CT MASH ADC Dither Injection for Quantization Noise Cancellation
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Solution Overview
Problem
In multi-stage noise shaping analog to digital converters (MASH ADCs), quantization noise cancellation is challenging due to leakage issues when injecting a dither signal, which can degrade noise performance and require accurate timing synchronization to prevent phase errors.
Innovation Solution
Injecting a dither signal at the output of the quantizer in a MASH ADC, using a dummy comparator to synchronize the dither signal with quantization noise, and replicating circuitry to match the timing of the quantizer, allowing for accurate estimation of transfer functions and cancellation of quantization noise through digital filters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a dither signal is injected at the input of the quantizer to estimate transfer functions, then the transfer function estimation can be performed, but quantization noise leakage occurs and noise performance degrades
Solution Approach 1:
The dither signal injection point is extracted from the quantizer input and moved to the quantizer output. This separation allows the dither signal to be added after quantization, preventing it from causing noise leakage while still enabling transfer function estimation through correlation with the quantizer output.
Solution Approach 2:
A delay element is introduced as an intermediary component between the dither signal source and the quantizer output. This delay element synchronizes the dither signal timing with the quantizer output, enabling accurate correlation-based transfer function estimation without requiring direct access to the quantizer internal timing.
2Device complexity
If dither signal timing is not synchronized with quantizer output, then implementation is simpler, but phase errors occur and estimation accuracy degrades
Solution Approach 1:
The delay element is configured to pre-adjust the dither signal timing based on the known quantizer output timing characteristics. This preliminary timing adjustment ensures that when the dither signal is correlated with the quantizer output, they are properly synchronized, eliminating phase errors without requiring complex real-time timing synchronization circuitry.
3Adaptability or versatility
If dither signal is injected early in the conversion process, then more stages can be characterized, but noise leakage affects multiple stages and degrades overall performance
Solution Approach 1:
The dither signal injection is localized to each individual quantizer stage's output rather than being injected globally at an earlier point. This localized injection ensures that the dither signal only affects the specific stage being characterized and does not propagate noise leakage to other stages, allowing independent characterization of multiple stages without mutual interference.
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AI summary
For continuous-time multi-stage noise shaping analog to digital converters (CT MASH ADCs), quantization noise cancellation often requires estimation of transfer functions, e.g., a noise transfer function of the front end modulator. To estimate the noise transfer function, a dither signal can be injected in the front end modulator. However, it is not trivial how the dither signal can be injected, since the dither signal can potentially leak to the back end modulator and cause overall noise degradation. To address some of these issues, the dither signal is injected post the flash analog to digital converter (ADC) of the front end modulator. Furthermore, dummy comparator structures can be used to synchronize the dither with the quantization noise of the targeted flash ADC.