Log-domain filters for low-voltage low-power applications

This paper focuses on the potential of the log-domain principle for implementing low-voltage (LV) and low-power (LP) continuous-time filters (CTFs). By means of examples, it is shown that log-domain filters allow substantial power saving at very low supply voltages (<2 V) compared to more traditional continuous-time filters. This can be partially explained by the fact that log-domain filters do not require any local linearization as traditional filters do. Although the internal voltages are compressed according to the nonlinear characteristic of the transistors, the output current of the filter is still related to its input current by a linear transfer function. For class AB implementations, this instantaneous companding (compressing/expanding) principle allows us even to process signals larger than the bias current. The dynamic range (DR) of such a filter exceeds its maximum signal-to-noise ratio (SNR), in a similar way it does for traditional syllabic companding systems. Finally, design examples are given and measured results are presented confirming the theoretical considerations and the feasibility of this new technique