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TPA3255-Q1: Distorted sine output

Zora Barabas
Prodigy 20 points
Part Number: TPA3255-Q1
Other Parts Discussed in Thread: TPA3255, TAS5634

Hi,

I am using the tpa3255-Q1 to amplify a 40 kHz signal in BTL mode. I wanted to connect a transducer array to the amplifier and the expected resistance of the array was calculated to be around 24 ohms. I designed my LC filter accordingly, and got an inductance of 42 uH and a capacitance of 47 nF.
Initially, I tested the amplifier with a 24 Ohm power resistor, later with a 30 Ohm as well because the array got smaller. Here are the outputs for the pure 40 kHz sine input:
24 Ohm

30 Ohm

The output is the pink signal and the input behind is the green one.

Since this looked good, I connected my transducer array, however the output was distorted. I realized it is probably because of the capacitance of the transducers (I used piezoelectric transducers with an overall capacitance of 42 nF). Therefore, I took off the output capacitors from the amplifier board but still got a distorted result:

I am unsure of why this happens. Any advice to improve?

Best regards,
Zora

  • 0
    TI__Expert 5530 points

    Hi Zora.

    I am not very familiar with transducer array. Can its equivalent circuit model be regarded as a capacitor?

    From your experimental results, it seems that the LC and your load are in resonance.

    You can use below tool to redesign a suitable LC filter.

    LCFILTER-CALC-TOOL Design tool | TI.com

    BR.

    Wei Qiu.

  • 0 in reply to Wei Qiu
    Prodigy 20 points

    Hi Wei,

    In the meantime I tried measuring the clip_otw and fault signals. I had a constant clip_otw signal no matter how low the input voltage was. I decided to replace the amplifier with a new one. Now the output cleared up but I am having trouble getting my desired signal level. At a 2.9 V peak to peak input and a 46V input on pvdd, I get some signal clipping. These inputs only give me 42 V peak to peak output to a purely resistive load, similarly to my transducer array.
    Is there a way I could get rid of the clipping while aiming for a 57 V peak to peak output?
    The amplifier should be able to have an input swing of up to 7V, but I am unsure how to solve the clipping.

  • 0 in reply to Zora Barabas
    TI__Expert 5530 points

    Hi Zora.

    It seems that the question you encountered before is solved.

    Zora Barabas said:
    Now the output cleared up but I am having trouble getting my desired signal level. At a 2.9 V peak to peak input and a 46V input on pvdd, I get some signal clipping. These inputs only give me 42 V peak to peak output to a purely resistive load, similarly to my transducer array.

    This device is not design for ultra sound application.

    Its bandwidth gain will have a relatively large attenuation after the input signal exceeds 20kHz, so when you input 40kHz, there will be more easy clipping.

    Zora Barabas said:
    Is there a way I could get rid of the clipping while aiming for a 57 V peak to peak output?

    This device can max support 53.5V PVDD, so a 57V peak to peak voltage is impossible.

    Hope this can help you.

    BR.

    Wei Qiu.

  • 0 in reply to Wei Qiu
    Prodigy 20 points

    Hi Wei,

    Thank you for your answer. Could you maybe recommend an amplifier that would be more suitable for the application?

    Best,
    Zora Barabas

  • +1 in reply to Zora Barabas
    TI__Mastermind 39280 points

    Hi Zora

       Sorry that for our devices, there's very few could support the voltage up to 57V, TPA3255 is already a good choice. Other device like TAS5634, is a very old device, the performance may not be good.