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TPA3220: Data sheet input impedance ambiguous

Part Number: TPA3220

What exactly are the differential and single-ended input impedances for the TPA3220?

The August 2018 data sheet section 9.3.5 quotes impedances of 48k, 24k, 12k, and 7.7k depending on gain selection. It also suggests, but does not explicitly state, that these are differential impedances i.e. a single (say) 48k from INx_P to INx_N.

The same section then recommends input capacitor values for high-pass corner frequencies that are only correct if the impedances are single-ended i.e. one 48k from INx_P to zero volts and a second 48k from INx_N to zero volts.

Which interpretation is correct?

Also, section 9.3.5 also refers to table 1 when I think it's meant to be table 3.

  • Hi Ross

    the input impedance is switched when gain is different.

    so when your config different gain, input impedance is different.

    i think you can mainly use table3 as your reference.

    sorry for table 1 typo in datasheet, we will let our team to modify this miss.



  • Thanks for the quick reply, but this was not the question I asked. I understand the impedance changes with gain changes. What I need to know is whether the quoted impedance is for each INx pin, or for the differential pair.

    In my application I'm configuring for +18dB gain, so the "impedance" is 48kohm. The data sheet reads "The TPA3220 input stage is a fully differential input stage and the input impedance changes with the gain setting", which suggests that this impedance is also differential i.e. 48kohm from INx_P to INx_N (or 24kohm from each to 0V, which behaves the same for a differential input signal).

    But table 3 shows that for +18dB gain and thus 48kohm impedance, using 4.7uF "capacitance" gives a high-pass corner at 0.7Hz. This is ambiguous at best. If the table means to use a 4.7uF capacitor on each pin, thus a total capacitance of 2.35uF, then the corner will actually be at 1/(2*pi*48kohm*4.7uF/2)=1.41Hz. If the table instead means to use 9.4uF on each pin for a total of 4.7uF then the corner is correct, but this seems unlikely since 4.7uF is an E12 value and the others aren't. It seems like the intent is to use one 4.7uF on each pin.

    If you do use one 4.7uF on each pin, then the input impedance to give a 0.7Hz corner is 96kohm across the pair. Thus the table would make sense if the quoted 48kohm is actually the single-ended impedance of each pin, and not the differential impedance of the pair. But the text implies the opposite.

    Does that make it clearer where the ambiguity lies?

  • Hi Ross

    those impedance are the impedance between pin and gnd.

    you can follow the table directly. that would be the definitely correct.



  • That's exactly what I needed to know, thank you.