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Delta Sigma Modulators with Beamformers for Application in Medical Ultrasound Imaging

Kaald, Rune
Doctoral thesis
Åpne
Fulltext not avialable (Låst)
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http://hdl.handle.net/11250/2451547
Utgivelsesdato
2017
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  • Institutt for elektroniske systemer [1831]
Sammendrag
Portable medical ultrasound solutions require a high degree of compactness

and low power consumption, without sacrificing image quality. In

this dissertation we investigate the feasibility of employing a continuous

time delta sigma (CTDS) analog-to-digital converter (ADC) based receive

architecture to simplify the front-end.

An important step towards the design of a high performance CTDS

ADC is to gain an overview of the trade-offs comprising the error sources

involved, and how to decide each source’s contribution in the overall design

specification. We provide a systematic design methodology using the

Simulink environment in Matlab. With an example design specification,

we show how each error source can be modeled, and determine its impact

on the final performance through high-level simulations.

In the quest of reducing the power consumption of a CTDS modulator’s

internal high frequency sampling quantizer, we explore the use of voltage

controlled oscillator (VCO) as a replacement. Because its use complicates

excess loop delay compensation, we propose applying a capacitive summing

solution to the preceding integrator. A prototype fifth order CTDS

modulator is designed and implemented in 180 nm CMOS, and achieves

a measured performance of 76 dB SNDR across 10 MHz bandwidth consuming

58 mW of power.

For the special case of cardiac ultrasound, we give design considerations

for per-channel digitization and beamforming. Further, we present

the design of a third order single-bit CTDS modulator with an integrated

mixer especially tailored for this application. Manufactured in 65 nm CMOS,

it demonstrates 67.4 dB SNDR across 1 MHz bandwidth consuming only

131 μW of power, corresponding to a figure of merit of 34.2 fJ/conversion

step. By adjusting the phase of each mixer, we show how to realize a highly

effective digitization and narrow-band beamforming structure.

Finally, we step into the digital beamforming domain and provide an

analysis of the distortion mechanism in delta sigma beamforming. From a per-channel consideration, analytical equations are derived describing that

during sample repetition an alias/imaging process occurs. Using the same

analytical approach, the insert zero compensation method is shown to provide

first order attenuation of the alias/images. Furthermore, by combining

the insert zero with a low complexity filter, an architecture results that

achieves power-efficient beamforming without any penalty to the dynamic

range.
Utgiver
NTNU
Serie
Doctoral theses at NTNU;2017:52

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