A 0.8 mm$^3$ wireless, ultrasonically powered, free-floating neural recording implant is presented. The device is comprised only of a 0.25 mm$^2$ recording IC and a single piezoceramic resonator that is used for both power harvesting and data transmission. Uplink data transmission is performed by analog amplitude modulation of the ultrasound echo. Using a 1.78 MHz main carrier, >35 kbps/mote equivalent uplink data rate is achieved. A technique to linearize the echo amplitude modulation is introduced, resulting in <1.2% static nonlinearity of the received signal over a $pm$10 mV input range. The IC dissipates 37.7 $mu$W, while the neural recording front-end consumes 4 $mu$W and achieves a noise floor of 5.3 $mu$V$_{rms}$ in a 5 kHz bandwidth. This work improves sub-mm recording mote depth by >2.5x, resulting in the highest measured depth/volume ratio by $sim$3x. Orthogonal subcarrier modulation enables simultaneous operation of multiple implants, using a single-element ultrasound external transducer. Dual-mote simultaneous power up and data transmission is demonstrated at a rate of 7 kS/s at the depth of 50 mm.
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