Analog Front End Variable Bias Voltage Correction for Sensor Characteristic Variations
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Solution Overview
Problem
Current analog front ends for current or charge output sensors face challenges in correcting characteristic variations of transistors and photodiodes, particularly due to large parasitic capacitance and transconductance variations, which affect operation speed and accuracy in image sensors, especially in large TFT liquid crystal panels for X-ray detection.
Innovation Solution
An analog front end that includes a transimpedance amplifier and a variable bias voltage generation circuit, along with a current source and integration circuit, to adjust and stabilize the bias voltage of the output signal line, thereby correcting sensor characteristics without reducing operation speed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a current output sensor is used in a large TFT liquid crystal panel for X-ray detection, then the sensor can detect X-ray signals, but large parasitic capacitance is added to the column line which slows down operation speed
Solution Approach 1:
The patent extracts the problematic parasitic capacitance effect by introducing a calibration circuit that separately measures and compensates for the capacitance impact. The calibration circuit isolates the sensor output during calibration mode, allowing measurement of the relationship between output current and cathode voltage without the influence of parasitic capacitance during normal operation.
Solution Approach 2:
The patent changes the operating parameters by implementing a calibration mode that temporarily modifies the circuit configuration. During calibration, the switch SW2 is turned on to connect the output to the calibration circuit, allowing measurement of sensor characteristics. This parameter change enables the system to adapt to the parasitic capacitance effect without permanently slowing down normal operation.
2Adaptability or versatility
If transistor GM is used to output current based on cathode voltage, then signal conversion is achieved, but transconductance variation causes characteristic differences between sensors
Solution Approach 1:
The patent implements feedback by measuring the actual output current during calibration and using this information to determine a correction value. The calibration circuit measures the relationship between cathode voltage and output current, and this measured data is fed back to adjust the reading operation, compensating for transconductance variations in transistor GM.
Solution Approach 2:
The patent performs preliminary calibration before normal reading operations. The calibration circuit pre-determines the correction value by measuring sensor characteristics in advance. This preliminary action allows the system to account for transistor variations before they affect normal operation, ensuring consistent characteristics across sensors.
3Device complexity
If multiple sensors are connected to one analog front end through one column line, then circuit integration is improved, but parasitic capacitance accumulates and affects performance
Solution Approach 1:
The patent introduces a calibration circuit as an intermediary between the sensor and the readout circuitry. This intermediary circuit allows the system to characterize and compensate for the cumulative parasitic capacitance effect without requiring separate processing for each sensor. The calibration circuit acts as a mediator that measures the collective impact of parasitic capacitance on the column line.
4Ease of operation
If switch SW2 is used to selectively output sensor current, then signal routing is achieved, but switch capacitance adds to the total parasitic capacitance
Solution Approach 1:
The patent performs preliminary calibration when switch SW2 is in a known state (turned on during calibration). This preliminary measurement captures the effect of the switch capacitance along with other parasitic elements. By measuring the total effect in advance, the system can compensate for the switch capacitance during normal operation without needing to dynamically adjust for it.
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 solution enables effective correction of transistor and photodiode variations, improving the accuracy and speed of image sensor operations by stabilizing the bias voltage and reducing noise, even with large parasitic capacitance, without compromising operation speed.
Implementation Method 1
the light charges generated by light reception can be accumulated in the cathode
Implementation Method 2
The integration circuit 110 is a circuit for integrating an output current from the sensor 91, and is configured with a capacitor CINT
Data Source
AI summary
An analog front end that reads from a sensor an output signal which is either a current output signal (IOUT) or a charge output signal corrects characteristics of the sensor by adjusting a bias voltage (VBIASIN) of an output signal line from the sensor.