1. An implantable stimulator device, comprising: a plurality of electrodes configured to provide electrical stimulation therapy to a tissue of a patient; and impedance monitoring circuitry configured to apply at least one AC measurement signal different from the electrical stimulation therapy to a plurality of combinations of the electrodes and to determine an impedance between each of the plurality of combinations of the electrodes using the at least one AC measurement signal, wherein the impedance monitoring circuitry is configured to apply the at least one AC measurement signal during quiet periods when electrical stimulation therapy is not occurring at any of the plurality of electrodes.