Single-Slope ADC Slope Trimming for Low DNL Conversion
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Solution Overview
Problem
Single slope analog-to-digital converters face challenges in achieving high accuracy due to the difficulty in realizing digital current sources with high accuracy, especially as the number of bits increases, leading to differential non-linearity errors and requiring very precise current source matching.
Innovation Solution
The implementation of a self-calibrating single slope ADC that uses a combination of a fixed current source and a voltage-to-current converter, along with a digital-to-analog converter and a sample and hold circuit, to generate a voltage slope and adjust the slope current, allowing for closed-loop calibration and compensation of errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a digital adjustable current source is used to generate the voltage slope, then the ADC can achieve high conversion accuracy, but the device complexity increases and manufacturing precision requirements become extremely difficult to meet
Solution Approach 1:
The current source is segmented into two independent parts: a fixed current source (Ifixed) that provides stable baseline current, and a variable current source (Ivar) controlled by a DAC that provides adjustable current. This segmentation allows the complex adjustable current requirement to be divided into a simple fixed current generation and a controlled variable current addition, reducing overall system complexity while maintaining accuracy
Solution Approach 2:
A DAC (digital-to-analog converter) is introduced as an intermediary component between the digital control signal and the variable current source. The DAC converts digital trim codes into analog voltage that controls the variable current, serving as a mediator that simplifies the control interface and reduces the complexity of directly generating precise adjustable currents from digital signals
2Measurement precision
If precise current source matching is implemented to reduce differential non-linearity errors, then conversion accuracy improves, but manufacturing precision requirements become excessively stringent
Solution Approach 1:
The system performs self-calibration by using the reference voltage conversion to automatically determine and adjust the slope current through a calibration loop. The ADC converts the reference voltage, compares the result with an expected value, and uses a PID controller to adjust the DAC trim code until the conversion matches the expected value. This self-service mechanism eliminates the need for high-precision manual current source matching during manufacturing
Solution Approach 2:
The slope current parameter is made dynamically adjustable through the DAC-controlled variable current source. Instead of relying on fixed precise current source matching, the system changes the slope current parameter during calibration to compensate for manufacturing variations, allowing the system to adapt to actual hardware characteristics and achieve high accuracy without stringent manufacturing requirements
3Device complexity
If a fixed slope current is used to simplify the circuit, then device complexity is reduced, but conversion accuracy deteriorates due to inability to compensate for process variations
Solution Approach 1:
The system merges a fixed current source and a variable current source to create a composite slope current. The fixed current source provides circuit simplicity and stability, while the variable current source adds adjustability. By combining these two current sources, the system maintains the simplicity of fixed current generation while gaining the accuracy benefits of adjustable current through the variable component controlled by the DAC
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 eliminates the need for a digital adjustable current source, reduces differential non-linearity errors, and stabilizes the calibration loop, enabling accurate analog-to-digital conversion without requiring precise current source matching, thus improving conversion accuracy and reducing errors.
Implementation Method 1
a voltage to current (VI) converter configured to convert the slope trim signal into the variable current
Data Source
AI summary
Various embodiments relate to a single slope analog to digital converter (ADC), including: a voltage slope generator configured to generate a voltage slope based upon a fixed current and variable current; an analog comparator configured to compare a voltage to a voltage output from the voltage slope generator; a first register configured to store a first count based upon a reference voltage being input into the analog comparator; a second register configured to store a second count based upon an input voltage being input into the analog comparator, wherein the input voltage is the voltage to be converted to a digital value by the ADC; and a digital to analog converter (DAC) configured to produce a slope trim signal based upon the voltage slope output by the voltage slope generator, the first count, and a count target associated with the voltage reference, wherein the variable current in the voltage slope generator is based upon the slope trim signal.