Log-domain fi lters are an intriguing form of externally linear, internally nonlinear current-mode circuits,

in which a compression stage is fi rst used to convert the input currents to the logarithmic domain,

then analogue processing is carried out on the resulting voltages, and fi nally input–output linearity is

restored by mapping the output voltages to current form through an expansion stage. The compressing

and expanding operations confer on log-domain fi lters a number of desirable features, but they may be

responsible for the loss of external linearity. In this paper, suffi cient conditions for the external linearity

of log-domain LC-ladders are established, and the local nature of this external linearity is highlighted.

Certain log-domain LC-ladders employing fl oating capacitors may exhibit externally nonlinear behaviour

even for zero input and very small initial conditions. We show how transistor parasitic capacitances are