Abstract
This paper presents a continuous-time third-order ΣΔ modulator designed for closing the feedback loop of a digital class-D audio amplifier. The closed-loop digital class-D amplifier fully exploits the potential of the used 40-nm CMOS technology to achieve at the same time the flexibility of digital implementations and the performance of analog solutions. The proposed ΣΔ modulator consumes 1.7 mW from a 1.1-V power supply, achieving 101-dB dynamic-range (DR) and 72-dB peak signal-to-noise and distortion ratio (SNDR). The active-RC implementation allows the 1.1-V ΣΔ modulator inputs to be directly connected to the 5-V class-D amplifier power stage outputs and inherently guarantees third-order anti-aliasing filtering.
Highlights
Mixed-signal systems-on-chip (SoC) for digital-input audio signal processing have to face challenging constraints for integration in deep sub-micron technologies
Most class-D amplifiers are used in a closed-loop configuration, so that the overall amplifier gain is only determined by the feedback factor and, it is constant and well defined, independently of the power stage supply voltage
The noise floor increase for large signals is due to cross-coupling between the digital output and the D/A converter (DAC) reference voltages, since the reference buffers have been undersized to save power
Summary
Mixed-signal systems-on-chip (SoC) for digital-input audio signal processing have to face challenging constraints for integration in deep sub-micron technologies (down to 40 nm and beyond) In such nodes low-noise and low-voltage blocks can be addressed, while medium power circuits such as hand-free amplifiers encounter severe problems to deliver the required acoustic output power. All the power stage non-idealities, such as delay mismatch, dead time, and rise/fall time mismatch, affect the output signal quality, degrading the total harmonic distortion (THD) [10], [11] To overcome these limitations, most class-D amplifiers are used in a closed-loop configuration, so that the overall amplifier gain is only determined by the feedback factor and, it is constant and well defined, independently of the power stage supply voltage.
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