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PCM4202: The recommended Input Buffer's impedance seems low for professional line levels?

Gabriel Boucher
Prodigy 40 points
Part Number: PCM4202
Other Parts Discussed in Thread: OPA1632

Hi,

in the PCM4202 data sheet, there's a recommended Input buffer schematic based around a OPA1632. (see attached screen capture)

The input impedance seems pretty low for professional line levels. We usually expect 10K and more.

Could you please explain how this circuit could offer optimal performances?

Thank you

https://i.postimg.cc/Hx4vzKTR/pcm4202-buffer.png

  • 0
    TI__Mastermind 23050 points

    Hi Gabriel,

    The PCM42xx family of devices is our highest performance family with SNR between 118-124dB. The full scale range of these devices is around 2Vrms, and with this level of performance the thermal noise generated by resistance in the signal path can actually start to become the dominant noise source. This is why the IC is designed with a low input impedance and I think you will find that most converters that support >115dB SNR will generally have <10kOhm input impedance.

    This is also why the input amplifier circuit is designed with a low noise amplifier and low resistance values in an attenuating configuration. This ensures the front end does not degrade the signal integrity. Since the inputs to the PCM4202 interface directly with the modulator, it is helpful to have a low impedance source like an op amp driving the inputs to ensure the signal is sampled accurately.

    Best,

    Zak