Negative-Resistance Input Cancellation for Low-Distortion Isolation

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Solution Overview

Problem

Existing input cancellation circuits face challenges in isolating inputs without introducing significant distortion and reducing bandwidth, as series switches with low resistance can cause parasitic capacitance and expose the oxide to damage when the input voltage exceeds the supply voltage.

Innovation Solution

Implementing a negative resistance path with a current or voltage controlled current source that matches the input resistance, allowing for effective signal cancellation by equalizing currents through the input and negative resistance paths, and using switches or transistors that can be turned off during operation to prevent damage.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Manufacturing precision

If the series switch resistance is made small to reduce distortion, then the distortion during normal operation is reduced, but the parasitic capacitance increases and bandwidth is reduced

Engineering Contradiction:

ImprovedistortionVSAvoidbandwidth

Core Design Contradiction:

Manufacturing precisionVSSpeed

Solution Approach 1:

The patent inverts the conventional approach by using a large resistance value in the isolation switch rather than a small one. This counterintuitive solution reduces parasitic capacitance effects and preserves bandwidth while still achieving effective isolation. The large resistance is compensated by using a parallel capacitor to maintain the isolation function.

Inventive Principle:

Principle #13The other way round (Inversion)

Solution Approach 2:

The patent changes the resistance parameter of the isolation switch from small to large values. This parameter change fundamentally alters the behavior of the circuit, reducing parasitic capacitance and preserving bandwidth. The combination of large resistance with parallel capacitance creates an effective isolation mechanism that differs from conventional small-resistance switches.

Inventive Principle:

Principle #35Parameter changes

2Manufacturing precision

If the series switch resistance is made small to reduce distortion, then the distortion during normal operation is reduced, but the physical size of the switch increases and parasitic capacitance is introduced

Engineering Contradiction:

ImprovedistortionVSAvoidparasitic capacitance

Core Design Contradiction:

Manufacturing precisionVSDevice complexity

Solution Approach 1:

Inventive Principle:

Principle #13The other way round (Inversion)

3Adaptability or versatility

If the input is allowed to exceed the supply voltage to improve dynamic range, then the dynamic range is improved, but the gate oxide may be exposed to damage

Engineering Contradiction:

Improvedynamic rangeVSAvoidoxide damage

Core Design Contradiction:

Adaptability or versatilityVSReliability

Solution Approach 1:

The patent introduces a large resistance element as an intermediary between the input and the switch, which limits the current and protects the gate oxide from damage. This intermediary component allows the input to exceed supply voltage without directly exposing the vulnerable oxide, thereby extending dynamic range while maintaining reliability.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The patent employs a large resistance element that acts as a protective barrier before the voltage can reach levels that would damage the gate oxide. This beforehand cushioning effect limits the stress on the oxide, allowing safe operation with inputs that exceed the supply voltage.

Inventive Principle:

Principle #11Beforehand cushioning (Prior cushioning)

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

This approach effectively isolates inputs without adding distortion and maintains the bandwidth of input circuits, while preventing switch damage by matching resistances and using current-controlled or voltage-controlled sources for signal cancellation.

Implementation Method 1

An input is coupled to an input resistance and a negative resistance. The outputs of the resistance and the negative resistance are coupled to an output.

Methodology Applied

Scientific EffectNegative resistance:

Implementation Method 2

a series resistance coupled to the input, a switch coupled to the series resistance

Methodology Applied

Scientific EffectElectrical Resistance: Electrical Resistance

Data Source

PatentUS20090267639A1Input cancellation circuit

Publication Date: 2009.10.29 ANALOG DEVICES INC

AI summary

A system and method are provided for isolating an input without adding significant distortion and without significantly adversely affecting the bandwidth of input circuits. In one embodiment, a single ended signal is substantially cancelled by an arrangement including an input resistance path in parallel with a negative resistance path wherein both paths substantially match in resistance. In another embodiment, a differential signal is substantially cancelled by a pseudo differential arrangement including two independent input resistance paths each in parallel with a corresponding negative resistance path, wherein the resistance paths substantially match the input resistance paths. In yet another embodiment, a differential signal is substantially cancelled by a differential arrangement including two resistance paths wherein a first negative resistance path is coupled between the first differential input and the second differential output and the second negative resistance path is coupled between the second input and the first output. In yet another embodiment, a current controlled current source may provide the negative amplification for the negative resistance path.