Coded data generation or conversion – Analog to or from digital conversion – With particular solid state devices
Reexamination Certificate
1999-11-04
2001-08-07
Williams, Howard L. (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
With particular solid state devices
C341S144000
Reexamination Certificate
active
06271779
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to current cells and to a digital-analog converter (DAC) wherein those current cells are used. A DAC outputs electrical currents according to numerical values indicated by digital signals. DACs are employed, for example, in integrated circuits and the like for processing video signals.
2. Description of Related Art
One known current cell used in DACs is the current cell described in U.S. Pat. No. 5,452,014.
As noted in this U.S. patent, a current cell, in principle, comprises one current source and two switches. One switch is a switch that controls current dumping, which is used for making and breaking connections between a constant current source and ground. The other switch is a switch that controls current output, which is used for making and breaking connection between the current source and a current output terminal. When the current dumping switch is closed and the current output switch is open, the current output from the current source is dumped to ground. When, on the other hand, the current dumping switch is open and the current output switch is closed, the current output from the current source is output from the current output terminal to the outside.
Switching transistors can be used for these two switches. In current cells wherein switching transistors are used, there are cases where the transistors are not in a completely on state at exactly the same time. In such cases, the current source cannot output currents at the design values, for which reason the output voltage of the current source rises abnormally. Accordingly, when, subsequent thereto, one of the switching transistors attains a completely on state, a very large current flows to that switching transistor. Parasitic inductance is present in integrated circuit lead wires and in patterns printed on circuit boards. For that reason, large current fluctuations cause cross-shorting and ringing, etc. In DACs mounted in integrated circuits used for video signal processing, cross-shorting and ringing cause deterioration in picture quality.
SUMMARY OF THE INVENTION
A first object of the present invention is to provide a current cell wherein current fluctuations in the constant current supply are suppressed.
A second object of the present invention is to provide a DAC wherewith the occurrence of cross-shorting and ringing can be prevented.
A current cell according to a first invention comprises: constant current supply means connected to a first power supply line; first switching means for dumping currents input from the constant current supply means when a first selection signal is at the on level, via an internal node, to a second power supply line; second switching means for outputting currents input from the constant current supply means when a second selection signal is at the on level, via the internal node, to the outside; first selection means for generating the first selection signal by inverting a digital signal, which are driven by potential differences appearing between the internal node and the second power supply line; and second selection means for generating the second selection signal by inverting an inverted digital signal, which are driven by potential differences appearing between the internal node and the second power supply line.
In the first invention, the first and second selection means are activated by potential differences between the internal node and the second power supply line. Accordingly, the activation timing for the first and second selection means is slower when changing from the on level to the off level than when changing from the off level to the on level. For that reason, the first and second selection means will not turn off simultaneously, wherefore current fluctuations in the constant current source are suppressed.
A digital-analog converter according to a second invention comprises: decoder means for converting a plurality of data bits to a plurality of digital signals; a plurality of current cells that comprise: constant current supply means connected to a first power supply line; first switching means for dumping currents input from the constant current supply means when a first selection signal is at the on level, via an internal node, to a second power supply line; second switching means for outputting currents input from the constant current supply means when a second selection signal is at the on level, via the internal node, to the outside; first selection means for generating the first selection signal by inverting a digital signal, which are driven by potential differences appearing between the internal node and the second power supply line; and second selection means for generating the second selection signal by inverting an inverted digital signal, which are driven by potential differences appearing between the internal node and the second power supply line; and an output terminal for outputting the sum of the currents output by the current cells.
The digital-analog converter according to the second invention employs current cells wherein the current from the constant current source is stable, wherefore the occurrence of cross-shorting and ringing can be prevented.
REFERENCES:
patent: 4393370 (1983-07-01), Hareyama
patent: 5331322 (1994-07-01), Cha et al.
patent: 5452014 (1995-09-01), Manley
patent: 6100830 (2000-08-01), Dedic
patent: 7-221642 (1995-08-01), None
Jones Volentine PLLC
Oki Electric Industry Co., Ltd
Williams Howard L.
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