ALTIROC1 is a 25-channel ASIC designed to read out the 5 x 5 matrix of 1.3 mm x 1.3 mm x 50 μm low Gain Avalanche Diodes (LGAD) of the ATLAS HGTD detector foreseen for the HL-LHC upgrade. The targeted combined time resolution of the sensor and the readout electronics is from 40 ps/hit (initial) to about 80 ps/hit (end of operational lifetime). Each ASIC channel integrates a RF preamplifier followed by a high speed discriminator and two TDCs for Time-of- Arrival and Time-Over-Threshold measurements as well as a local memory. This front-end must exhibit an extremely low jitter noise while keeping a challenging power consumption of less than 4.5 mW. Detailed measurements are presented.