An Element-Level Sigma-Delta ADC for Ultrasound Imaging

Abstract

This work presents an architecture capable of digitizing every channel of an ultrasound transducer array independently and simultaneously. This feature is achieved by exploiting the frequency response of the piezoelectric transducer in order to save area and reduce to a minimum the required building blocks of the ADC. The transducer is used as an electro-mechanical resonator, which is embedded in a band-pass continuous-time, multi-bit ΣΔ modulator. This converter relies entirely on the noise shaping provided by the transducer, which proves to be sufficient to reach the required specifications. Each converter only includes an inverter-based transimpedance amplifier, a variable gain amplifier for time-gain compensation and a 3-bit tracking quantizer, which comprises two comparators and two DACs. A prototype chip has been fabricated in TSMC 0.18𝜇𝑚 technology, featuring 20 channels, with one ADC per channel, and 20 parallel high-speed LVDS transmitters to convey the bitstreams out to the measurement system. The whole modulator, clocked at 200𝑀𝐻𝑧, achieves an area of 150𝜇𝑚 × 150𝜇𝑚, a power consumption of 800𝜇𝑊 and an SNR of 47dB in a 75% bandwidth around a transmit frequency of 5𝑀𝐻𝑧. The measurements clearly show the desired noise shaping behaviour, thus proving that the proposed concept is valid.