A Wideband Monolithic RMS-DC Converter Using Micromachined Diaphragm Structures

This paper reports a monolithic rms-dc converter for measuring wideband rms signals. 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 a thermal time constant less than 5 ms. The on-chip control circuitry is realized using a standard 3µm 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 Vrms, handles crest factors in excess of 5, exhibits a typical nonlinearity of less than 1%, and achieves a 3 dB bandwidth greater than 20 MHz.