Bandgap Reference Circuit Offset Voltage Removal
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Solution Overview
Problem
Bandgap reference circuits in CMOS image sensors face challenges due to offset voltages in operational amplifiers, which vary from chip to chip, affecting the accuracy and stability of reference voltages needed for high-resolution analog-to-digital converters, and conventional compensation methods like high-frequency chopper techniques and auto-zeroing can reduce accuracy by introducing high-frequency currents or voltages.
Innovation Solution
A bandgap reference circuit that includes an offset capacitor to sample and cancel the offset voltage of the operational amplifier, using a source follower transistor and bipolar transistors with different current densities to regulate the reference voltage, and synchronizes the refresh of the reference voltage with the row readout of the image sensor to minimize high-frequency switching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If operational amplifiers are used to improve reference voltage accuracy, then measurement precision is improved, but offset voltage variation causes reliability to deteriorate
Solution Approach 1:
The patent applies preliminary action by sampling and storing the offset voltage on a capacitor before the main measurement process. The offset capacitor captures the operational amplifier's offset voltage in advance, allowing it to be subtracted from the final measurement result, thereby eliminating the reliability issue caused by offset variation while preserving measurement precision.
2Measurement precision
If high-frequency chopper techniques are used to compensate offset voltages, then measurement precision is improved, but high-frequency currents introduce harmful factors that reduce accuracy
Solution Approach 1:
The patent extracts the offset voltage compensation function from the high-frequency chopper technique and implements it separately using a dedicated offset capacitor and switching mechanism. This separates the offset compensation process from the main reference voltage generation, eliminating the harmful high-frequency current coupling while retaining the precision benefits of offset compensation.
3Measurement precision
If auto-zeroing techniques are used to compensate offset voltages, then measurement precision is improved, but high-frequency switching generates voltages in common ground paths that reduce accuracy
Solution Approach 1:
The patent segments the offset compensation process into distinct phases controlled by separate switches. The offset sampling phase is separated from the reference voltage output phase, with each phase activated at different times. This segmentation prevents high-frequency switching voltages from coupling into the common ground paths during the critical reference output period, while still achieving offset compensation during the sampling phase.
Data Source
AI summary
An example bandgap reference circuit includes an amplifier, a first, a second, and a third switch, and a capacitor. The first switch is coupled between an inverting input and an output of the amplifier to provide a negative feedback loop around the amplifier when the first switch is closed. The capacitor has a first end coupled to the inverting input, and a second end coupled to the second switch, where the capacitor is charged to a voltage substantially equal to an offset voltage of the amplifier when the second switch is closed. The third switch is coupled to a second end of the capacitor, where the voltage across the capacitor is subtracted from an input loop of the reference circuit to cancel the offset voltage of the amplifier when the third switch is closed.