Design of operational amplifiers with feedforward multistage architecture in CMOS low voltage technology for biomedical instrumentation applications

摘要

Magnetic Resonance (MRI) is an imaging technique used to obtain anatomical and structural features of a patient’s body. The creation of a measurable signal is based on the varying water concentration throughout the body. The atoms which participate actively in such event are the hydrogen atoms from water, which create a varying magnetic field in response to a radiofrequency pulse. Such radiofrequency pulse is applied perpendicular to a main magnetic field created by a gradient coil, which is in charge of aligning the magnetic moments of hydrogen atoms. Upon application of the radiofrequency pulse, magnetic moments of hydrogen atoms rotate 90º with respect to the main field, creating a varying magnetic field which can be converted into a current through the use of coils.The detection of such current is achieved through the use of sensors, and constitutes a crucial step for the creation of an image based on the information emanating from the magnetic resonance equipment. Bearing in mind that a 1.5 Tesla MRI contains around 80 sensing components, the key element in MRI is the data acquisition block which forms part of each individual sensor. This data acquisition block contains a sensor, a band pass filter, and a band pass Analog to Digital Converter (ADC). From these three elements, the one in charge of the final resolution so that the digital part can appropriately process each sensor’s information is the ADC. Therefore, the accuracy of the ADC’s performance influences image processing steps to be performed afterwards for the final creation of an image.An Analog to Digital Converter can be built in several ways, either through a subsampling pipeline or band pass Sigma Delta. In both cases, the key elements which influence the resolution of the ADC are the operational amplifiers with which they are built up. Subsampling pipelines make use of operational amplifiers in order to sample the input signal in successive steps, whereas band pass Sigma Delta circuits use them to filter the quantification noise around a specified frequency.The ADC which will be used for the purpose of this work is a band pass Sigma Delta modulator whose bandwidth and center frequency are respectively 1MHz and 140MHz. The main objective of this work is to build a model of a real operational amplifier to be used in the filter of the ADC.