Thermometer DAC Switching Layout for Low-Spike Fast Output
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Solution Overview
Problem
Thermometer DACs face high silicon 'real estate' costs due to extensive control logic and experience significant output spikes during fast switching, which can lead to malfunction and increased current consumption.
Innovation Solution
An n-bit thermometer DAC design with interconnected resistor strings and an up/down counter to minimize control signals and reduce output spikes by ensuring banksel switches move to the closest position for the next DAC code, thereby canceling charge injection effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If each switch is controlled separately with dedicated control signals, then the DAC achieves precise control over each resistor bank, but the amount of control logic and routing area increases significantly
Solution Approach 1:
The patent combines multiple control signals into fewer signals by using an up/down counter that generates sequential control patterns. Instead of requiring separate control signals for each of the 96 switches, the system uses a unified counter that cycles through all switch states, merging the control function into a single logical unit that reduces routing complexity while maintaining precise control over all resistor banks.
Solution Approach 2:
The up/down counter serves multiple functions: it controls the selection of resistor banks within a string, manages the selection between different strings via muxsel switches, and generates the sequential timing for all control operations. This multi-functional approach eliminates the need for separate dedicated control logic for each switch, reducing overall device complexity while preserving precision.
2Area of stationary object
If the number of control signals is reduced to save routing area, then the DAC area is decreased, but output spikes occur during switch transitions
Solution Approach 1:
The system prepares the next control state in advance by using the up/down counter to predict and pre-position the control signals. When transitioning between resistor banks, the counter has already positioned the control signals to minimize the distance between consecutive switch states, thereby preventing large charge injections and output spikes before they occur.
Solution Approach 2:
The up/down counter implementation inherently prevents output spikes by designing the control sequence to avoid large transitions. By counting sequentially through the switch states rather than allowing random or distant jumps, the system preemptively counteracts the harmful charge injection effect that would otherwise occur during abrupt switch transitions.
3Speed
If fast switching is implemented for rapid DAC output changes, then the response speed increases, but large charge injection occurs causing output spikes
Solution Approach 1:
The system dynamically adjusts the switching sequence to minimize charge injection while maintaining fast response. The up/down counter provides a dynamic control pattern that adapts to the current state, always transitioning to the nearest adjacent switch state rather than making fixed or random jumps. This dynamic approach enables fast switching speeds while continuously minimizing harmful charge injection effects.
Data Source
AI summary
A thermometer-coded Digital to Analog Converter (DAC) is described, whose output is changed with fast speed, and reduced output overshoot or undershoot. The thermometer-coded DAC has selection switches and an up/down counter, with DAC codes separated into higher and lower bits. The lower bits increase up to a maximum code, then decrease. The configuration of resistors in the DAC reduces output spike, especially at the DAC code changing point.