Segmented Resistive DAC With Equalized Switch Resistance
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Solution Overview
Problem
Conventional segmented Resistive Digital-to-Analog Converters (R-DACs) are limited in speed and accuracy due to high equivalent series resistance, requiring a power amplifier for reasonable operation and facing challenges in maintaining monotonicity with large switch-on resistances.
Innovation Solution
A digital-to-analog converter (DAC) design that includes a bridge switch with adjustable switch-on resistance, allowing for the use of PMOS, NMOS, or CMOS transistors to mitigate mismatch between branches and improve accuracy and monotonicity, while using a binary-to-thermometer decoder to decode binary input signals into thermometer codes for precise output generation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If switches with low on-resistance are used to maintain monotonicity, then accuracy is improved, but device area increases significantly
Solution Approach 1:
The DAC is divided into two independent sub-DACs (MSB sub-DAC and LSB sub-DAC) with separate switch groups. Each sub-DAC handles a portion of the conversion, allowing switches to be optimized for their specific function rather than requiring all switches to have uniformly low on-resistance. This segmentation reduces the overall area requirement while maintaining monotonicity.
Solution Approach 2:
Different switch groups are assigned different functions: the first group of switches controls MSB contributions and the second group controls LSB contributions. This allows each switch group to be optimized locally - the first group can use larger switches for lower on-resistance where needed, while the second group can use smaller switches where high speed is more critical, achieving monotonicity without uniform area expansion.
2Speed
If a power amplifier is used to achieve reasonable speed, then operating speed is improved, but device complexity and power consumption increase
Solution Approach 1:
The patent uses dynamic switching of capacitor connections rather than a static power amplifier configuration. The switches dynamically connect capacitors to different voltage references (VrefH or VrefL) based on the input digital code, enabling high-speed operation through rapid switching action rather than through amplifier bandwidth, thus avoiding the complexity of high-speed power amplifiers.
Solution Approach 2:
The patent replaces the need for a power amplifier (an active electronic component requiring complex biasing and compensation circuits) with a passive capacitor-switch network. The capacitors store and transfer charge directly, and the switches control the charge redistribution, eliminating the need for a power amplifier entirely while achieving high-speed operation through the inherent speed of capacitive charge transfer.
3Measurement precision
If the number of resistors in sub-DAC is increased to improve resolution, then accuracy is improved, but equivalent series resistance increases and speed decreases
Solution Approach 1:
The patent transitions from a resistor-based voltage division approach to a capacitor-based charge storage and transfer approach. Instead of using multiple resistors in series to achieve high resolution (which increases equivalent series resistance and reduces speed), the patent uses capacitors that can be rapidly charged and discharged to represent digital codes, achieving high resolution through the number of capacitor stages rather than resistor divisions, thereby maintaining high speed operation.
Data Source
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AI summary
A digital to analog converter (DAC) that receives a binary coded signal and generates an analog output signal includes a binary-to-thermometer decoder and a resistive network. The decoder receives the binary coded signal, and decodes it into thermometer coded signals. The resistive network has branches that are coupled to an output terminal of the DAC in response to the thermometer coded signals. Each of the branches includes first and second resistors, and a switch. The first resistor is coupled between a first reference voltage and the switch, and the second resistor is coupled between a second reference voltage and the switch. The switch couples either the first resistor or the second resistor to the output terminal in response to a corresponding thermometer coded signal.