Reference Network for Pipelined ADC MDAC Stability

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Solution Overview

Problem

Pipelined analog-to-digital converters (ADCs) face accuracy degradation due to errors in reference signals, particularly in high-speed charge-injection and charge-extraction currents, which are critical for maintaining conversion accuracy in switched-capacitor multiplying digital-to-analog converters (MDACs).

Innovation Solution

A reference network with a feedback structure, including transistors and resistors, is designed to provide stable reference signals by reducing differences between feedback and predetermined voltages, using cascode transistors to isolate circuit paths and enhance control over common-mode and differential signal levels, and employing capacitors to stabilize reference signals against disturbances.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Productivity

If high-speed charge-injection and charge-extraction currents are used in switched-capacitor MDACs, then conversion speed and sample rate are improved, but reference signal stability and conversion accuracy deteriorate due to errors being directly transferred into the gained-up residue signal

Engineering Contradiction:

Improvesample rateVSAvoidconversion accuracy

Core Design Contradiction:

ProductivityVSMeasurement precision

Solution Approach 1:

A feedback network is implemented that senses the actual reference signal levels and automatically adjusts them to compensate for errors introduced by high-speed charge-injection and charge-extraction currents. The feedback mechanism continuously monitors and corrects reference signal drift, maintaining accuracy even at high sample rates up to 250 MSPS.

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

Cascode transistors are introduced as intermediary devices between the reference signal sources and the switched-capacitor MDAC circuits. These cascode structures act as buffer stages that isolate the reference signals from the high-speed switching transients, preventing direct coupling of errors while maintaining signal integrity.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Measurement precision

If reference signal levels are maintained with high precision, then conversion accuracy is improved, but circuit complexity increases due to the need for feedback networks and cascode transistors

Engineering Contradiction:

Improvereference signal accuracyVSAvoidcircuit structure

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The reference network is segmented into functionally independent modules: reference signal generation units, feedback sensing units, and adjustment units. Each module performs a specific function and can be independently optimized. The cascode transistors are also segmented into separate stages that can be independently designed and tuned.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Different parts of the reference network are designed with different properties optimized for their specific functions. The feedback network uses high-precision components for sensing, while the cascode transistors use optimized geometry for isolation. This localized optimization achieves high overall accuracy without uniformly increasing complexity throughout the entire circuit.

Inventive Principle:

Principle #3Local quality

Data Source

PatentUS7268720B1Converter networks for generation of MDAC reference signals

Publication Date: 2007.09.11 ANALOG DEVICES INC

AI summary

Reference network embodiments are disclosed that provide reference signals to, for example, switched-capacitor multiplying digital-to-analog converters (MDACs) in pipelined analog-to-digital converters (ADCs). These embodiments are configured to maintain accuracy of the levels of the reference signals in the presence of high speed charge-injection and charge-extraction currents which are presented by the MDACs.