CMOS Temperature-to-Digital Converter With Sub-nW Charge-Time Sensing
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Solution Overview
Problem
Current temperature sensing techniques fail to achieve sub-nW power consumption necessary for next-generation near-zero-power sensing nodes, with existing methods requiring tens of nW of power and often relying on external constant with temperature frequency sources.
Innovation Solution
A sub 1 V and sub-nW temperature-to-digital converter is developed using a CMOS integrated circuit with two paths charged by a sub-nanowatt current reference, where ramp voltages or currents are generated and compared to constant and proportional to absolute temperature references, allowing for digitization of temperature by matching charging times via digital feedback.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If BJT-based temperature sensing is used to achieve accurate temperature measurement, then measurement precision is improved, but power consumption increases to nA-μA range which exceeds near zero power requirements
Solution Approach 1:
The patent replaces the conventional BJT-based sensing mechanism with a CMOS-based sensing mechanism that uses capacitive charging/discharging cycles. Instead of relying on BJT voltage characteristics that require nA-μA bias currents, the invention uses charge transfer between capacitors driven by sub-nW current sources, fundamentally substituting the sensing physics to achieve picowatt-level power consumption while maintaining temperature measurement capability
Solution Approach 2:
The patent changes the operating parameters from continuous analog voltage comparison (requiring steady-state bias currents) to time-domain charge transfer measurements. By measuring the time required for capacitors to charge/discharge through sub-nW current sources, the system achieves temperature sensing with power consumption in the picowatt range, representing a dramatic parameter change from nA-μA to pW regime
2Use of energy by moving object
If MOSFET-based temperature sensing is used to reduce power consumption, then power consumption decreases to tens of nW, but measurement precision and accuracy are compromised
Solution Approach 1:
The patent replaces MOSFET-based sensing with a capacitive charge-transfer sensing mechanism. Instead of exploiting MOSFET threshold voltage or current characteristics, the invention uses direct charge transfer between capacitors controlled by sub-nW current sources, substituting the sensing mechanism to achieve both lower power (pW vs tens of nW) and improved accuracy through direct time-domain measurement
Solution Approach 2:
The patent implements a feedback mechanism where the charging/discharging times of two capacitors are compared, and the capacitor array is adjusted based on this comparison to achieve balanced charging times. This feedback loop enables precise temperature measurement by iteratively optimizing the charge transfer process, improving measurement accuracy while maintaining sub-nW power operation
3Stability of the object's composition
If conventional voltage reference generators are used to provide stable references, then stability is improved, but power consumption increases to tens of nW
Solution Approach 1:
The patent replaces conventional voltage reference generators with a current reference-based system that operates in the time domain. Instead of generating stable voltages that require continuous power dissipation, the invention uses sub-nW current sources to charge capacitors and measures temperature through charging time comparisons, substituting voltage-domain reference stability with time-domain measurement precision at picowatt power levels
Solution Approach 2:
The patent employs periodic charging and discharging of capacitors instead of continuous voltage generation. The sub-nW current sources charge capacitors in controlled cycles, and temperature is determined by comparing the periodic charging times. This periodic operation allows stable reference behavior to be achieved through repeated measurement cycles rather than continuous power-consuming voltage generation
4Measurement precision
If external constant with temperature frequency sources are used for digitization, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The patent merges the sensing, reference generation, and digitization functions into a single integrated CMOS circuit. The capacitive charge-transfer sensor, sub-nW current references, and time-domain digitization logic are all combined on one chip, eliminating the need for external frequency sources and reducing system complexity while achieving precise temperature measurement through unified on-chip operation
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 achieves improved accuracy and conversion time with negligible power overhead, demonstrating a maximum temperature error of ±1.38° C. and average power consumption of 763 pW, significantly reducing energy consumption while maintaining high precision.
Implementation Method 1
Two paths including on-chip capacitors charged with a sub nanowatt current reference such that a first ramp voltage or current is generated in one of the two paths and a second ramp voltage or current is generated in a second of the two paths
Data Source
AI summary
A charging to digital converter sensor in a CMOS integrated circuit digitizes a sensed property by comparing charging time between two paths and adjusting the charging rate of one of the two paths by increasing or decreasing the amount of capacitance in that path, until both of the two paths have the same charging time to reach respective constant with sensed property and proportional with sensed property reference voltages or currents. Sub nanowatt operation is achieved with preferred circuits. A method directly digitizes, on-chip, a charging time comparison of two ramp voltages that are compared to respective constant with sensed property and proportional with sensed property reference voltages or currents.