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A successive approximation analog-to-digital converter (ADC) is presented, whose components make use of effective ultralow-power techniques to enable wireless sensing with passive and semi-passive sensor nodes. Compared with prior publications, new layout enhancements were applied to the capacitive array of the integrated digital-to-analog converter (DAC) to achieve less distortion caused by mismatch. In addition, it is shown that the digital circuit parts consume most of the available energy. Therefore, digital near-threshold operation is proposed to minimize their consumption. The applicability of the ADC is demonstrated in a UHF RFID system. Within this system, it is applied as a core of a sensor interface integrated into a passive multistandard RFID transponder. The EPC protocol used for communication ensures the compatibility with standard UHF RFID readers while the sensor data is acquired using custom commands. Single sensor readings are demonstrated as well as continuous sensor data transmission without further interaction of the reader device. A stable transponder reading distance of 6.5 m is achieved. The integrated ADC consumes only 525 nA at 40 kSps and 0.9-V supply voltage. Under these conditions, an ENOB of 7.23 is achieved and thus a FOM of 79 fJ/conversion-step.