Exponential DAC Current Mirror for Stable Step Uniformity
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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 generate an exponential analog output signal, where the ratio between adjacent steps is kept constant, reducing the impact of component mismatch and ensuring stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a positive feedback loop is used in an exponential DAC, then the exponential transfer function can be achieved, but the system becomes inherently unstable and sensitive to component mismatch
Solution Approach 1:
The patent inverts the conventional feedback approach by using a negative feedback loop instead of a positive feedback loop. The output current is fed back through a resistor to the inverting input of the operational amplifier, creating a stable feedback mechanism that achieves the exponential transfer function without the inherent instability of positive feedback systems.
Solution Approach 2:
The patent introduces an operational amplifier as an intermediary element that mediates between the digital-to-analog conversion process and the feedback mechanism. The op-amp's high gain and differential input structure allow precise control of the exponential relationship while isolating the feedback loop from direct impact on the core conversion elements, thereby improving stability and reducing sensitivity to component variations.
2Device complexity
If a 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 changes the fundamental parameter relationship in the DAC by implementing an exponential transfer function where the output current changes exponentially with the digital input code rather than linearly. This is achieved through the feedback resistor and operational amplifier configuration, which creates a relationship where each step has a constant relative impact (percentage increase) across the entire DAC range, from small increments at low values to large increments at high values.
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
The solution provides a robust and stable exponential digital to analog conversion with reduced sensitivity to component variations, allowing for precise control of the output current and achieving a nearly constant step size across the entire range, thus improving the overall conversion accuracy and stability.
Implementation Method 1
The digital to analog conversion circuit comprises a plurality of transistors in a current mirror configuration, and the exponential transfer function is defined by the mirror ratio of said current mirror.
Data Source
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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.