| Summary: | 碩士 === 國立臺灣大學 === 電子工程學研究所 === 95 === This research investigates a sigma-delta modulator for audio analog-to- digital converter. The goal of this research is to improve the high quantization noise and enhance the signal-to-noise ratio (SNR) when converting analog signal to digital in use of sigma-delta modulator.
Oversampling techniques based on Σ-Δ modulation are wildly used to implement the interfaces between analog and digital signals in VLSI systems. This approach is relatively insensitive to imperfections in circuit components. In particular, oversampling architectures are potentially power-efficient means of implementing high resolution A/D converters because they reduce the number and complexity of the analog components in comparison with Nyquist-rate converters. Furthermore, they allow the performance requirements, and thus most of the power dissipation, to be concentrated in the input stage of a converter.
In this thesis, we pay attention on using higher oversampling rate (128) than general Σ-Δ modulator, increasing the based-band width of audio signal to 25 kHz, and utilizing the architecture of Multi-Stage Sigma-Delta modulator to increase the swing of input signals, in order to achieve higher resolution. The specification of the operational amplifier, the mismatch and imperfection of circuit component will face the severer requirement with such oversampling rate. It is really a great challenge when implementing high-quality audio nowadays.
This Multi-Stage Sigma-Delta Modulator is fabricated in TSMC 0.35μm Mixed-Signal 2P4M Polycide process. The area of this chip is 1.2 x 1.2 mm2 and power dissipation is 13mW under 3.3V supply voltage. In the Pre-layout simulation, the peak SNDR in the basband (25 kHz) can achieve 96.12dB; in the Post-layout simulation, the peak SNDR can be 89dB. Experimental result shows that the peak SNR is 70dB within the baseband of 25 kHz (SNDR is 60dB), which might be caused by the layout imperfection, making the analog circuit be disturbed by the noise from digital circuit.
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