A fully-synchronous offset-insensitive structure is proposed for implementing level-crossing analog-to-digital converters (LC-ADCs). The proposed structure is designed and implemented for high-precision compressed electrocardiogram (ECG) monitoring applications. Synchronous implementation leads to less implementation complexity compared to the conventional asynchronous implementations. Also the major source of error, viz. the difference in comparators offsets is eliminated which additionally leads to considerable saving in silicon area. Designing and simulating in a 0.18 μm CMOS technology, the LC-ADC achieves an ENOB of 8.45 bits and occupies 0.038 mm² silicon area. The average sampling rate is about 120 S/s when applied to the whole MIT/BIH arrhythmia database. Simulation results show a power consumption of 81 nW with a 1.8 V supply voltage, by testing the ADC using Tape 100 of the MIT/BIH arrhythmia database.@en