This book develops voltage-to-frequency converter (VFC) solutions integrated in standard CMOS technology to be used as a part of a microcontroller-based, multisensor interface in the environment of portable applications, particularly within a WSN node. Coverage includes the total design flow of monolithic VFCs, according to the target application, as well as the analysis, design and implementation of the main VFC blocks, revealing the main challenges and solutions encountered during the design of such high performance cells. Four complete VFCs, each temperature compensated, are fully designed and evaluated: a programmable VFC that includes an offset frequency and a sleep/mode enable terminal; a low power rail-to-rail VFC; and two rail-to-rail differential VFCs.