Electronic digital logic circuitry – Interface – Current driving
Reexamination Certificate
2000-11-15
2002-03-26
Tokar, Michael (Department: 2819)
Electronic digital logic circuitry
Interface
Current driving
C326S086000, C326S030000, C326S090000, C327S108000, C327S112000
Reexamination Certificate
active
06362655
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of integrated circuits; more particularly, to methods and apparatus for implementing active resistors for use in driver circuitry.
BACKGROUND OF THE INVENTION
Advances in integrated circuit technology have produced a trend toward the use of low voltage differential signaling and high-speed terminated buses. This development, in turn, has placed greater importance on the implementation of precise, on-die linear resistors and circuit designs capable of driving signals with an output impedance that matches the characteristic impedance of the terminated buses. In the past, linear resistors have been used in integrated circuits as constant impedance drivers and terminators for buses. In addition, linear resistors find use as resistive elements for digital-to-analog converters (DACs) and as active loads for amplifiers.
In prior applications requiring constant impedance drivers, it has been the practice to use discrete resistors coupled in series with a switching device that has almost zero resistance. The drawback of this approach is that the driver transistor that forms the switching device is required to be very large in order to minimize “on” resistance. Obviously, a very large device size is costly in terms of silicon layout area. Additionally, the added gate and source-drain capacitance associated with a large device hinders high-speed performance.
In standard complementary metal oxide semiconductor (CMOS) technology, on-die resistors can be created using transistors operated in the so-called “linear region”. In reality, however, the degree of linearity that can be achieved is limited by the inherent non-linearity present in the drain-source current equation of the transistor. That is, the physical nature of the field-effect transistor causes the current versus voltage relationship of the device to deviate from an ideal straight line. As bus speeds increase and signal swings decrease, deviations from linearity in driver and terminator circuits can become a major problem affecting signal integrity. By way of example, even though the driver circuitry described in the related applications listed above provide an overall improvement in linearity, the percentage deviation of the active resistor designs from the ideal (i.e., a straight line) remains unacceptably large at low drain-source voltages.
Therefore, what is needed is a circuit that achieves better linearity, and which could be used to create on-die termination resistors for conventionally terminated bus schemes and/or low voltage differential signaling (LVDS) technologies, or any analog application requiring precision resistors.
REFERENCES:
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patent: 5841702 (1998-11-01), Kim
patent: 6069491 (2000-05-01), Muller et al.
patent: 6087853 (2000-07-01), Huber et al.
patent: 6194945 (2001-02-01), Bahramzadeh
patent: 6222389 (2001-04-01), Williams
Abraham Isaac P.
Johnson David R.
Blakely , Sokoloff, Taylor & Zafman LLP
Intel Corporation
Tan V.
Tokar Michael
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