DAC Distortion Correction for Harmonic and Timing-Skew Errors
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Solution Overview
Problem
Current-steering digital-to-analog converters (DACs) suffer from third order harmonic distortion due to charge transfer effects and instantaneous output impedance changes during switching, as well as higher order distortions caused by timing skew between DAC components, which limit their signal-to-noise and distortion ratio (SNDR) and spurious free dynamic range (SFDR).
Innovation Solution
The implementation of a distortion correction signal generated from the input digital signal, applied to the output analog signal or input digital signal of the DAC, using a combination of delay modules, distortion correction modules, and sin(x)/x or T(jω) compensation filters to reduce third and higher order harmonic distortions. This involves a correction DAC with a 180-degree phase shift and a sin(x)/x compensation filter to align and filter the correction signal, effectively canceling out distortions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If current-steering segments are used to achieve high conversion rate and differential output, then conversion speed and output power are improved, but third order harmonic distortion increases due to charge transfer effects during switching
Solution Approach 1:
The patent generates a distortion correction signal that deliberately replicates the charge transfer effect, then subtracts it from the output signal. By converting the harmful distortion into a useful correction signal, the system eliminates third order harmonic distortion while preserving the high conversion rate capability of current-steering segments
Solution Approach 2:
The distortion correction signal acts as an intermediary between the input digital signal and the output analog signal. It mediates the harmful effect of charge transfer by introducing a compensating signal that cancels the distortion, allowing the system to maintain both high speed and low distortion
2Adaptability or versatility
If differential transistor pair switching is used to steer current to output terminals, then current steering capability is improved, but output impedance drops instantaneously causing distortion
Solution Approach 1:
The system performs preliminary action by generating the distortion correction signal in advance based on the input digital signal characteristics. This allows the correction to be applied before the actual switching distortion occurs, preventing impedance-related distortion rather than merely compensating for it
3Productivity
If timing skew between DAC components is present to enable parallel operation, then productivity is improved, but higher order distortions increase
Solution Approach 1:
The system employs feedback by using the input digital signal to generate a distortion correction signal that is fed back to cancel higher order distortions. This feedback mechanism continuously compensates for timing skew effects, allowing parallel operation of DAC components without sacrificing signal quality
Data Source
AI summary
An apparatus and method for reducing distortion from an output of a digital-to-analog converter (DAC). In one implementation, the distortion being reduced is third order harmonic distortion caused by charge transfer effects and instantaneous output impedance drop during switching. Each of the embodiments includes a module adapted to generate a distortion correction signal being a function of the input digital signal, the full scale code of the DAC, the load resistance, the conversion rate of the DAC, and a proportionality constant. In another implementation, the distortion being reduced is higher order harmonic distortion caused by timing skew between a main DAC and a least significant bit (LSB) DAC. Each of the embodiments includes a module adapted to generate a distortion correction signal being a function of the input digital signal, the number of levels covered by the LSB DAC, the conversion rate of the DAC, and the timing skew.