Chopped Current Mirror Temperature Sensor for Low 1/f Noise
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Solution Overview
Problem
Existing digital-output temperature sensors face challenges in achieving high accuracy due to high levels of low-frequency noise, particularly 1/f noise, generated by transistors in the current mirror circuit, which hinder their ability to provide precise temperature detection results.
Innovation Solution
The implementation of a chopping circuit that modulates the 1/f noise into a higher frequency by switching the flow of mirror currents through transistors in the current mirror circuit, preventing noise transmission to the A/D conversion circuit and enhancing accuracy through differential pair A/D conversion of difference currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a current mirror circuit is used in the temperature sensor, then temperature detection can be performed, but 1/f noise is generated by transistors causing high low-frequency noise levels
Solution Approach 1:
The patent applies periodic chopping action to the current mirror circuit transistors, switching them between ON and OFF states at a specific chopping frequency. This periodic switching modulates the 1/f noise generated by the transistors to a higher frequency range, effectively removing the low-frequency noise component that degrades temperature measurement accuracy while preserving the useful temperature detection signal.
Solution Approach 2:
The patent converts the harmful 1/f noise generated by the current mirror transistors into a beneficial high-frequency signal through chopping modulation. By switching the transistors periodically, the noise is shifted from the low-frequency range (where it harms measurement) to the high-frequency range (where it can be filtered out), thus transforming a harmful effect into a manageable one that enables accurate temperature detection.
2Measurement precision
If amplifier circuits are used for signal processing, then signal amplification is achieved, but 1/f noise is generated increasing low-frequency noise
Solution Approach 1:
The patent replaces the traditional amplifier circuit with a chopping-based modulation system. Instead of using an amplifier that would introduce 1/f noise, the invention uses periodic switching (chopping) to modulate the signal and noise, then uses correlation detection or filtering to recover the signal. This substitution eliminates the amplifier-induced 1/f noise while maintaining signal detection capability.
3Measurement precision
If standard A/D conversion is used, then digital output is obtained, but noise affects conversion accuracy reducing measurement precision
Solution Approach 1:
The patent performs preliminary chopping modulation of the analog signal before it reaches the A/D converter. By switching the current mirror transistors at a chopping frequency, the signal is modulated to a higher frequency range before conversion. This preliminary action ensures that when the A/D conversion occurs, the signal is less susceptible to low-frequency noise, thereby improving conversion accuracy and overall measurement precision.
Data Source
AI summary
The digital-output temperature sensor includes a temperature sensor circuit, a current mirror circuit which makes a mirror current of a temperature detection current flow and pulls in a mirror current of a reference current to thereby output a first difference current from a first output node and output a second difference current from a second output node, a chopping circuit, and an A/D conversion circuit. The chopping circuit performs a chopping operation of making the mirror current of the reference current flow in a second state through a transistor of the current mirror circuit through which the mirror current of the temperature detection current flows in a first state, and making the mirror current of the temperature detection current flow in the second state through the transistor of the current mirror circuit through which the mirror current of the reference current flows in the first state.