DAC Chopper Noise Reduction via Digital Filter Zeros
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Solution Overview
Problem
Chopper amplifier output stages in digital-to-analog converters (DACs) face challenges in reducing broadband noise, particularly noise sources near multiples of the chopping frequency, which are aliased to in-band frequencies, increasing noise levels.
Innovation Solution
A digital filter with non-uniformly weighted switched current elements is introduced, forming a finite impulse response (FIR) filter that includes extra zeros at the chopping frequency and its odd harmonics, effectively attenuating noise and artifacts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a chopper amplifier output stage is used in a DAC, then offset voltages and flicker noise are reduced, but broadband noise near multiples of the chopping frequency is aliased to in-band frequencies, increasing noise levels
Solution Approach 1:
The digital filter is placed before the chopper amplifier to preemptively remove noise components at multiples of the chopping frequency from the digital signal. This preliminary action prevents the noise from being aliased to in-band frequencies by the chopper, solving the contradiction by addressing the noise issue before it can harm the output.
Solution Approach 2:
A digital filter is introduced as an intermediary component between the digital input and the chopper amplifier. This intermediary selectively attenuates frequency components at multiples of the chopping frequency before they reach the chopper stage, preventing the aliasing problem while preserving the offset reduction benefit.
2Object-affected harmful factors
If a digital filter with non-uniform weighting is used to reduce noise, then noise attenuation at chopping frequencies is improved, but filter design complexity increases
Solution Approach 1:
The digital filter uses non-uniform weighting where only specific filter coefficients are adjusted to create zeros at the chopping frequency multiples. This local quality approach modifies only the necessary parts of the filter (specific coefficients) rather than redesigning the entire filter, achieving noise attenuation with minimal additional complexity.
Solution Approach 2:
The filter coefficients are modified by changing their weighting values to introduce zeros at specific frequencies (multiples of the chopping frequency). This parameter change approach allows the filter to target specific noise frequencies without requiring a complete redesign, balancing performance improvement with design simplicity.
Data Source
AI summary
A method and apparatus for reducing noise in a digital-to-analog converter (DAC) having a chopper amplifier output stage provides improved DAC performance. A switched-current output provided from a digital filter is coupled to the input of a chopper amplifier. The current switches are non-uniform, so that extra zeros are provided at the chopping frequency of the chopping amplifier, thereby reducing noise that would otherwise be aliased in-band at the output of the chopper amplifier. The current switches may include a first set of half-magnitude switches followed by a set of full-magnitude switches and finally by another set of half-magnitude switches having a size equal to that of the first set.