Differential Output Driver With Source Termination for Signal Integrity
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Solution Overview
Problem
As differential serial data rates increase, minor impedance discontinuities in transmission lines cause significant reflection, negatively impacting the slew rate of signals in programmable logic devices, particularly in FPGAs, due to the transmission line effect.
Innovation Solution
An output driver configuration that includes a differential driver coupled with single-ended drivers to provide source termination resistances, allowing the differential driver to operate in an open-drain mode, thereby reducing impedance discontinuities and reflection by trimming the source termination impedance to match the transmission line impedance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If differential driver operates in conventional push-pull or open-drain mode, then device complexity is reduced, but impedance discontinuities cause significant reflection and degraded signal quality at high data rates
Solution Approach 1:
The patent combines single-ended drivers with the differential driver by coupling their output nodes together. The single-ended drivers provide source termination resistances that work in conjunction with the differential driver to reduce impedance discontinuities and signal reflection, thereby improving signal quality without requiring a completely separate termination circuit.
Solution Approach 2:
The single-ended drivers serve multiple functions: they provide source termination resistance for impedance matching, contribute to differential signal generation, and reduce reflection. This multi-functionality allows the circuit to achieve better signal integrity without proportionally increasing complexity, as the same components perform multiple roles.
2Reliability
If source termination resistance is added to reduce reflection, then signal integrity improves, but circuit complexity increases
Solution Approach 1:
Instead of adding separate termination resistors, the patent merges the termination function with the existing single-ended drivers. The output nodes of the single-ended drivers are coupled to the differential driver output nodes, and these drivers inherently provide the source termination resistance needed for impedance matching.
Solution Approach 2:
The single-ended drivers are configured to simultaneously provide differential signaling and source termination. This multi-functionality eliminates the need for dedicated termination components, achieving signal integrity improvement without proportional complexity increase.
3Speed
If transmission line impedance is matched to reduce reflection, then slew rate improves, but device complexity increases
Solution Approach 1:
The patent merges impedance matching functionality into the driver circuitry itself. By coupling single-ended drivers to the differential driver output nodes, the source termination resistance is provided directly at the output stage, enabling transmission line impedance matching without adding separate matching networks.
Solution Approach 2:
The driver circuit is designed to simultaneously perform differential signaling, provide source termination for impedance matching, and maintain fast slew rates. This multi-functionality allows the circuit to achieve high-speed performance with impedance matching without requiring additional dedicated components for each function.
Data Source
AI summary
Integrated circuits for an output driver and an output interface, as well as a method for operating an output driver, are described. In an embodiment of an integrated circuit for an output driver, a differential driver is coupled to a first single-ended driver at a first output node of the first single-ended driver and the differential driver. A second single-ended driver is coupled to the differential driver at a second output node of the second single-ended driver and the differential driver. The first single-ended driver provides a first source termination resistance for an open-drain mode of the differential driver, and the second single-ended driver provides a second source termination resistance for the open-drain mode of the differential driver.