Offset elimination in ΣΔ analog to digital converters by ƒs/2 modulation

This paper derives f_s/2 modulation from signal and system theory of complex signals. It is presented, how the In phase and the Quadrature phase are reduced to a single phase, while all properties of complex signaling are maintained. Further, it is derived, that the remaining phase is predestinated for ΣΔ modulation. Finally, simulation results prove the theoretically derived offset-elimination property.

I. Introduction

Dc-offset and low frequency noise elimination is an important design issue in integrated analog to digital converters (ADC). The requirements are especially stringent, if signals with small amplitudes and low frequencies have to be converted, as in the case of sensor read-out circuits. One of the main offset sources in ADCs is mismatch between the input differential pair transistors of operational amplifiers (opamp) or comparators. Different circuit techniques are used to reduce these opamp imperfections, such as chopping or correlated double sampling [1].