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Microring resonators can be used as passband filters for wavelength-division demultiplexing in electronic-photonic integrated circuits. For applications such as analog-to-digital converters, the resonant frequency of the filter has to be held at a certain value to allow minimum timing errors in the sampling of the signal. Thermal tuning is used to compensate for any fabrication errors or environmental temperature fluctuations that might lead to a shift in the resonant frequency. With an optimized heater design, we demonstrate efficient on-chip thermal tuning for a second-order silicon-rich silicon nitride microring resonator at 80 mu W/GHz. A closed-loop feedback circuit controls a resistive heater which is also used as a temperature sensor to maintain the resonant frequency within 280 MHz of the target value.