R-2R DAC Switch Control for Temperature-Stable Linearity
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Solution Overview
Problem
Conventional digital-to-analog converters (DACs) face nonlinearity issues due to mismatched resistances in resistor ladders and switch on-resistances, limiting their accuracy to 16-bit integral nonlinearity (INL), which is not effectively addressed by existing solutions.
Innovation Solution
A DAC circuit with a control circuit that dynamically adjusts the on-resistance of switches in an R-2R ladder network to maintain a predetermined total resistance, using a replica branch to regulate gate voltages and compensate for temperature and voltage changes, ensuring the sum of resistances matches the required 2R resistance ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If MOS transistors are used as switches in DAC, then device simplicity is improved, but on-resistance increases causing nonlinearity
Solution Approach 1:
The patent dynamically changes the gate voltage parameter of the MOS transistor switch to adjust its on-resistance. By varying VGS, the on-resistance is tuned to compensate for resistor ladder mismatches, thereby maintaining output linearity while keeping the simple MOS switch structure
Solution Approach 2:
The patent implements a feedback mechanism where the actual resistor values are measured and used to adjust the MOS switch on-resistance accordingly. This closed-loop control compensates for manufacturing variations in the resistor ladder, maintaining precision without complicating the basic switch structure
2Device complexity
If conventional resistor ladders are used, then device complexity is reduced, but resistance mismatch causes nonlinearity errors
Solution Approach 1:
The patent employs self-service techniques where the system automatically measures its own resistor values and adjusts the MOS switch parameters to compensate for detected mismatches. This self-calibration maintains precision without requiring external calibration equipment or complex additional circuitry
Solution Approach 2:
The patent transforms the static resistor ladder into a dynamically adjustable system by allowing the MOS switch on-resistance to vary. This dynamic adjustment compensates for fixed manufacturing mismatches in the resistor values, maintaining linearity without changing the physical resistor structure
3Manufacturing precision
If switch on-resistance is increased to compensate for resistor mismatch, then linearity improves, but voltage losses increase
Solution Approach 1:
The patent dynamically adjusts the MOS switch on-resistance to match the actual resistor ladder values rather than using a fixed high resistance. This dynamic tuning achieves linearity compensation while minimizing voltage losses by using only the necessary amount of resistance
4Stability of the object's composition
If temperature compensation is implemented, then temperature invariance is improved, but device complexity increases
Solution Approach 1:
The patent merges the temperature compensation function with the existing linearity compensation mechanism. The same MOS switch parameter adjustments that correct resistor mismatches also compensate for temperature effects, as both issues manifest as resistance variations. This unified approach achieves temperature invariance without separate compensation circuitry
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
This approach enhances DAC linearity beyond 16-bit INL, achieving high-resolution output with improved accuracy and reduced nonlinearity errors in a compact integrated circuit area.
Implementation Method 1
a switch including a transistor coupled to one of the first and second resistors. Further, the DAC circuit includes a control circuit operable to adjust an on-resistance of the transistor
Implementation Method 2
Where μn is the electron mobility in the channel, Cox is the gate oxide capacitance per unit area, W and L are the effective width and length of the transistor, respectively, VGS is the gate-source voltage, and VTH is the threshold voltage of the transistor
Data Source
AI summary
Circuitry is disclosed herein that dynamically (temperature-invariant and voltage-invariant) adjusts the Ron of switches in a resistive Nyquist-rate digital to analog converter (DAC) to thereby reduce DAC nonlinearity errors and improve INL results of greater than 16b. Consistent with the present disclosure, the DAC includes an R-2R ladder in which each bit corresponds to a switch. A control circuit is provided for generating signals applied to the gate of the switch to cause the on-resistances of the switch to be a particular value, such that the on-resistance of the switch plus the sum of two resistors, one having the resistance R, and the other having a resistance R′ is equivalent to the resistance of the 2R-size resistors or twice the resistance of the R-sized resistors in the ladder.