Switched-Capacitor Thermistor Sensing for Low-Noise High-Rate Readout
Find Innovative SolutionsGenerate Solutions
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 development of circuitry and techniques utilizing a switched capacitor network with a digital Sigma-Delta modulator, chopping circuitry, and a pseudo-differential VCO-based analog-to-digital converter to efficiently convert temperature-dependent resistive changes into digital codes, while mitigating 1/f noise and circuit offsets, and using a micromachined thermistor structure as the temperature sensitive device.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a bipolar transistor-based temperature sensing circuit is used, then robust temperature signal is achieved across process variations, but noise performance deteriorates and area/power consumption increases
Solution Approach 1:
The patent replaces the bipolar transistor-based temperature sensing mechanism with a resistor-based sensing mechanism combined with a switched-capacitor network and Sigma-Delta modulator. This substitution fundamentally changes the sensing approach from voltage-based (bipolar) to resistance-based measurement, achieving lower noise floor while maintaining temperature signal robustness through the micromachined thermistor structure and digital modulation techniques.
Solution Approach 2:
The patent changes the operating parameters by using a micromachined thermistor with specifically engineered resistance-temperature characteristics instead of bipolar transistor Vbe characteristics. The thermistor provides higher temperature sensitivity (larger dR/dT) compared to bipolar transistors, enabling better signal-to-noise ratio. The switched-capacitor network dynamically adjusts effective resistance values to optimize sensing across different temperature ranges.
2Productivity
If sampling rate is increased to achieve high sample rates (>100 Hz), then productivity is improved, but noise performance deteriorates
Solution Approach 1:
The patent employs periodic switching action through the switched-capacitor network operating at high frequencies (e.g., 64 kHz or higher). The periodic charging and discharging of capacitors during each sampling cycle enables high sample rate operation while the periodic nature of the switching allows for noise averaging and filtering. The Sigma-Delta modulator uses periodic modulation to shape noise spectrum, pushing quantization noise to higher frequencies where it can be filtered out.
Solution Approach 2:
The patent maintains continuous temperature monitoring through overlapping sampling windows and continuous operation of the switched-capacitor network. The feedback loop continuously adjusts the Sigma-Delta modulator input based on error data, ensuring uninterrupted temperature measurement. This continuous operation enables high sample rates while maintaining low noise through persistent averaging and filtering operations.
3Object-affected harmful factors
If chopping circuitry is added to mitigate 1/f noise, then noise performance is improved, but device complexity increases
Solution Approach 1:
The chopping circuitry operates by periodically switching the polarity of the sensing signal at a frequency much higher than the 1/f noise corner frequency. This periodic modulation shifts the 1/f noise to higher frequencies where it can be easily filtered by the low-pass characteristics of the Sigma-Delta modulator and subsequent digital filtering. The chopping frequency is typically chosen to be several times higher than the sampling frequency to effectively separate the signal from 1/f noise.
Solution Approach 2:
The switched-capacitor network serves as an intermediary between the micromachined thermistor and the Sigma-Delta modulator. It provides impedance transformation, signal conditioning, and noise filtering functions. The capacitor network acts as a mediator that converts the resistive temperature signal into a form suitable for digital modulation while filtering out 1/f noise through its switching action and the inherent low-pass filtering of the Sigma-Delta architecture.
4Measurement precision
If micromachined thermistor structure is used, then temperature sensing sensitivity is improved, but manufacturing precision requirements increase
Solution Approach 1:
The micromachined thermistor is designed with specific geometric parameters (dimensions, layout, material composition) that are optimized to achieve desired temperature sensing characteristics. By carefully controlling the thermistor's physical dimensions and material properties during fabrication, the resistance-temperature relationship can be precisely engineered. The high temperature sensitivity arises from the thermistor's specific material composition and geometric configuration, which are established during the micromachining process.
Solution Approach 2:
The Sigma-Delta modulator implements a feedback loop that continuously monitors the temperature signal and adjusts the modulator input based on error data. This feedback mechanism compensates for variations in thermistor characteristics caused by manufacturing tolerances. The digital feedback loop ensures accurate temperature measurement even when there are slight variations in thermistor resistance values due to fabrication process variations, effectively decoupling measurement precision from manufacturing precision requirements.
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, enabling accurate temperature sensing with compact design, suitable for various applications including MEMS-based systems.
Implementation Method 1
the temperature dependent characteristics (and/or changes therein) of the temperature sensitive device is resistance or change in resistance of the temperature sensitive device
Implementation Method 2
a pseudo-differential VCO-based analog-to-digital converter (ADC) structure, or other ADC topology, to efficiently convert the analog error between the MEMS-based resistance value and the effective resistance of the switched capacitor network into a digital code
Data Source
AI summary
A temperature to digital converter circuitry to generate output data which is representative of one or more temperature dependent characteristics of a temperature sensitive device (for example, MEMS thermistor having a resistance that correlates to its temperature), the temperature to digital circuitry comprising a switched capacitor network to generate a effective reference resistance in response to a switching signal, a signal generator to generate the switching signal, wherein the switching signal has a switching frequency which is controlled, at least in part, via control data, comparator circuitry to generate error data using the effective reference resistance and the resistance of the temperature sensitive device, and converter circuitry to generate the output data which is representative of one or more temperature dependent characteristics of the temperature sensitive device.