Sigma-Delta DAC Output Resistor Linearity for Distortion Reduction
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Solution Overview
Problem
Sigma-delta digital-to-analog converters (DACs) implemented in integrated circuits often introduce undesirable distortion due to non-linear resistors, particularly polysilicon resistors, which can affect the precision of applications such as inertial guidance systems and audio equipment by degrading signal quality.
Innovation Solution
The implementation of a sigma-delta DAC system that includes a translator stage and a filtering stage with an approximately linear output resistor, where the filtering stage is located off-chip to reduce distortion, and an error correction path to generate and add an error correction signal to the output signal, thereby correcting for non-linearity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If polysilicon resistors are used in the filtering stage of an integrated circuit SD DAC, then the device can be implemented on-chip with compact size, but the output signal exhibits undesirable distortion due to non-linear resistance
Solution Approach 1:
The patent extracts the output resistor from the integrated circuit chip and places it externally. This allows the filtering stage to use a linear resistor outside the chip while keeping other components on-chip, thereby eliminating the distortion caused by non-linear polysilicon resistors while maintaining compact on-chip integration.
Solution Approach 2:
The patent applies different resistance linearity requirements to different parts of the circuit. The output resistor, which critically affects signal linearity, is specified to be a linear resistor (metal film or carbon film) placed externally, while other resistors on-chip can tolerate non-linearity. This localized quality improvement resolves the contradiction between on-chip integration and output distortion.
2Manufacturing precision
If an approximately linear output resistor is used in the filtering stage, then the linearity of the output signal is improved, but the device complexity increases due to off-chip components
Solution Approach 1:
The critical linear output resistor is extracted from the on-chip filtering stage and placed externally. This extraction improves signal linearity while minimizing the increase in device complexity, as only one component is moved off-chip and the on-chip filtering architecture remains intact.
Solution Approach 2:
The patent changes the resistance linearity parameter of the output resistor by selecting linear resistor materials (metal film or carbon film) and placing them externally. This parameter change improves signal linearity while the overall device complexity increase is managed by keeping the filtering stage topology unchanged.
3Object-generated harmful factors
If error correction path is added to correct non-linearity, then the distortion in output signal is reduced, but the translator stage complexity increases
Solution Approach 1:
The patent implements an error correction path that generates a correction signal based on the non-linearity of the output resistor and adds it to the translator stage output. This feedback mechanism reduces output distortion by compensating for resistor non-linearity, while the added complexity is concentrated in the error correction circuitry rather than the core translator stage.
Solution Approach 2:
The patent segments the distortion correction function into a separate error correction path that is distinct from the main translator stage. This segmentation allows the translator stage to focus on signal conversion while the error correction path handles non-linearity compensation, distributing the complexity rather than concentrating it in one block.
Data Source
AI summary
A sigma-delta digital-to-analog converter (SD DAC) exhibits undesirable distortion when implemented in an integrated circuit due to the non-linearity of polysilicon resistors used in the filtering stages of the SD DAC. By using resistors other than polysilicon for the output resistor of an SD DAC, distortion can be reduced or eliminated. Additionally or alternatively, by generating an error correction signal, the distortion can be corrected.