A monolithic RMS-DC converter using planar diaphragm structures

A monolithic RMS-DC converter for measuring wideband RMS signals is presented. Two identical thermoelements for thermal RMS-DC conversion are formed using planar diaphragm structures and micromachined dielectric windows. Each thermoelement consists of two polysilicon heaters and a thin-film temperature sensor located on a window and operated at constant temperature using on-chip control feedback circuitry. The thermoelements achieve a thermal efficiency greater than 7°C/mW with thermal time constant less than 5 ms. The on-chip control circuitry is realized using a standard 3-μm p-well CMOS process with minor modifications for process compatibility with the dielectric window formation. This converter measures RMS signals over a full scale range of 1 V RMS, handles crest factors in excess of five, exhibits a typical nonlinearity of less than 1%, and achieves a 3-dB bandwidth greater than 20 MHz.