Differential Current-Mode SD DAC for Lower Gain and Offset Error
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Solution Overview
Problem
Current sigma-delta digital-to-analog converters (SD DACs) suffer from gain and offset errors that require additional hardware, firmware, and software corrections for high precision operation, which can be cumbersome and inefficient.
Innovation Solution
A differential current-mode SD DAC design that includes a current-mode translator with a differential current source and a current-to-voltage converter configured to operate at a virtual ground voltage level, allowing for improved current matching and reduced voltage compliance range, thereby minimizing gain and offset errors through resistance matching rather than transistor matching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a two-state voltage translator is used in an SD DAC, then the converter can generate two different voltages corresponding to logic one and logic zero, but gain errors and offset errors occur that reduce precision
Solution Approach 1:
The patent replaces the conventional two-state voltage translator with a differential current-mode translator. This substitution changes the fundamental operating mode from voltage-based to current-based translation, enabling better error performance. The differential current translator uses current sources and transconductance amplifiers instead of voltage switches, fundamentally changing how the translation is achieved while improving precision.
Solution Approach 2:
The patent changes the operating parameters of the translator by operating at a virtual ground voltage level with differential currents. By changing from single-ended voltage operation to differential current operation, the system achieves better symmetry and matching, reducing both gain and offset errors. The virtual ground operation point is a specific parameter change that enables improved precision.
2Measurement precision
If digital domain modifications are applied to correct gain and offset errors, then precision can be improved, but additional digital hardware, firmware, and software are required
Solution Approach 1:
The patent applies preliminary action by correcting gain and offset errors at the translator stage before the signal proceeds through the rest of the DAC system. By addressing the errors early in the signal path through improved translator design, subsequent stages don't require additional correction hardware or software, reducing overall system complexity.
Solution Approach 2:
The differential current-mode translator is self-correcting in that its symmetric differential architecture inherently reduces offset errors, and its current-mode operation with virtual ground provides inherent gain accuracy. The design serves its own correction needs through its fundamental architecture rather than requiring external correction mechanisms.
3Measurement precision
If current sources are designed with high current matching, then gain and offset errors are reduced, but the voltage compliance range is reduced
Solution Approach 1:
The patent uses virtual ground operation to create equipotential conditions at the input nodes of the transconductance amplifiers. By maintaining both inputs at virtual ground potential, the system achieves high current matching without requiring a large voltage compliance range, as the voltage swing requirements are minimized through the virtual ground technique.
Data Source
AI summary
In general, this disclosure is directed to a differential current-mode sigma-delta digital-to-analog converter (SD DAC) with improved accuracy and reduced offset and gain errors. In one example, the SD DAC may include a current source configured to provide a differential current. The SD DAC may further include a switching network configured to adjust a polarity of the differential current according to a bit within the bit-stream to produce a differential current signal. The SD DAC may further include a current-to-voltage converter configured to convert the differential current signal to a differential voltage signal. In additional examples, the differential current source may include one or more source degeneration resistances. In further examples, the current-to-voltage converter may include a fully-differential operational amplifier. A low pass filter may be included within the current-to-voltage converter and/or coupled to the output of the current-to-voltage converter.