Ambient Light Sensor Readout Circuit for Offset Error Cancellation
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Solution Overview
Problem
Ambient light sensors, such as photodiodes, suffer from parasitic impedance and operational amplifier offset voltages that affect output accuracy, particularly at high temperatures.
Innovation Solution
A circuit is introduced that compensates for parasitic impedance and offset voltages using an integrator assembly with operational amplifiers, capacitors, and switch circuits to adjust the charge balance, ensuring accurate readings by alternating the connection polarity of compensation circuits during integration phases.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If ambient light sensor is used with operational amplifier, then the sensor can amplify and process the light signal, but offset voltage of the operational amplifier introduces measurement errors
Solution Approach 1:
The patent applies preliminary action by performing offset compensation before the main measurement process. The circuit pre-charges capacitors with compensation charges that counterbalance the operational amplifier's offset voltage, ensuring that the offset error is eliminated before the actual light signal measurement begins. This preliminary compensation action ensures accurate measurements without sacrificing signal amplification capability.
2Adaptability or versatility
If parasitic impedance of the sensor is present, then the sensor can function in practical conditions, but it affects the output accuracy particularly at high temperatures
Solution Approach 1:
The patent converts the harmful effect of parasitic impedance into a beneficial compensation mechanism. By measuring the voltage drop across the parasitic impedance and using this information to calculate and apply compensating charges, the circuit transforms the previously harmful parasitic effect into useful data that enables accurate compensation. This approach maintains practical operability while eliminating the negative impact on output accuracy.
3Measurement precision
If compensation circuit is added to cancel offset errors, then measurement accuracy improves, but device complexity increases
Solution Approach 1:
The patent merges the compensation function with the existing operational amplifier circuitry by integrating capacitor-based compensation networks directly into the feedback paths of the operational amplifiers. This merging approach allows offset compensation to be achieved without adding completely separate compensation circuits, thereby improving measurement accuracy while minimizing the increase in device complexity. The compensation components are seamlessly incorporated into the signal processing path.
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
The solution effectively cancels out offset errors by averaging opposite error currents, improving the accuracy and reliability of ambient light measurements, especially at high temperatures.
Implementation Method 1
an integrator assembly comprising an operational amplifier, a first capacitor, and a circuit adapted to compensating for an offset voltage of the operational amplifier. A first electrode of the first capacitor is coupled to an inverting input of the operational amplifier and a second electrode of the first capacitor is coupled to a non-inverting output of the operational amplifier
Implementation Method 2
A preset circuit is adapted to receive a first voltage equal to a preset voltage multiplied by a coefficient and to charge the first capacitor
Data Source
AI summary
A circuit can be used for reading out a light sensor. The circuit includes an operational amplifier. A first capacitor has a first electrode coupled to an inverting input of the operational amplifier and a second electrode coupled to a non-inverting output of the operational amplifier. A compensation circuit is coupled between the operational amplifier and the first capacitor. A preset circuit has an input coupled to a first voltage node and an output coupled to the first capacitor. The first voltage node configured to carry a first voltage equal to a preset voltage multiplied by a coefficient.