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Original question:

PGA460: What is the correct waveform from outA and outB?

PGA460: Working of PGA460's internal transistors

Aravind Balasubramanian
Prodigy 60 points
Part Number: PGA460

Hi,

This is a follow up to the response in the thread https://e2e.ti.com/support/sensors/f/1023/t/623106.

Thanks in advance!

Best Regards,

Aravind

  • 0
    Prodigy 60 points

    My original post did not fully appear. Hence reposting below. Sorry for the inconvenience!

    ----------------- Original Post Below ---------------------------

    Hi,

    This is a follow up to the response in the thread https://e2e.ti.com/support/sensors/f/1023/t/623106. I have been able to verify that the OUTA/B waveforms are sinusoidal when connected to a transformer load and

    rectangular when the direct drive circuitry is used, similar to the scope traces in the above link. I wanted to ask if the transistors in Figure 3 - Echo Generation Block Diagram in the PGA460's datasheet (also attached below), is operated as a linear amplifier allowing the ability to wave shape depending on the load connected, while also limiting the current through and amplifying the voltage at the PGA460's OUTA/B terminals?

    Thanks in advance!

    Best Regards,

    Aravind

  • 0 in reply to Aravind Balasubramanian
    TI__Genius 11015 points

    Aravind,

    We have received your question & will answer it before Wednesday.

  • 0 in reply to Aravind Balasubramanian
    TI__Mastermind 27950 points

    Aravind,

    The OUTA/OUTB are nothing more than open-drain outputs to the internal FETs. The FETs' gates are driven with a square wave at the user set frequency for n-number of pulses. There is no internal wave shaping, only user set current limiting of the drain-to-source sink current through the FET. The value of the user set current limit is able to affect the shape of the external drain load (transformer or transducer) in certain cases.

    In the transformer driven case, keep in mind that this is effectively an inductive load that resists changes to current, so the voltage will appear sinusoidal at OUTA/B. In the bridge driven case, the open-drain is connected directly to the transducer, which is primarily capacitive, resisting changes in voltage. So the transducer, during burst, is more likely to follow the square wave pattern of the internal FET driver in a bridge driven case.

    The difference between these two load types is what accounts for the difference in the driver mode OUTA/B waveforms.