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A successive approximation (SAR) ADC is presented, whose components make use of effective ultra low-power techniques to enable wireless sensing with passive and semi-passive sensor nodes. Compared to prior publications new layout enhancements were applied to the integrated capacitive DAC array to gain 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 an UHF RFID System. Within this system it is applied as core of a sensor interface integrated into a passive multi-standard RFID tag. The EPC protocol used for communication ensures the compatibility with standard UHF RFID readers while the sensor data is acquired using custom commands. The ADC consumes only 525 nA at 40kSps and 0.9 V supply voltage. Under these conditions an ENOB of 7.23 is reached and thus a FOM of 79 fJ/conversion-step.