Analog Multiplexer Well Biasing for Current Injection Protection
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Solution Overview
Problem
As CMOS transistor geometries shrink, safe operating voltages decrease, leading to current injection issues that degrade the accuracy of analog-to-digital converters (ADCs) and cause errors in adjacent analog channels, with existing analog multiplexers struggling to handle injected currents while minimizing leakage current and noise coupling.
Innovation Solution
The implementation of contesting well biasing in analog multiplexers, which uses hardening transistors to dissipate or source current injection, keeping the transistors relatively small to maintain low leakage current and improve ADC accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If larger geometry transistors are used in I/O circuits to protect from excessive voltages, then device protection is improved, but leakage current increases
Solution Approach 1:
The protection function is divided between two types of transistors: large geometry transistors for voltage protection and small geometry contesting well biasing transistors for leakage current control. This segmentation allows each transistor type to be optimized for its specific function, resolving the contradiction between protection capability and leakage current.
Solution Approach 2:
The patent changes the operating parameters of the contesting well biasing transistors by adjusting their geometry to be smaller than traditional protection transistors while maintaining protection effectiveness through contesting well biasing mechanism. This parameter change reduces leakage current while preserving the protection function.
2Measurement precision
If contesting well biasing is used to handle current injection, then ADC accuracy is improved, but device complexity increases
Solution Approach 1:
The contesting well biasing transistors serve multiple functions: they protect against current injection, control leakage current, and maintain low noise coupling. This multi-functionality reduces the need for separate circuits for each function, thereby limiting the increase in device complexity while achieving improved ADC accuracy.
Solution Approach 2:
The patent combines the current injection protection function with the leakage current control function into a single contesting well biasing circuit structure. This merging of functions achieves ADC accuracy improvement without proportionally increasing device complexity.
3Loss of energy
If hardening transistors are made smaller to reduce leakage current, then leakage current is reduced, but current injection handling capability deteriorates
Solution Approach 1:
The patent changes the geometry parameters of the hardening transistors to be smaller, which reduces leakage current. Simultaneously, it changes the biasing mechanism to contesting well biasing, which compensates for the reduced size and maintains current injection handling capability. This dual parameter change resolves the contradiction.
Solution Approach 2:
The contesting well biasing mechanism acts as an intermediary that enables small geometry transistors to handle current injection effectively. By introducing this intermediary biasing mechanism, the patent allows smaller transistors to achieve the same current injection handling capability as larger transistors would provide alone.
Data Source
Figure 1~2
Figure 3
AI summary
A multi-branch analog multiplexer (anamux) includes protection circuitry to help dissipate both positive and negative injected current without increasing the size of hardening transistors in each branch, thereby avoiding increased leakage current and enabling an analog to digital converter to operate with the required accuracy. The protection circuitry is tied to the body of the hardening transistor to lower the threshold voltage of the hardening device, thereby enabling the hardening device to handle more of the injected current.