Differential Transmitter Impedance Matching via Averaged Voltage Sampling
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Solution Overview
Problem
In high-speed differential signaling, determining proper driver impedance is challenging due to noise on transmitter chips and leakage in capacitances and Field Effect Transistors, which affects signal integrity and impedance matching between transmitter and receiver.
Innovation Solution
A method and apparatus that sample voltage levels at a differential transmitter output, averaging multiple cycles of a slow repeating signal to mitigate noise and select optimal pullups and pulldowns to match impedance with a terminator, using controllable impedance segments like PFETs and NFETs to adjust transmitter impedance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If voltage samples are taken at transmitter output to determine impedance, then impedance matching accuracy is improved, but noise and leakage from capacitances and FETs cause measurement errors
Solution Approach 1:
The patent applies periodic action by taking multiple voltage samples over several cycles of a slow repeating signal and averaging them. This periodic sampling approach allows the system to capture voltage levels at consistent points in each cycle while the averaging process mitigates the effect of random noise and leakage variations that occur at different times
Solution Approach 2:
The patent implements feedback by comparing the averaged measured voltage levels against target voltage levels and using this information to adjust the number of pullups and pulldowns. This closed-loop feedback mechanism continuously refines the impedance matching by selecting the configuration that produces measured levels closest to the targets, thereby overcoming noise and leakage effects
2Adaptability or versatility
If multiple pullups and pulldowns are used to adjust impedance, then impedance matching flexibility is improved, but device complexity increases
Solution Approach 1:
The patent applies segmentation by dividing the impedance adjustment function into multiple independent pullup and pulldown elements that can be individually controlled. Each pullup and pulldown represents a discrete segment of the overall impedance control system, allowing fine-grained adjustment of the transmitter output impedance to achieve precise matching with the receiver termination
Solution Approach 2:
The patent uses partial action by implementing a finite set of discrete pullup and pulldown elements rather than continuous control. This provides sufficient impedance adjustment granularity for practical purposes while avoiding the complexity of fully continuous control mechanisms. The system selects from a limited number of configurations to achieve adequate matching without requiring excessive control elements
Data Source
AI summary
A method and apparatus to adjust a differential transmitter output impedance is disclosed. Output voltage is sampled and averaged to mitigate noise and leakage. Averaged uplevel and downlevel voltages are used to select a number of pullup devices and a number of pulldown devices to control the differential transmitter output impedance to match a distal termination resistance or a transmission line impedance.