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Optimized serial line termination for high speed circuits push-pull drivers

IP.com Disclosure Number: IPCOM000014012D
Original Publication Date: 2001-Mar-01
Included in the Prior Art Database: 2003-Jun-19
Document File: 2 page(s) / 39K

Publishing Venue

IBM

Abstract

Disclosed is a circuit that optimises the serial line termination for high speed push-pull drivers. The invention consists in replacing a single series resistor by a network made of two resistors and one Schottky diode. A point to point transmission line is made of one driver , the transmission line, the receiver and the termination. The classic series termination consist in a serial resistor Rs located at the front end of the line back to the driver (see Figure 1). DRIVER Vdd

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Optimized serial line termination for high speed circuits push-pull drivers

Disclosed is a circuit that optimises the serial line termination
for high speed push-pull drivers. The invention consists in
replacing a single series resistor by a network made of two
resistors and one Schottky diode.

 A point to point transmission line is made of one driver , the
transmission line, the receiver and the termination. The classic
series termination consist in a serial resistor Rs located at the
front end of the line back to the driver (see Figure 1).

DRIVER

Vdd

 N Gnd

Ri

Ri

PRs

Z0

Figure 1

The series termination resistor "Rs" added to the internal resistor
"Ri" of the driver is supposed to match the line impedance Zo.
"Rs+Ri= Zo". A perfect match will deliver a very clean signal at
the receiver input with no ringing (overshoot/undershoot) at the
receiver input. However In very fast circuits (rise time/fall time
<500 picoseconds), this desirable condition of perfect match is
never met due to dissymmetry of the Pushpull driver itself.

 The invention consists in replacing the single series resistor Rs
by a network made of two resistor R1 and R2 and one Schottky diode

1

RECEIVER

Classical series termination

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SD (see Figure 2). This arrangement may be chosen to provide a
perfect impedance match of the driver to the line impedance for
rising edge as well as falling edge reducing ringing for high-speed
circuits .This deliver cle...