Then why does Murata recommend driving it with a weak little ordinary CD4049 Cmos inverter/buffer IC? They show two inverters paralleled for each half-bridge and a 12V supply. The supply current is only 15mA and the output is 17Vp-p on one schematic and is 20Vp-p on another schematic. Then the piezo transducer load is 1133 or 1333 ohms.My pocket calculator also tells Xc = 1560 ohm.
But more important, the input impedance at the resonance frequency isn't given by the sensor capacitance. It's a real value at the parallel and series resonance. A transmitter would be operated at the series resonance which has typically a few 100 ohms real impedance.
Murata 40 kHz air ultra sonic transducers (also similar products from other manufacturers) have moderate bandwidth of about 1 kHz, so meeting the resonance frequency is not so critical. Most processor based distance measurement systems are operating it just at the nominal frequency.
For a simple drive circuit, you can refer to figure 14 in the application manual. I couldn't find it presently at murata.com, so I uploaded a backup copy.
Please do not mention the CD4069 that has much less output current than the CD4049 that Murata recommended.
How about one of those dual mosfet gate driver chips ?
They would drive 2nF or more, rail to rail easily, without raising a sweat.
There is far more to it than just generating 20 volts of ac across the transducer terminals.
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