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TPA3255: TPA3255 and TAS5630 INPUT_X series resistor

Timothy Alberg
Prodigy 140 points
Part Number: TPA3255
Other Parts Discussed in Thread: TAS5630, TAS5630B

Hello All,

The datasheet recommended design and EVM schematic for both the TPA3255 and TAS5630 show a 100 ohm resistor in series with the INPUT_X pins of the IC.  In looking through the schematics, EVM manuals, and other application notes we could not find any mention for why these would be required. 

We are currently experimenting with an evaluation board in PBTL configuration containing the TAS5630 and made the following observation: The 100 ohm series resistor forms a resistor divider with internal circuitry that attenuates the signal by half.  In addition, the DC component of the signal gets set to PVDD/25.5319 (~0.94V for a 24V supply and ~ 1.88V for 48V).

If the internal resistance of the TAS5630 is 33k Ohms, why is the signal attenuating by 2x?

Does putting the IC in PBTL configuration result in the Analog loop filter setting a 200 ohm differential terminator across the A and B inputs?

Will the TPA3255 exhibit this same behavior? 

Thank You.

  • 0
    TI__Mastermind 19080 points

    Hi,

    1. Let's take the Figure 21 on TAS5630B datasheet as an example. The 100R input series resistor formed a high-pass R-C filter for DC input blocking. Input signal is not divided by the R-C filter and the input impedance of the audio amp.

    Could you explain a little bit more on the attenuation you observed? What's the input frequency?

    2. To configure the device in PBTL mode, INPUT_C and INPUT_D should be connected to VREG and GND respectively, as shown in Figure 19 on TAS5630B datasheet. The device will take INPUT_A and INPUT_B as its input, exactly the same behavior with BTL mode.

    TPA3255 is also analog input device. It's similar with TAS5630 on the input configuration.

    Thanks!

    Regards,

    Sam

  • 0 in reply to Sam(MPAA) Zhang
    Prodigy 140 points

    Hello Sam,

    Thank you for the response.  We took a look at our measurements again this morning and realized we made a measurement error.  We were using two oscilloscope probes and the math function for performing a differential measurement (CHA-CHB) on the inputs and using the cursors for getting the peaks.  Unfortunately at the time, we did not notice that our cursors were set to CH1 (500mV/div) instead of the math output (1V/div).  This is where our factor of 2 came from. 

    Thank you for taking the time to read and respond to us and we apologize for not noticing this before posting.

    Kind Regards,

    -Tim