ideal characteristics of the output so as to achieve accurate and reliable digital-to-analog conversion. The DAC can be recalibrated from time to time to take into account drifts in the characteristics of the output. The design of the DAC is such that variances in the manufacturing process for the converter and other factors do not degrade accuracy or reliability. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, a method, or a computer readable medium such as a computer readable storage medium or a computer network wherein program instructions are sent over optical or electronic communication lines. Several inventive embodiments of the present invention are described below.
The DAC generally comprises a plurality of analog weights, a weight table adapted to store digital sizes of the plurality of analog weights, and a converter for searching for selected weights from the plurality of analog weights using the digital sizes stored in the weight table and for mapping a binary input to the selected analog weights. The digital sizes of all except for at least two of the analog weights are successively approximated using the assigned sizes of at least two of the analog weights.
The method for digital-to-analog conversion comprises receiving the binary input, searching for selected weights from analog weights using a weight table storing digital sizes of the analog weights, mapping the binary input to the selected weights, and outputting a sum of the selected analog weights.
In another embodiment, the method comprises successively comparing the digital signal to weights stored in an order according to size of each weight in a weight table, each comparing being between the digital signal and one of the weights, starting with a highest analog weight. The method further includes selecting the weight being compared that is no greater than the digital signal, subtracting the selected weight from the digital signal to generate a digital signal difference, repeating successively comparing, selecting, and subtracting with the digital signal difference instead of the digital signal and with the analog weights not previously compared until after the lowest analog weight is compared, and setting the analog signal as a sum of the selected weights.
These and other features and advantages of the present invention will be presented in more detail in the following detailed description and the accompanying figures which illustrate by way of example the principles of the invention.
REFERENCES:
patent: 4212074 (1980-07-01), Kuno et al.
patent: 4336526 (1982-06-01), Weir
patent: 4451821 (1984-05-01), Domogalla
patent: 4852130 (1989-07-01), Draxelmayr
patent: 4970514 (1990-11-01), Draxelmayr
patent: 5107377 (1992-04-01), Ballard
Jean-Pierre Peguy
LSI Logic Corporation
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