Thermistor Measurement Circuit Using DAC Mismatch Reduction
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Solution Overview
Problem
Existing temperature measurement systems using NTC thermistors face challenges in achieving high resolution due to the exponential dependence of impedance on temperature, requiring high bit resolution ADCs, which is impractical and introduces measurement errors.
Innovation Solution
A system and method utilizing a digital-to-analog converter (DAC) with mismatch reduction techniques, coupled with an analog-to-digital converter (ADC) and a reference impedance, to accurately measure thermistor impedance by equal engagement of DAC elements and minimizing voltage differences, allowing for lower resolution ADCs to achieve precise temperature measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a high-resolution ADC is used to achieve 0.01°C temperature resolution across a wide temperature range, then measurement precision is improved, but device complexity and cost increase
Solution Approach 1:
The temperature measurement process is segmented into multiple measurement cycles with different drive current levels. Instead of requiring a single high-resolution ADC reading, the system performs multiple lower-resolution measurements at different current levels (first current and second current) and combines them through computational processing to achieve the equivalent of high-resolution temperature measurement.
Solution Approach 2:
The system changes the drive current parameter between measurement cycles to optimize the measurement process. By measuring at different current levels and using the differential information, the system can extract temperature data with high precision while using a lower-resolution ADC, effectively changing the electrical parameters to overcome the ADC resolution limitation.
2Reliability
If drive voltage or drive current is maintained constant across thermistor and reference resistor measurements, then measurement reliability is improved, but measurement flexibility and error correction capability are reduced
Solution Approach 1:
The system performs preliminary measurements at different current levels before final temperature calculation. By measuring the thermistor voltage at both first and second current levels, and similarly measuring the reference resistor at different current levels, the system gathers preliminary data that enables subsequent error correction and compensation during the calculation phase.
Solution Approach 2:
The system uses feedback from multiple measurement points to correct errors. By comparing measurements taken at different current levels and using the known reference resistor characteristics, the system can detect and correct for non-ideal behaviors such as ADC offset errors, gain errors, and thermal drift, thereby improving reliability while maintaining flexibility.
3Ease of manufacture
If the number of ADC bits is reduced to lower device complexity, then ease of manufacture is improved, but measurement precision deteriorates
Solution Approach 1:
The system introduces an intermediary computational process that acts as a mediator between the low-resolution ADC and the final high-precision temperature measurement. The intermediary algorithm processes multiple low-resolution measurements taken at different current levels, combining them mathematically to produce a high-precision temperature result, effectively bridging the gap between simple hardware and complex measurement requirements.
Solution Approach 2:
The system adds another dimension to the measurement process by introducing a time dimension with multiple measurement cycles at different current levels. Instead of relying on a single high-resolution measurement in one dimension, the system distributes the measurement across multiple time steps with varying electrical conditions, using the additional temporal and electrical dimensions to achieve high precision through computational synthesis.
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
Enables accurate temperature measurement with reduced errors by using DAC mismatch reduction techniques, enabling lower bit resolution ADCs to achieve 0.01°C resolution across a wide temperature range.
Implementation Method 1
A type of temperature sensor often employed in electrical and electronic systems is a negative temperature coefficient (NTC) thermistor, which has an electrical impedance R that varies exponentially with variance in temperature
Implementation Method 2
a digital-to-analog converter (DAC) comprising a plurality of DAC elements... enable a first number of the DAC elements of the DAC in order to generate the first current... enable a second number of the DAC elements of the DAC in order to generate the second current
Implementation Method 3
an analog-to-digital converter (ADC) path... measure a test voltage across the thermistor responsive to the test current via the ADC path... measure a first voltage across the thermistor responsive to the first current via the ADC path... measure a second voltage across the reference impedance responsive to the second current via the ADC path
Implementation Method 4
while measuring the first voltage, cause the plurality of DAC elements to be equally engaged via a mismatch reduction technique... while measuring the second voltage, cause the plurality of DAC elements to be equally engaged via the mismatch reduction technique
Data Source
AI summary
A current digital-to-analog converter may be used in a system for measuring temperature of a thermistor, with mismatch reduction techniques applied to digital-to-analog converter elements of the digital-to-analog converter in order to maximize accuracy and precisions of the temperature measurement.