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TPA3255: Implementing Post-Filter Feedback with differential input stage

Tomasz Chumiecki
Genius 3060 points
Part Number: TPA3255
Other Parts Discussed in Thread: PCM1795, OPA1632

Hi TI Experts,

In my design I drive TAS3255 directly from PCM1795 DA converter. My D/A output low-pass filter use OPA1632 fully differential amplifier:

I would like to implement also PFFB directly in this design without using additional buffers / summing amplifiers between low-pass section and TPS3255 input. Could you tell me how to adjust / calculate the components? The component values in low-pass filter stage are taken directly from PCM1795 datasheet. Can I simple adjust PFFB components and R138/R139/R158/R159? Or I need to redesign my low impedance low-pass filter stage?

Thanks for your support, Tomasz

  • 0
    TI__Mastermind 27910 points
    Hi Tomasz,

    The biggest concern with PFFB is keeping the amplifier stable, and so I think the only concern/change to the input circuitry might be to the cutoff frequency to manage any high frequency instability. I don't have any exact values, and the circuit would need to be tested on the bench. The worst case for the amplifier would be open load high frequency cases.

    Justin
  • 0 in reply to Justin Bohr
    Genius 3060 points

    Hi Justin,

    Thanks for your reply. I analyzed a bit PFFB application note any tried to understand the concept:

    I understood, the important parameter is the feedback factor and the right change of closed loop gain. Can I just use 2k7 resistors for R138 / R139 / R158 / R159 - this is the value from the datasheet ;) Would it work also for my application?

    What is the primary function of the opamp input stage? 10k / 10k suggest it is like unity gain buffer with limited bandwidth - probably for the stability reason... The R_in resistor should have any influence on the input opamp stage, shoudn't it? The most important will be the keep the right ration of R_in and R_fb resistors to keep feedback factor low and set the right closed loop gain...

    Are the values in the application note calculated or estimated and than optimized doing lab measurements?

    Cheers,Tomasz

  • 0 in reply to Tomasz Chumiecki
    TI__Mastermind 27910 points
    Hi Tomasz,

    I believe you could use 2k7 for the series input resistor, as well as the differential resistor although I am not sure the impact the differential resistor will have on the PFFB. The input op amp stage was partly because of its presence in our EVM design, as well as limiting the gain of the higher frequencies, which the PFFB circuit will be more sensitive to.

    I am not sure about the room to adjust R_in, but if changed I agree R_fb would need to change. This would need to be tested on the bench.

    These values were more on the estimated and tested side of development.

    Justin