Current-Output DAC with Auxiliary Current Range Compensation
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Solution Overview
Problem
Current-output DACs are highly sensitive to process and temperature variations, leading to an unreliable output current range, which can cause errors in downstream processing and affect the functionality of applications such as photon counting systems.
Innovation Solution
The proposed solution involves a current-output DAC design that includes a voltage-output DAC coupled with a transconductance stage and an auxiliary current generator. The auxiliary current generator produces a second current that is correlated with the voltage-output DAC and summed with the first current to extend the output current range and compensate for temperature and process variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a current-output DAC uses fully decoded steered current sources, then it can drive low output impedances and be used in high speed applications, but the output current range becomes highly sensitive to process and temperature variations
Solution Approach 1:
The DAC is segmented into two independent parts: a voltage-output DAC that generates a voltage signal, and a separate transconductance stage that converts the voltage to current. This segmentation isolates the sensitive current generation from the digital control logic, allowing the voltage DAC to operate with stable digital-to-voltage conversion while the transconductance stage handles the current output. The segmentation enables high-speed operation through optimized voltage switching while improving reliability by separating the functions that are sensitive to process and temperature variations.
Solution Approach 2:
A voltage-output DAC serves as an intermediary between the digital input and the current output. Instead of directly generating current from digital codes, the system first converts digital codes to voltage with high precision and stability, then uses a transconductance stage to convert the stable voltage to current. This intermediary voltage stage acts as a buffer that isolates the current output from digital control variations, thereby improving output current range stability while maintaining high-speed capability.
2Reliability
If the output current range is extended to compensate for temperature and process variations, then reliability improves, but device complexity increases due to additional circuitry
Solution Approach 1:
The voltage-output DAC serves multiple functions: it acts as both the digital-to-analog conversion stage and as a reference for the transconductance stage. The same voltage signal is used both for high-precision voltage output and as the input to the transconductance amplifier that generates the current output. This multi-functionality eliminates the need for separate voltage and current DAC circuits, extending the output current range while avoiding the complexity of fully decoded steered current sources.
Solution Approach 2:
The patent replaces the traditional electrical current-steering mechanism with a voltage-based control mechanism. Instead of using complex current mirrors and steered current sources that are highly sensitive to process variations, the system uses a voltage-output DAC followed by a transconductance amplifier. This substitution of the control mechanism from direct current steering to voltage-mediated current generation simplifies the circuit while improving reliability against process and temperature variations.
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 design effectively extends the output current range of the DAC without modifying the transfer characteristic gain or the least significant bit (LSB) size, ensuring reliability across varying conditions and maintaining consistent LSB granularity and gain over a large output current range.
Implementation Method 1
a transconductance stage to convert the voltage output from the voltage-output DAC to a current
Implementation Method 2
The auxiliary current generator includes a second transistor having a gate node coupled to a gate node of the first transistor in the buffer stage of the voltage-output DAC
Implementation Method 3
The buffer stage includes a first transistor designed to provide a string current across the resistor string
Data Source
AI summary
A circuit includes a digital-to-analog converter (DAC) configured to receive a binary input signal and to provide an output voltage that corresponds to a magnitude of the binary input signal. The circuit further includes a transconductance stage configured to receive the output voltage from the DAC and to generate a DAC current based on a magnitude of the output voltage. The circuit also includes an auxiliary current generator configured to generate an auxiliary current. The DAC current and the auxiliary current are summed together to produce an output current.