This paper presents an ultra-low power incremental $${\varDelta {\Sigma }}$$ ADC with flexible sampling frequency, accuracy, and operational duty-cycle. The flexibility and low leakage power enable efficient scaling of average power together with performance. This allows simultaneous optimization of the sensor system (1) for various multiplexed, both on-chip and off-chip sensor interfaces, and (2) for a wide range of available harvested energy. The architecture allows further flexibility as it can be used in regular continuous $${\varDelta {\Sigma }}$$ mode as well, without trading off accuracy. The ADC was implemented in a 180 nm CMOS process, on the same ASIC with a temperature sensor, pressure sensor and energy harvesting functionalities. The ADC has a nominal power consumption of $$1.3\,\upmu$$ W, SNDR of 68 dB and BW of 200 Hz, denoting a $$FOM_w =1.58$$ pJ/conv.