AFE Feedback Loop With Quantization Noise Cancellation
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Solution Overview
Problem
Analog front ends in ECG measurement systems face challenges in minimizing power consumption while maintaining a fixed noise budget, particularly due to thermal noise from resistors and limited dynamic range in continuous-time gains, which can be overcome by using capacitive gain elements and feedback digital-to-analog converters.
Innovation Solution
The solution involves using capacitive gain elements for both input and feedback signals, and integrating a digital-to-analog converter with the gain stage to achieve extended dynamic range and reject sampling noise, while also recombining quantized filtered ADC outputs to reduce quantization noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If continuous-time gain is taken before ADC sampling, then sampling noise impact is reduced, but amplifier dynamic range is limited and realizable gain is constrained
Solution Approach 1:
The patent segments the gain function into two parts: a first gain stage before ADC sampling and a second gain stage after ADC sampling. This segmentation allows the first stage to provide sufficient gain to reduce sampling noise impact while the second stage provides additional gain without being constrained by the amplifier's dynamic range, thereby resolving the contradiction between improving signal-to-noise ratio and maintaining dynamic range adaptability
Solution Approach 2:
The patent introduces an intermediary digital signal processing stage between the two gain stages. The ADC converts the analog signal to digital, allowing a digital gain stage to operate on the digitized signal. This intermediary digital stage acts as a mediator that can provide gain without the dynamic range constraints of analog amplifiers, enabling the system to achieve high overall gain while maintaining full dynamic range capability
2Power
If resistors are used for continuous-time gain, then gain is achieved, but thermal noise increases and power consumption increases
Solution Approach 1:
The patent replaces resistive gain mechanisms with capacitive gain mechanisms in the analog front end. Capacitors store energy in electric fields rather than dissipating it as thermal noise through resistance. This substitution eliminates the thermal noise generation inherent in resistive circuits while maintaining the ability to provide continuous-time gain, thereby resolving the contradiction between achieving gain capability and minimizing thermal noise
Solution Approach 2:
The patent changes the fundamental parameter used for gain from resistance to capacitance. By using capacitive dividers and switched capacitor circuits, the system achieves gain through charge redistribution rather than resistive voltage division. This parameter change from resistance to capacitance fundamentally alters the noise characteristics, eliminating thermal noise while preserving gain functionality
3Extent of automation
If quantization is applied to filtered ADC output, then digital processing is enabled, but quantization noise is introduced
Solution Approach 1:
The patent applies feedback techniques where a portion of the quantized output is fed back and subtracted from the original ADC output. This feedback mechanism allows the system to compensate for quantization noise by canceling out the quantization error components, thereby maintaining high measurement precision while enabling digital signal processing through quantization
Solution Approach 2:
The patent combines multiple signal paths with different processing characteristics: an analog path that preserves continuous signals and a digital path that enables programmable processing. By compositing these paths and selectively combining their outputs, the system achieves both digital processing capability and high precision, as the analog path preserves signal fidelity while the digital path provides processing flexibility
Data Source
AI summary
An analog front end (AFE) system for substantially eliminating quantization error or noise can combine an input of an integrator circuit in the AFE system with an input of the digital-to-analog converter (DAC) circuit in the feedback loop of the AFE system. By combining the input of the integrator with the input of the DAC circuit in the feedback loop, the in-band quantization noise of the filter can be substantially eliminated, thereby improving measurement accuracy.