Capacitor-Referenced Temperature Sensing With Chopped Low-Noise Readout
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Solution Overview
Problem
Existing temperature sensors face challenges in achieving low noise performance with high sample rates, low integrated area, and low power operation, particularly in MEMS-based clock applications, where high accuracy and low Allan deviation are crucial.
Innovation Solution
The use of a switched capacitor network with a digital Sigma-Delta modulator, chopping circuitry, and a pseudo-differential VCO-based analog-to-digital converter to mitigate 1/f noise and circuit offsets, along with a feedback loop that adjusts the Sigma-Delta modulator input based on error data, enables efficient temperature sensing using a micromachined thermistor structure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional temperature sensing circuits are used, then temperature measurement is achieved, but noise performance deteriorates at high sample rates
Solution Approach 1:
The patent employs periodic chopping action at a frequency significantly higher than the temperature signal bandwidth to modulate the temperature-sensitive resistor and reference resistor. This periodic switching moves the measurement signal to a higher frequency domain where 1/f noise is reduced, while the low-pass filter reconstructs the original temperature signal, achieving low noise performance at high sample rates
Solution Approach 2:
The patent introduces a switched capacitor network as an intermediary to synthesize a high-precision reference resistor with temperature compensation. This intermediary component enables accurate differential measurement by providing a reference that tracks temperature variations, thereby improving signal-to-noise ratio without requiring larger capacitors that would increase area
2Object-affected harmful factors
If larger sampling capacitors are used to reduce noise, then noise performance improves, but integrated area increases
Solution Approach 1:
The patent replaces the traditional mechanical approach of using large physical capacitors with a switched capacitor network that synthesizes equivalent capacitance through rapid switching. This substitution achieves the same noise filtering effect with much smaller physical components, reducing integrated area while maintaining low noise performance at high sample rates
Solution Approach 2:
The periodic switching action of the switched capacitor network creates an effective large capacitance value through time-averaging, allowing the system to achieve low noise performance without requiring physically large capacitors that would occupy excessive integrated area
3Productivity
If higher sample rates are implemented, then measurement speed improves, but noise performance deteriorates
Solution Approach 1:
The patent uses periodic chopping at a frequency much higher than both the temperature signal bandwidth and the desired sample rate. This allows the system to operate at high sample rates while the low-pass filter reconstructs the temperature signal at the lower effective bandwidth, maintaining low noise performance regardless of the high intermediate sampling frequency
Solution Approach 2:
The patent employs dynamic element matching and differential switching that adapts to high sample rate operation. The switched capacitor network and differential circuitry are designed to maintain stability and accuracy across a wide range of switching frequencies, enabling high sample rates without degrading noise performance
4Reliability
If bipolar transistors are used for temperature sensing, then robust temperature signal is achieved, but temperature sensitivity is limited
Solution Approach 1:
The patent changes the sensing parameter from the small ΔVbe voltage difference of bipolar transistors to the resistance change of a temperature-sensitive resistor. This parameter change enables the use of a simple voltage divider configuration where the temperature signal appears as a significant voltage variation, achieving high temperature sensitivity while maintaining robustness through differential measurement and chopping techniques
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 approach results in improved noise performance, higher sensitivity to temperature changes, and reduced power consumption, achieving accurate temperature measurements with compact design.
Implementation Method 1
the temperature dependent characteristics (and/or changes therein) of the temperature sensitive device (for example, a micromachined thermistor structure)
Data Source
AI summary
The temperature-dependent resistance of a MEMS structure is compared with an effective resistance of a switched CMOS capacitive element to implement a high performance temperature sensor.