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PGA460: High voltage transducer issues

Adrien CASTEL
Prodigy 150 points
Part Number: PGA460

Hi everyone,

I ‘am working with the PGA460 device. I ’am using the demo board hardware with a custom software. I have also installed a custom board for the transformer and the ultrasonic transducer. With this transformer I reach 250V peak-peak driving voltage for the transducer. Everything went well until I lost all my signals on the PGA460 reading. After an investigation I find out that the resistor on the INP pin was dead (R41). I think that the resistor could not withstand the higher voltage. What do you think about that?

This problem raises another question. I ‘am planning to raise my voltage up to 1000V. What is the maximum voltage that the PGA460 is able to support on his INP pin? (On the datasheet the value is 2V, I think that is very low). What kinds of protection are implemented in the PGA460 for the INP and INN pin which are directly exposed to the high driving voltage of the transducer?

Thank you.

  • 0
    TI__Mastermind 27780 points
    Hi Adrien,
    INP and INN are specified to the correct ratings of -0.3V to 2.0V. There is some internal circuitry that has a clipping like effect, but this effect is dependent on the value of the AC-capacitive coupling capacitors (C_INP and C_INN). This is why calculating the AC-capacitive coupling values (per the equations given in the PGA460 datasheet) is important to prevent damage to the INP and INN pins. Applying a fixed DC value beyond 2.0V without the external AC coupling capacitor will damage the pins, which is why the coupling capacitors are required to eliminate/remove DC bias.

    To eliminate any Vpp risk on the INP return path, be sure the C_INP capacitor and R_INP resistor are appropriately rated . The C_INP (C10) capacitor on the EVM is rated for 100V, so a 250Vpp transducer driver is likely derating the C_INP cap. Have you replaced R_INP? If so, did replacing R_INP restore the PGA460's ability to detect an echo? What is the new value of R_INP?

    What are the part numbers or specifications of the custom transducer and transformer your are using? We have only tested the return path operation for transducer excitation voltage up to approximately ~300Vpp. If you anticipate 1000Vp at the transducer, I recommend that you use a two-diode-ring to ground (see figure "a" of www.wtec.org/.../fh3_12.gif for an example of a diode ring) in parallel to the INP pin to externally clamp the driver voltage to +/-1~2V at the INP pin after the signal goes through the R_INP and C_INP path. This will prevent damage to the INP pin.
  • 0 in reply to Akeem Whitehead
    Prodigy 150 points

    Hi Akeem,

    I replaced R_INP by 2512 1.2k resistor (with a rating of 500V). It has restored the ability of the PGA460 to detect an echo. I agree with you, I have to find a capacitor that is rated for at least 1000V for my final application.

    My transducer is an Airmar ATK75 (75kHz transducer with a 1000V max driving voltage). For my test I use a MuRata 55050C (Ratio 1:1:50). To reach 1000V I plan to build a custom transformer based on the specification that Airmar gave to me.

    I attached to my post a schematic of the protection that I plan to add on the INP pin (based on your recommendations). Do you think that would be suitable?

  • 0 in reply to Adrien CASTEL
    TI__Mastermind 27780 points
    Hi Adrien,
    Your schematic proposal to protect the INP pin appears to be suitable. Since you are using a center-tap transformer, be sure not to exceed a 15V centertap voltage (V_CT). OUTA and OUTB are only rated for 30V, and will peak at V_CT * 2 in a center-tap configuration.