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BOOSTXL-PGA460: Voltage Issue

Alexandre Becker Filho
Prodigy 220 points
Part Number: BOOSTXL-PGA460
Other Parts Discussed in Thread: PGA460, TUSS4470

Hi,

I'm developing an ultrasonic sensor to measure the depth of the sludge in large tanks. But for now i'm trying to measure water depth. My transducer is the TD40200D, it's recommended 1000vpp supply but it should works fine with 300vpp for my needs.

 I guess that my problem is that i can't reach such high voltage. I'm supplying BOOSTXL-PGA460 a voltage of 12V and it supplies the transducer with 260vpp, when the frequency is set to 58,6kHz. But when i configure it to 40khz the voltage degreses to 100vpp. Why is this happening ? 

Another question is, should i always do the tests underwater? Because of the impedance of water.

OBS: I have unsolder the resistor (R38) and capacitor (C15) as recommended in PGA460 Ultrasonic Module Hardware and Software Optimization.

  • 0
    TI__Expert 3450 points

    Hello,

    Transducers are high-Q devices that operate near specific frequencies, so when exciting them too far from their resonant frequency or using an untuned transformer driver, they exhibit a low SNR, as you saw reflected in the transducer voltage. I see that the TD40200D transducer's center frequency is listed as 40kHz and 200kHz, but I also see a reference to 1 MHz. First, I would recommend determining the resonant frequency of the transducer either by contacting the manufacturer or taking a measurement. As you have removed the damping resistor (R38), the ringing decay should be easy to observe on the positive terminal of the transducer. E.g. If you can excite the transducer at 40 kHz or 200kHz and see a decaying sine wave at the end of the excitation waveform around the same frequency, then the transducer can resonate well at that frequency. If the ringing waveform does not look right or oscillates at a different frequency than the input waveform then the excitation frequency is not close enough to the resonant frequency of the transducer. Taking this measurement and testing the transducer while submerged (as true to your application), is also a good idea, especially if the transducer is designed for underwater use. 

    Since it sounds like the transducer's resonant frequency is not as desired (not uncommon), you will need to tune the circuit with C15. To do this, you can successively measure the excitation voltage across the transducer while iterating through different capacitor values for C15. Try values from 100pF to 3000pF in 100pF steps and be sure the capacitor(s) is rated for over 150V. Once a good tuning capacitor value is determined and in place, use a trial and error method to determine the damping resistor (R38). A good value should be between 1 and 20 kOhms. 

    If you continue to have issues using the TD40200D, another option which the PGA460 supports and is designed for submerged, long distance depth sensing is the Prowave 200LM450 200kHz transducer. 

    Also, if you determine that more than 300Vpp for this transducer is required then the TUSS4470 AFE can be configured to achieve a higher output voltage. 

    Regards,

    Gabriel

  • 0 in reply to Gabriel Lake1
    Prodigy 220 points

    Thanks for the answer, 

      I measure the TD40200D resonant frequency and it's 38Khz. The transducer datasheet gives an Impedance (in water) 600 ± 30% Ω and a capacitance 2700 ± 15% pf. Are it the right values for tuning capacitor and dumping resistor ? (https://www.chinalevelsensor.com/product/ultrasonic-depth-sensor-td40200d/) 

  • 0 in reply to Alexandre Becker Filho
    TI__Expert 3450 points

    To better understand these transducer datasheet values and the tuning passive components, have a look at the transducer BVD model in the PGA460 Ultrasonic Module Hardware and Software Optimization App Report (page 8) and read section 3.4. 

    The 2700pF static capacitance value in the TD40200D datasheet is CT in Fig. 5, and the product website gives CPT    

    as 2.278 nF for their testing. The C-TUNE and R-DAMP values should be experimentally determined. 

    Regards,

    Gabriel

  • 0 in reply to Alexandre Becker Filho
    TI__Expert 3450 points

    To better understand these transducer datasheet values and the tuning passive components, have a look at the transducer BVD model in the PGA460 Ultrasonic Module Hardware and Software Optimization App Report (page 8) and read section 3.4. 

    The 2700pF static capacitance value in the TD40200D datasheet is CT in Fig. 5, and the product website gives CPT    

    as 2.278 nF for their testing. The C-TUNE and R-DAMP values should be experimentally determined. 

    Regards,

    Gabriel