Segmented DAC Resistor Network for Lower Thermal Noise
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Solution Overview
Problem
Conventional digital-to-analog converters (DACs) with resistor-2resistor (R2R) ladders suffer from thermal noise at low output current levels, which degrades signal resolution and increases noise in transmitters due to low output impedance, affecting signal-to-noise-and-distortion ratio (SNDR) and receive-band noise (RxBN) performance.
Innovation Solution
Implementing a segmented resistor architecture with a first resistor network coupled to binary code-controlled current sources and a second resistor network with higher resistance, allowing thermometer code-controlled current sources to inject current into a node, thereby increasing the output impedance and reducing thermal noise transfer to the DAC output.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a conventional R2R ladder is used in a DAC, then the circuit structure is simple and easy to manufacture, but the output impedance is low which causes thermal noise to transfer directly to the output, degrading signal resolution and SNDR performance
Solution Approach 1:
The resistor network is divided into two separate resistor networks: a first resistor network coupled to binary code-controlled current sources, and a second resistor network coupled between the first resistor network and the DAC output. This segmentation allows the second resistor network to provide higher resistance and increased output impedance, thereby reducing thermal noise transfer to the output while maintaining manufacturing feasibility through modular design
Solution Approach 2:
The patent changes the resistance parameter by introducing a second resistor network with higher resistance values than the conventional R2R ladder. This parameter change increases the output impedance of the DAC, which reduces the transfer of thermal noise to the output, thereby improving signal resolution and SNDR performance
2Measurement precision
If the output current level is reduced to improve signal resolution, then the output signal becomes more precise, but thermal noise becomes more substantial relative to the signal, further reducing DAC resolution
Solution Approach 1:
By changing the resistance parameter to higher values in the second resistor network, the output impedance is increased. This allows the system to operate at lower current levels for improved signal precision while the higher impedance prevents proportional increase in thermal noise, thus maintaining signal-to-noise ratio at low current levels
3Object-affected harmful factors
If the resistance of the resistor network is increased to improve output impedance and reduce thermal noise, then noise power at the output is reduced, but the current steering capability and power consumption may be affected
Solution Approach 1:
The segmented resistor network architecture separates the high-resistance second resistor network from the current steering function, which is handled by the binary code-controlled and thermometer code-controlled current sources. This allows the high resistance to reduce thermal noise while the current sources maintain effective current steering capability, resolving the trade-off between noise reduction and power consumption
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The segmented resistor architecture enhances the effective number of bits (ENOB) and improves SNDR/RxBN performance by reducing noise power at the DAC output, increasing output impedance by a factor of four compared to traditional R2R ladders.
Implementation Method 1
individual resistors of the R2R ladder produce thermal noise that can build and transfer toward the output of the DAC along with the output current
Data Source
AI summary
The present disclosure describes aspects of segmented resistor architecture for digital-to-analog converters (DACs). In some aspects, a DAC circuit is implemented with a first resistor network coupled to a set of binary code-controlled current sources and a second resistor network that includes a resistor coupled between the first resistor network and an output of the DAC circuit. A set of thermometer code-controlled current sources are coupled to a node of the second resistor network and provide varying amounts of current. This current is scaled based on a resistance of the second resistor network's resistor, which is higher than a resistance of the first resistor network and effective to increase a combined output impedance of the first and second resistor networks. The increase of output impedance reduces noise of the resistor networks that transfers to the output of the DAC circuit, thereby improving signal-to-noise performance of the DAC circuit.