  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.
