PGA450Q1EVM: Ultrasonic sensor resonance tuning with fixed IFT

Hi Arun,

Responding to your questions:

We do not have an accurate formula to calculate the value of the external matching capacitor. Section "8.2.2.1 Hardware" of the PGA450-Q1 datasheet shows equation-11, which is intended to approximate the ideal value of tuning capacitor based on the transformer's inductance and Butterworth-Van *** model equivalent of the transducer. We recommending using this calculated value as a starting point, but manually checking values within a +/- 500pF range from the calculated value. The difference in electrical versus mechanical resonance of a transducer will always counter performance of ring-decay or long range; it is up to the user to optimize which to improve and which to sacrifice based on min/max range requirements. This passive selection process is also described in the "PGA460 Ultrasonic Module Hardware and Software Optimization" ( www.ti.com/.../slaa732 ).

Assuming nominal capacitance of the transducer, and fixed tuning capacitor value is selected, any deviation could be potentially be compensated by the configurable driver frequency. It is assumed that the ring-decay performance of transducer with a fixed tuning cap, but min or max sensor capacitor specification will always yield better performance compared to having no fixed tuning cap in place.

Exact external passive matching can be difficult if the sensor capacitance drifts by +/-20%, but compensation through the driver frequency typically resolves this concern. Instead of always using 58kHz, you may find some transducer to have improved performance at 58.5kHz or 57.5kHz for example.