Nested Feedback Offset Cancellation in Wireline Receivers

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Solution Overview

Problem

Wireline receivers face challenges in optimizing DC offset cancellation across multiple tap points, leading to asymmetrical saturation and reduced signal-to-noise ratio due to introduced offsets at various stages of the receiver path.

Innovation Solution

The implementation of multiple offset compensation digital-to-analog converters at strategic stages, with downstream offset information used to set targets for upstream cancellation loops, and the use of digital accumulators to apply offset compensation codes, ensuring error-free operation even during changes in D/A codes, by summing and comparing offset compensation codes against a digital reference.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If multiple offset compensation D/A converters are applied at different stages, then offset cancellation effectiveness is improved, but device complexity increases

Engineering Contradiction:

Improveoffset cancellation effectivenessVSAvoidnumber of D/A converters and control loops

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The offset cancellation function is segmented into multiple independent D/A converters placed at different stages of the receiver path. Each converter handles offset compensation locally at its specific stage, dividing the overall cancellation task into manageable segments that can operate semi-independently, thus improving effectiveness without overwhelming complexity

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The offset compensation system is structured as nested feedback loops where downstream offset information feeds into upstream cancellation loops. The inner loops handle local offset compensation while outer loops provide broader control, creating a hierarchical structure that manages complexity through organized nesting of control functions

Inventive Principle:

Principle #7Nested doll (Nesting)

2Adaptability or versatility

If D/A converter codes are updated dynamically, then offset compensation adaptability is improved, but bit error rate increases during code transitions

Engineering Contradiction:

Improveoffset compensation adaptabilityVSAvoidbit error rate during code changes

Core Design Contradiction:

Adaptability or versatilityVSReliability

Solution Approach 1:

Before updating D/A converter codes, the system performs preliminary actions including validating the new codes, preparing transition sequences, and ensuring synchronization across all converters. This preliminary preparation prevents erroneous code transitions that would cause bit errors, allowing dynamic adaptability while maintaining reliability during transitions

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The system implements protective measures beforehand during code updates, such as using guarded transition states, validation checks, and rollback mechanisms. These cushioning measures prevent harmful effects from incomplete or erroneous code transitions, ensuring that adaptability updates do not compromise bit error-free operation

Inventive Principle:

Principle #11Beforehand cushioning (Prior cushioning)

3Measurement precision

If offset compensation range is expanded, then cancellation effectiveness is improved, but voltage amplifier saturation occurs

Engineering Contradiction:

Improveoffset cancellation effectivenessVSAvoidvoltage amplifier saturation

Core Design Contradiction:

Measurement precisionVSObject-affected harmful factors

Solution Approach 1:

Each D/A converter is configured with appropriate compensation ranges tailored to the specific offset conditions at its stage. Rather than using a uniform large range throughout the system, each converter applies locally-optimized compensation parameters that are sufficient for its position in the signal path, achieving effective cancellation without excessive ranges that would cause amplifier saturation

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The system applies offset compensation in controlled partial amounts at each stage rather than attempting to cancel all offsets in a single large correction. Each D/A converter contributes a partial compensation action that accumulates through the signal path, achieving complete cancellation through multiple small steps that avoid overwhelming any single voltage amplifier

Inventive Principle:

Principle #16Partial or excessive action

Data Source

PatentUS11509338B2Nested feedback for offset cancellation in a wireline receiver

Publication Date: 2022.11.22 ANALOG DEVICES INC

AI summary

Systems and methods are provided for optimizing offset compensation in a receiver with multiple offset compensation D/A converters. At each stage where offset cancellation is applied, there is a fan-out of two or more. At the final stage, comparator offset compensation codes are summed and compared against a digital reference. In one version the digital reference is zero. A second implementation has a non-zero digital reference which is the sum of comparator offsets stored from start up. The difference between the sum of offsets and digital reference is applied to a digital accumulator. The most significant bits of the digital accumulator are applied to a digital D/A converter, which cancel analog offsets in an intermediate stage of amplifiers. The summation of offsets feeding into an accumulator is implemented for all preceding stages.