Ultrasonic Piezo passive component matching
I do not understand the inductor L3 or its value. It would only make sense if you were trying to create resonance at 640kHz in which case it is the wrong value.

You should not need to do this however because the transducer will be mechanically resonant at 640kHz. Take the inductor out.

It will be difficult to hit resonance with the output of a FPGA it might be better to use a switched self oscillating circuit as is described here
 How can i make a ceramic piezo operate at a specific frequency?