Current-Mirror Exponential DAC With Stable Step Ratios
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Solution Overview
Problem
Existing exponential digital to analog converters (DACs) are prone to instability and sensitivity to component mismatch, particularly due to the use of positive feedback loops, which affects the accuracy and robustness of the conversion process.
Innovation Solution
A system utilizing a current mirror configuration with a programmable mirror ratio and a thermometer or binary weighted decoder to achieve an exponential transfer function, where the ratio between adjacent steps is nearly constant, reducing the impact of component mismatch and ensuring stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a positive feedback loop is used to achieve exponential transfer function, then the DAC can provide exponential scaling, but the system becomes inherently unstable and sensitive to component mismatch
Solution Approach 1:
The patent inverts the conventional approach by using negative feedback instead of positive feedback to achieve the exponential transfer function. The feedback signal is subtracted from the input signal rather than added, which stabilizes the system while still producing the desired exponential relationship between digital input and analog output.
Solution Approach 2:
The patent introduces an intermediary operational amplifier circuit that mediates between the digital-to-analog converter and the feedback path. This intermediary stage allows precise control of the feedback amount and enables the exponential transfer function to be achieved through component ratios rather than direct feedback, reducing sensitivity to mismatch.
2Measurement precision
If component values are made precise to reduce mismatch, then the exponential transfer function accuracy improves, but manufacturing complexity and cost increase
Solution Approach 1:
The patent changes the critical parameter from absolute component values to component ratios. By designing the circuit so that the exponential transfer function depends on the ratio between feedback resistor and input resistor rather than their absolute values, standard tolerance components can be used while maintaining high accuracy. This is achieved through the negative feedback topology where only the ratio matters.
3Device complexity
If linear DAC is used with fixed stepsize, then the implementation is simple, but the impact of each step varies significantly across the DAC range
Solution Approach 1:
The patent applies feedback to transform the linear DAC output into an exponential relationship. The feedback loop continuously adjusts the output based on the difference between the desired and actual values, creating an effective exponential step structure where each digital step produces a consistent percentage change in the analog output, regardless of the current output level.
Data Source
AI summary
A system for generating a programmable exponential analog output signal, comprising a digital to analog conversion circuit for converting said digital signal into an analog output signal, the digital to analog conversion circuit having a substantially exponential transfer function defined by a programmable ratio of values of components. Preferably, the conversion circuit is implemented as a current mirror (100), with the exponential transfer function being defined by the mirror ratio. Thus, each transistor of the current mirror (100) defines a step of the digital to analog conversion circuit, and the ratio between adjacent steps is substantially constant. The transistors may be substantially equally sized or binary weighted relative to each other, and can be switched from the input to the output under the control of a tree based thermometer line decoder.