Electronic digital logic circuitry – Signal sensitivity or transmission integrity – Bus or line termination
Reexamination Certificate
2003-03-26
2004-11-02
Wamsley, Patrick (Department: 2819)
Electronic digital logic circuitry
Signal sensitivity or transmission integrity
Bus or line termination
C326S024000
Reexamination Certificate
active
06812735
ABSTRACT:
FIELD OF INVENTION
The present invention relates generally to integrated circuits and specifically to termination resistor circuits.
DESCRIPTION OF RELATED ART
Output driver circuits for driving cables which interconnect integrated circuits (ICs) are well-known. For example,
FIG. 1
shows a well-known driver
100
fabricated using CMOS technology. NMOS transistors MN
1
and MN
2
form a differential pair which, in response to a differential voltage signal V
2
−V
1
, steers a bias current I
bias
between terminating resistors
102
and
104
, respectively, to produce a differential output signal between output nodes OUT_
1
and OUT_
2
. Resistor
102
sets the minimum voltage at node OUT_
1
, and thus controls the voltage swing at node OUT_
1
. Similarly, resistor
104
sets the minimum voltage at node OUT_
2
, and thus controls the voltage swing at node OUT_
2
. The output signals on output nodes OUT_
1
and OUT_
2
may be used to drive a load
106
via transmission lines T
1
and T
2
, which have a characteristic impedance Z
T
of between 50 ohms and 300 ohms.
To minimize signal reflections on transmission lines T
1
and T
2
, terminating resistors
102
and
104
, as well as the resistance of load
106
, is chosen to match the characteristic impedance Z
T
of transmission lines T
1
and T
2
. Typically, resistors
102
and
104
are passive resistive elements such as, for example, polysilicon thin film resistors. However, because of process variations inherent in the fabrication of semiconductor circuits (e.g., imprecise doping and photolithographic techniques), as well as temperature-dependent operating characteristics, such passive resistors may vary as much as 20%, which may be unacceptable for some communication applications.
For improved precision, passive resistors
102
and
104
may be replaced by active resistive elements such as, for example, NMOS transistors
202
and
204
, as shown in FIG.
2
. As well-known in the art, resistive transistors
202
and
204
are operated in the triode region as voltage-controlled resistances having gates to receive a control voltage V
CTL
. However, although more accurate than polysilicon resistors
102
/
104
, transistors
202
and
204
may vary as much as 10% because of process and temperature variations. In addition, the p
junctions within transistors
202
and
204
(e.g., source/well and drain/well junctions) may add significant capacitance loading to output nodes OUT_
1
and OUT_
2
, which in turn undesirably limits circuit speed.
Accordingly, there is a need for precise termination resistors fabricated using current CMOS processes that are insensitive to temperature and process variations and which have a minimal impact upon circuit speed. In addition, for applications where the characteristic impedance of the transmission lines is not known, it would be desirable for a user to be able to change the value of the termination resistors.
REFERENCES:
patent: 4972098 (1990-11-01), Boudewijns
patent: 5617064 (1997-04-01), Gorecki
patent: 5757264 (1998-05-01), Petit
patent: 5955911 (1999-09-01), Drost et al.
patent: 6356106 (2002-03-01), Greeff et al.
patent: 6414512 (2002-07-01), Moyer
Paradice III William L
Silicon Bridge Inc.
Wamsley Patrick
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