Translinear Read-Out Circuit for Resistive Sensor
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Solution Overview
Problem
Resistive sensors, particularly metal oxide-based sensors, face challenges in accurately reading resistance values across a large dynamic range and require effective temperature compensation, especially with on-chip heat sources causing significant temperature changes.
Innovation Solution
A translinear current-to-voltage converter with a logarithmic amplifier and feedback loop is used, where the output is proportional to the logarithm of the resistance of the sensing element scaled by a reference element, stabilized through capacitance and resistor feedback, allowing for precise measurement of high resistive sensors with low current levels and temperature independence.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional read-out circuits are used for resistive sensors, then the circuit can operate with standard components, but the measurement precision deteriorates for high resistance values (tens of GΩ) and temperature compensation becomes inadequate
Solution Approach 1:
The patent implements feedback mechanisms in the translinear I-V converter circuit to stabilize the measurement process. The feedback loop compensates for temperature variations and ensures accurate resistance measurements across the dynamic range by continuously adjusting the operating point based on the actual sensor resistance value.
Solution Approach 2:
The patent utilizes the exponential current-voltage relationship of bipolar junction transistors to transform the linear resistance measurement problem into a logarithmic domain. By changing the measurement parameter from linear resistance to logarithmic resistance, the circuit achieves high precision across a wide dynamic range (10kΩ to tens of GΩ) while maintaining manageable circuit complexity.
2Speed
If on-chip heat sources are used to generate temperature changes for sensing, then the sensor response time improves, but temperature compensation becomes more difficult and measurement precision deteriorates
Solution Approach 1:
The patent introduces a reference resistive element as an intermediary component that experiences the same temperature variations as the sensing element. By comparing the sensing element resistance with the reference element resistance through the translinear circuit, the system automatically compensates for temperature effects without requiring additional temperature sensors or complex compensation algorithms.
Solution Approach 2:
The patent replaces traditional temperature compensation mechanisms (such as separate temperature sensors and software-based compensation) with an analog circuit-based compensation approach using translinear principles. This substitution enables real-time temperature compensation to occur automatically through the circuit's inherent exponential characteristics, maintaining measurement precision despite rapid temperature changes from on-chip heating.
3Device complexity
If high resistive sensors (tens of GΩ) are measured with standard voltage biasing, then the circuit design remains simple, but the current signal becomes too weak (picoampere level) for accurate measurement
Solution Approach 1:
The translinear I-V converter employs feedback mechanisms that automatically adjust the bias conditions based on the actual current flowing through the high-resistance sensor. This feedback ensures that sufficient current is maintained for accurate measurement while preventing excessive current that would simplify the circuit but overload the sensor.
Solution Approach 2:
The patent transforms the measurement from the current domain to the voltage domain through the translinear I-V conversion. By measuring voltage across the transistor rather than directly measuring the tiny current, the system achieves high precision for picoampere-level currents while maintaining relatively simple circuit design. The exponential I-V relationship of the BJT provides natural gain for these weak signals.
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 solution enables accurate measurement of high resistive sensors with low current levels and reduces temperature dependency, improving signal-to-noise ratio and resolution for high resistance values, while maintaining stability and minimizing noise.
Implementation Method 1
Translinear circuits are known as such and generally characterized through the use of elements exhibiting an exponential current-voltage relations. Such relations are found for example in diodes and bipolar junction transistors (BJTs)
Implementation Method 2
The converter is best stabilized through a feedback loop, preferably through a capacitance or a capacitance and a resistor in the feedback loop of the log amp.
Data Source
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AI summary
A resistive sensor, particularly a chemical sensor with at least one layer of a metal oxide arranged between two electrodes, is described with a resistive sensing element (Rsens,10-1) and a resistive reference element (Rref,10-2) each biased by a voltage such as a common on-chip voltage is described wherein the currents (Is, Ir) though the elements a converted by a translinear I-V convertor (12-1,12-2) to generate an output proportional to the logarithm of the resistance of the sensing element scaled be the resistance of the reference element, with a temperature dependence removed optionally in an analogue-digital (A-D) convertor (16) using a PTAT voltage as reference voltage.