Current-Steering DAC Interface for Class D Common-Mode Ripple
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Solution Overview
Problem
Audio amplifiers, particularly those using class D amplifiers with current-steering DACs, face challenges due to noise and common mode ripple, which necessitate wide signal amplitude ranges and limit the use of low voltage power supplies.
Innovation Solution
A system comprising a class D amplifier directly connected to a current-steering DAC, with a common mode servo circuit and a feed-forward common-mode compensation circuit that includes resistors and a current mirror to reduce AC ripple, providing an alternative path for common mode ripple currents to ground.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a current-steering DAC is directly connected to a class D amplifier, then circuit complexity is reduced, but common mode ripple voltage increases
Solution Approach 1:
A common mode feedback circuit is introduced as an intermediary between the DAC and class D amplifier. This circuit includes a feedback amplifier that senses the common mode voltage at the amplifier input and generates a compensating signal to cancel the ripple, effectively mediating the harmful interaction while maintaining the direct connection architecture
Solution Approach 2:
The patent implements a common mode feedback mechanism where the feedback amplifier continuously monitors the common mode voltage at the class D amplifier input and adjusts the DAC output accordingly. This feedback loop dynamically compensates for common mode ripple, allowing the direct connection to be maintained without suffering from unchecked ripple effects
2Object-affected harmful factors
If common mode ripple voltage is reduced through feedback circuits, then noise is minimized, but device complexity increases
Solution Approach 1:
The feedback circuit is designed to address only the specific local problem of common mode ripple at the class D amplifier input, rather than attempting to control all signal parameters. The feedback amplifier is configured with specific gain and bandwidth characteristics tailored to suppress common mode voltages while leaving the differential audio signal path unaffected
Solution Approach 2:
The common mode feedback circuit applies partial action by focusing solely on suppressing common mode voltages rather than controlling the entire signal spectrum. The feedback amplifier is designed with selective frequency response and gain characteristics that target only the problematic common mode ripple frequencies, leaving other signal components untouched
Data Source
AI summary
A system includes a class D amplifier and a current steering digital-to-analog converter (DAC) directly connected to the class D amplifier. The system also includes a common mode servo circuit coupled to a node interconnecting the current steering DAC to the class D amplifier. The common servo circuit amplifies a difference between a common mode signal determined from the node and a reference voltage and generates a feedback current to the node based on the amplified difference. A feed-forward common-mode compensation circuit is included to reduce an alternating current (AC) ripple from the class D amplifier. The feed-forward common-mode compensation circuit includes first and second resistors coupled to respective outputs of the class D amplifier. A current mirror is coupled to the first and second resistors and is configured to sink a current from the node to ground that approximates a common mode feedback current of the class D amplifier.