Current Steering DAC Cell Layout for Switchless Dual Outputs
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Solution Overview
Problem
Conventional Digital-to-Analog Converters (DACs) require external switches to redirect output signals to multiple destinations, which can degrade signal quality due to on-resistance and added capacitance, violating specifications like VESA Video Signal Standard and limiting bandwidth.
Innovation Solution
A current steering DAC design with multiple unit cells connected in parallel, each featuring three output switches forming differential pairs that share a negative switch, eliminating the need for external switches by creating two positive output paths, thus reducing circuit area and relaxing design requirements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If an external switch is added to redirect the output signal to multiple destinations, then the output signal can be directed to multiple destinations, but the output signal quality is degraded due to on-resistance and added capacitance
Solution Approach 1:
The DAC is divided into multiple independent unit cells (e.g., first and second unit cells), each capable of generating differential output signals independently. This segmentation allows each unit cell to drive separate output destinations without requiring external switching, thereby maintaining signal quality while achieving multi-destination output capability.
Solution Approach 2:
Each DAC unit cell is designed with universal functionality to drive multiple output destinations directly. The differential output structure of each unit cell can be independently connected to different destinations (e.g., VGA connector and docking connector), eliminating the need for external switches and preserving signal integrity across multiple outputs.
2Adaptability or versatility
If an external switch is added to redirect the output signal, then multiple output destinations can be served, but the output bandwidth is limited due to extra capacitance
Solution Approach 1:
By segmenting the DAC into multiple unit cells with independent differential outputs, each unit cell can directly drive specific destinations without sharing a common external switch. This eliminates the capacitance bottleneck and preserves high-frequency signal integrity, maintaining full output bandwidth across multiple destinations.
3Device complexity
If a conventional DAC design is used, then the circuit structure is simple, but external switches are required which increase overall circuit area
Solution Approach 1:
The DAC unit cells are designed with universal output capability that can directly serve multiple destinations without external switches. This multi-functional design integrates the switching functionality within the DAC structure itself, eliminating the need for separate external switch components and reducing overall circuit area.
4Adaptability or versatility
If an external switch is placed behind the DAC, then output path selection is enabled, but the DAC must compensate for the switch on-resistance
Solution Approach 1:
The differential output structure segments the signal paths, allowing each unit cell to maintain precise voltage control independent of external switch resistance. This segmentation enables direct connection to multiple destinations without requiring compensation for switch on-resistance, thereby meeting output compliance voltage specifications.
Data Source
AI summary
A current steering Digital-to-Analog Converter (DAC), a video adapter including a current steering DAC, and a video circuit including a current steering DAC are described. In one embodiment, a current steering DAC includes multiple DAC unit cells that are connected in parallel with each other. Each of the DAC unit cells includes three output switches connected to a current source. The three output switches are configured to form two differential pairs of switches that are placed in parallel with each other and share a negative output switch of the three output switches. The current steering DAC also includes at least one switch control circuit configured to receive digital input data and to control the three output switches of each of the DAC unit cells to generate differential analog output data based on the digital input data and a current from the current source. Other embodiments are also described.