Continuous-Time Sigma-Delta ADC Input Circuit for Stable Wideband Conversion
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Solution Overview
Problem
Conventional discrete-time Sigma-Delta Analog-to-Digital Converters (ADCs) are vulnerable to disturbances and limited by the need for fast opamps, leading to instability and distortion, especially under high input signals, and require additional circuitry for stability and anti-aliasing, which complicates system design and increases noise floor.
Innovation Solution
A continuous-time Sigma-Delta ADC with a single directional voltage-to-current converter that translates differential signal voltage to current, preventing current generation during non-linear disturbances and providing capacitive input impedance, thereby stabilizing the system and reducing noise, and allowing for variable transconductance to adjust the dynamic range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If discrete-time Sigma-Delta ADC uses clocked loop filters, then signal bandwidth is limited by opamp speed, but stability can be maintained through sampling action
Solution Approach 1:
The patent replaces the mechanical/clocked sampling system with a continuous-time system. Instead of using clocked switches and discrete-time sampling to achieve stability, the invention uses continuous-time loop filters with inherent stability properties, allowing the system to operate at higher speeds without the stability compromises of discrete-time approaches
Solution Approach 2:
The patent changes the fundamental operating parameter from discrete-time sampling to continuous-time operation. By transitioning from sampled signals to continuous signals processed through continuous-time integrators, the system achieves both high bandwidth and stability simultaneously, overcoming the speed-stability tradeoff
2Device complexity
If conventional integrator uses resistor and op-amp, then circuit is simple, but non-linear disturbances at op-amp input produce distortion at integrator output
Solution Approach 1:
The patent extracts and removes the problematic resistor component from the integrator circuit. By eliminating the resistor and using only capacitive elements in the feedback path, the system avoids the non-linear distortion issues that arise from resistive loading and op-amp input disturbances, while maintaining circuit functionality
Solution Approach 2:
The patent changes the impedance characteristics of the integrator from resistive to purely capacitive. By using a capacitive integrator instead of a resistive one, the system achieves linear operation even under non-linear disturbances, as capacitive elements do not generate the same type of distortion as resistive elements in this context
3Measurement precision
If DAC output uses large step size, then quantization resolution is improved, but integrator is excited causing op-amp slewing and non-linear behavior
Solution Approach 1:
The patent replaces the resistive integrator that suffers from slewing with a capacitive integrator. The capacitive integration mechanism does not exhibit the same slewing behavior as resistive-integrator op-amps when subjected to large step inputs, thereby maintaining linear operation and avoiding non-linear distortion while preserving quantization resolution
4Device complexity
If continuous-time loop filter is used, then separate anti-aliasing filter is not needed, but loop filter must handle wide bandwidth signals
Solution Approach 1:
The patent designs the continuous-time loop filter to serve multiple functions simultaneously. The same filter structure provides both the noise shaping function required for Sigma-Delta conversion and the anti-aliasing function that would otherwise require a separate filter, while handling wide bandwidth signals through its continuous-time operation
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 solution enhances stability and reduces noise by preventing current generation during non-linear disturbances and providing capacitive input impedance, allowing for wider signal bandwidth and lower noise floors, making the ADC more reliable and efficient.
Implementation Method 1
a single directional voltage-to-current converter coupled to the input terminal. The single directional voltage-to-current converter translates a differential signal voltage only to a differential signal current
Implementation Method 2
at least one capacitor coupled to the operational amplifier and the feedback mechanism
Data Source
AI summary
A continuous time sigma-delta analog-to-digital converter (CT ΣΔADC) including an integrator, which includes an operational amplifier having at least one input terminal that receives an input signal, a feedback mechanism operatively connected to the operational amplifier, at least one capacitor coupled to the operational amplifier and the feedback mechanism, a reset switch coupled to the at least one capacitor, the operational amplifier, and the feedback mechanism, and a single directional voltage-to-current converter coupled to the input terminal. The single directional voltage-to-current converter translates a differential signal voltage only to a differential signal current. The reset switch resets the feedback mechanism. The single directional voltage-to-current converter behaves a one-directional resistor. The integrator prevents current generation when there is a non-linear disturbance at the input terminal of the operational amplifier. The single directional voltage-to-current converter clips an input current that exceeds a threshold value.