Sigma-Delta Converter Dither Injection at Virtual Ground
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Solution Overview
Problem
Sigma-delta converters face challenges in effectively addressing quantization noise, particularly due to its non-white, correlated nature causing audible noise and high peak-to-RMS ratios, and existing dithering techniques either require large signals, limit dynamic range, or are inefficient in terms of silicon area usage.
Innovation Solution
A method to add a dither signal at the output of the last integrator of a sigma-delta converter by feeding it to the virtual ground node of the amplifier, using a digital-to-analog converter to generate an amplified analog replica of the dither signal, which does not require additional dedicated circuitry, thus saving silicon area and allowing precise amplification control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a dither signal is added using a dedicated adder circuit in cascade to the last integrator, then quantization noise is effectively reduced, but silicon area is significantly occupied
Solution Approach 1:
The patent merges the dither signal addition function with the existing virtual ground node of the integrator amplifier. Instead of using a separate adder circuit, the dither signal is injected directly at the virtual ground node where the feedback capacitor connects to the inverting input. This combines the dither injection function with the existing integrator structure, eliminating the need for dedicated adder circuitry and reducing silicon area while maintaining effective quantization noise reduction.
Solution Approach 2:
The virtual ground node of the integrator amplifier is made multi-functional by using it both for its original purpose (summing the feedback signal and input signal) and for injecting the dither signal. This universal usage of the virtual ground node allows the same circuit point to serve multiple functions: signal summation, virtual ground maintenance, and dither signal injection, thereby eliminating the need for separate dedicated circuitry.
2Object-affected harmful factors
If large dither signals are used to remove periodicity in quantization noise, then noise attenuation is improved, but dynamic range of the converter is limited
Solution Approach 1:
The patent changes the parameter of dither signal amplitude by using a programmable gain amplifier (PGA) to provide variable amplification of the dither signal. The gain can be adjusted according to the input signal level, allowing the dither signal amplitude to be optimized for different operating conditions. This enables effective noise attenuation without permanently limiting the dynamic range, as the gain can be adapted to match the signal level.
Solution Approach 2:
The dither signal amplification is made dynamic through the use of a programmable gain amplifier that can adjust its gain based on operating conditions. This dynamic adjustment allows the system to use larger dither signals when needed for noise reduction while maintaining smaller dither signals when the input level is high, thereby preserving the converter's dynamic range across different operating conditions rather than being limited by a fixed large dither amplitude.
Data Source
AI summary
A single-ended or differential single-stage, or multi-stage sigma-delta analog-to-digital converter includes at least one switched-capacitor integrator comprising a switched-capacitor network receiving as input a signal to be sampled, and an amplifier coupled in cascade to the switched-capacitor network. A circuit is coupled to the amplifier for feeding an analog dither signal to a virtual ground of the amplifier.