Current mode continuous time sigma delta converter design

Abstract

Most sigma delta modulators are realized in the discrete-time domain using circuit techniques such as switched capacitor. In typical discrete time implementations, there are some speed constraints. For example, maximum clock rate is limited to op amp bandwidths. Therefore, high resolution can be achieved only for low bandwidth signals. However, continuous-time circuits do not suffer from such speed restrictions, hence, clock rates can be increased by building the loop filter out of continuous-time circuits and high resolution converters with wider bandwidths could be implemented. Secondly, in most sigma delta modulators where input signal is current, quantizers are generally implemented in voltage-mode and a digital/analog converter is used in the feedback loop to convert voltage signals to current signals. However, if all blocks are designed in current-mode, an extra digital/analog converter requirement can be relaxed. Furthermore, current mode circuits have attractive features such as high speed and low voltage operation. This thesis is mainly about designing current mode circuits for a first order continuous time sigma delta modulator. Main blocks of a modulator such as integrator, quantizer and comparator have been designed in current mode. Also, current mode design of digital circuits such as full adder and delay element which is widely used in digital filters has been made.