A fully differential layout placement paradox: Matching vs. full symmetry

The use of a fully differential architecture has numerous advantages in a switched capacitor delta-sigma modulator, but the efficient use of such an architecture requires a completely symmetrical layout and routing, which may contradict with the requirements of component matching. A fully symmetrical placement, for example, requires division of the capacitor arrays and possibly placement of the two halves in distant locations on the chip. Hence full symmetry requirements conflict with the matching of capacitor arrays. This paper discusses the effect of possible separation of the capacitor array, based on the assumption of a systematic capacitance variation along the chip surface due to process variations. The result of such a separation is observed in simulated circuits.