R2R Ladder DAC Trim Circuit for DNL and Headroom Limits
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Solution Overview
Problem
Digital-to-analog converters (DACs) face challenges in achieving high precision and voltage range while managing headroom effectively, with existing designs often suffering from differential non-linearities due to resistor mismatches.
Innovation Solution
The DAC design incorporates a first stage with thermometric circuit arms and a second stage with series-connected resistors, featuring a current DAC trim circuit that selectively injects current into specific circuit arms based on a digital current code to trim differential non-linearities, using a combination of current sources and switches to adjust the analog output voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a high precision DAC (e.g., 20-bit) is designed, then the precision is improved, but the headroom requirement increases
Solution Approach 1:
The DAC is divided into two stages: a first stage with circuit arms for coarse conversion and a second stage with circuit arms for fine conversion. The IDAC trim circuit is connected to only some of the second stage circuit arms, segmenting the trimming function to reduce overall headroom requirements while maintaining high precision.
Solution Approach 2:
The IDAC trim circuit applies local correction only to specific circuit arms in the second stage that exhibit differential non-linearities due to resistor mismatches. This localized approach allows precision improvement without requiring increased headroom across the entire DAC structure.
2Adaptability or versatility
If the voltage range of the DAC is increased, then the voltage range capability is improved, but the headroom requirement increases
Solution Approach 1:
The two-stage architecture segments the voltage conversion function, allowing the DAC to achieve extended voltage range capability through the combination of first and second stage circuit arms without proportionally increasing the headroom requirement of the entire system.
3Manufacturing precision
If resistor mismatches are present in the R2R ladder, then the manufacturing precision deteriorates, but device complexity increases when trim circuits are added
Solution Approach 1:
The IDAC trim circuit provides local compensation for resistor mismatches in specific second stage circuit arms. By targeting only the circuit arms that require correction rather than implementing a comprehensive trim circuit across all circuit arms, the solution addresses manufacturing precision issues without excessive increase in device complexity.
Solution Approach 2:
The trim circuit is applied to only some of the second stage circuit arms rather than all of them. This partial application is sufficient to correct the differential non-linearities caused by resistor mismatches while avoiding the complexity of a full-coverage trim circuit.
Data Source
AI summary
A digital-to-analog converter (DAC) includes a first stage comprising a plurality of first circuit arms coupled together, each first circuit arm including a resistor. A second stage includes a plurality of second circuit arms coupled together, each second circuit arm comprising a first resistor and a pair of series-connected resistors. The first resistors of the second circuit arms are connected in series. A current digital-to-analog converter (IDAC) trim circuit is connected to a plurality, but not all, of the second circuit arms of the second stage. The IDAC trim circuit includes a plurality of first current sources. Each first current source is coupled to a respective node between a pair of the series-connected resistors of a corresponding second circuit arm, and each of the first current sources is configured to produce a same current level as the other first current sources.