Thermometer DAC Calibration for Current Offset Cancellation
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Solution Overview
Problem
Current digital-to-analog conversion circuits suffer from non-ideal signal amplitude and shape offsets due to process variations, affecting system performance.
Innovation Solution
A digital-to-analog conversion circuit and method utilizing thermometer-controlled current sources and a control circuit to retrieve and sort current offset values, creating a turn-on order where neighboring current offset groups have opposite signs, canceling each other to keep the total current offset value below half of the largest absolute value, thereby improving linearity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional DAC circuit is used, then the circuit structure is simple, but the signal amplitude and shape have non-ideal offsets affecting system performance
Solution Approach 1:
The patent applies preliminary action by pre-calibrating and storing offset compensation values in a lookup table before actual signal conversion. The control circuit retrieves pre-computed compensation data based on digital input codes, eliminating the need for real-time complex calculations and reducing circuit complexity while improving signal accuracy.
Solution Approach 2:
The patent introduces an intermediary calibration mechanism that acts as a mediator between the digital input and analog output. The control circuit serves as an intermediary that retrieves compensation values and adjusts the current source activation sequence, thereby canceling offset effects without requiring hardware modifications to the current sources themselves.
2Manufacturing precision
If current offset compensation is implemented, then the linearity is improved, but the control complexity increases
Solution Approach 1:
Offset compensation values are pre-calculated and stored in a lookup table during the design or calibration phase. The control circuit simply retrieves these pre-computed values based on the digital input code, avoiding the need for complex real-time offset calculation algorithms and reducing control complexity.
Solution Approach 2:
The system performs self-calibration by automatically retrieving appropriate compensation values from the lookup table based on the input digital code. The control circuit autonomously adjusts the current source activation sequence without requiring external intervention or complex control algorithms, thereby improving linearity while maintaining simple control.
3Speed
If thermometer-controlled current sources are used, then the conversion speed is fast, but the process variation causes signal offset
Solution Approach 1:
The patent applies local quality by providing individualized offset compensation for each digital input code. The lookup table stores specific compensation values tailored to each code, allowing the system to address local variations in current source characteristics caused by process variation while maintaining the fast conversion speed of thermometer-coded DAC.
Solution Approach 2:
The patent changes the activation sequence parameter of the current sources based on the digital input code and corresponding offset compensation values. By dynamically adjusting which current sources are activated and in what sequence, the system compensates for process variation effects while maintaining fast conversion speed characteristic of thermometer-coded architecture.
Data Source
AI summary
The present invention discloses a DAC method having signal calibration mechanism used in a DAC circuit having thermometer-controlled current sources generating an output analog signal according to a total current thereof and a control circuit. Current offset values of the current sources are retrieved. The current offset values are sorted to generate a turn-on order, in which the current offset values are separated into current offset groups according to the turn-on order, the signs of each neighboring two groups being opposite such that the current offset values cancel each other when the current sources turn on according to the turn-on order to keep an absolute value of a total offset not larger than a half of a largest absolute value of the current offset values. The current sources are turned on based on the turn-on order according to a thermal code included in an input digital signal.