| Ultrasound equipments have been widely used in the medical diagnostics domain. But their bulkiness and high-power consumption significantly limit the traditional ultrasonic systems inside hospitals. With the continuing progress in microelectronics technology, the miniaturization of ultrasonic systems attracts great interests. In such a system, analog to digital converter (ADC) acts as the exact interface between analog and digital signal processing and often contributes the system bottleneck.; The aim of this master thesis is to design a Sigma-Delta ADC dedicated to front-end ultrasonic receiver. This ADC features high resolution, low power consumption and adequate programmability which are necessary for hand-held multi-function ultrasonic devices.; This thesis examines the options of Sigma-Delta ADC techniques and proposes a fourth-order continuous-time bandpass modulator. The Gm-cell, which is the basic unit of the Gm-C filter, can be programmed within a wide range through coarse and fine tunings. This programmability, together with the flexibility of the proposed fourth-order filter, guarantees that the modulator is capable of being programmed between different central frequencies with different bandwidth. A Sigma-Delta ADC for 3 MHz application has been implemented in a 0.18 mum CMOS and consumes 2.5 mW drawn from a 1.8V power supply. Post-layout simulations in Cadance give 78 dB SNR. The fabricated chip is measured after being tuned from 3 MHz to 750 kHz and a 56 dB SNR is achieved. |
