Fully Differential Pixel Sensing Circuit for Drive Current Accuracy
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Solution Overview
Problem
Variations in drive transistor characteristics in display pixels lead to degraded image quality, necessitating a system and method for accurate measurement and compensation.
Innovation Solution
A fully differential sensing circuit is employed to measure and compensate for variations in drive transistor currents, utilizing a low-pass current filter and integrator with wideband common mode feedback to reduce noise and improve accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single-ended sensing circuit is used to measure drive transistor current, then the device complexity is reduced, but the measurement precision deteriorates due to ground noise coupling
Solution Approach 1:
The patent uses a pseudo-differential sensing approach where an off-pixel is copied and used as a reference to mimic the common-mode noise conditions of the on-pixel. This reference pixel's signal is subtracted from the on-pixel signal to eliminate ground noise coupling, achieving noise rejection without requiring a fully complex differential circuit architecture
Solution Approach 2:
The patent introduces an intermediary reference voltage (Vref) that represents the common-mode noise level. This reference voltage is derived from the off-pixel measurement and used to cancel out the ground noise component in the on-pixel measurement, thereby improving measurement precision without substantially increasing circuit complexity
2Measurement precision
If a fully differential sensing circuit is used to reduce noise, then the measurement precision is improved, but the device complexity increases
Solution Approach 1:
The patent employs a pseudo-differential architecture that copies the off-pixel configuration to create a reference signal. This approach achieves noise rejection similar to fully differential circuits but with reduced complexity by using the existing pixel structure as the reference rather than requiring a complete differential pair
Solution Approach 2:
The sensing circuit is designed to perform multiple functions: it measures the on-pixel current, generates a reference signal from the off-pixel, and performs common-mode noise rejection all within a unified circuit structure. This multi-functionality reduces the need for separate dedicated circuits for each function, thereby reducing overall device complexity
3Measurement precision
If differential sensing is used to reject common mode noise, then the measurement precision is improved, but leakage currents increase
Solution Approach 1:
The patent uses periodic switching of the pixel states (on/off) combined with correlated double sampling. The off-pixel reference is taken during a specific time window, and the differential measurement is performed by subtracting this reference from the on-pixel measurement taken in a subsequent window. This periodic action allows noise rejection while minimizing the duration that differential pairs are active, thereby reducing leakage current
Solution Approach 2:
The patent performs preliminary measurement of the off-pixel state before the on-pixel measurement. This preliminary reference measurement is stored and then used to cancel common-mode noise in the subsequent on-pixel measurement. By performing the reference measurement first and using it to correct the main measurement, the system achieves noise rejection with minimal additional leakage current
Data Source
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AI summary
A system and method for sensing drive current in a pixel. In some embodiments, the system includes: a first pixel, a second pixel, a differential sensing circuit, a reference current source, and a control circuit. The differential sensing circuit may have a first input, a second input, and an output, the first input being connected to a node at which a reference current generated by the reference current source is subtracted from a first pixel current, the first pixel current including a current generated by the first pixel. The second input may be configured to receive a second pixel current, the second pixel current including a current generated by the second pixel. The output may be configured to produce an output signal based on a difference between a current received at the first input and a current received at the second input.