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RC4580-Q1: low SNR issue

Dongbao Zhou
Expert 4130 points
Part Number: RC4580-Q1
Other Parts Discussed in Thread: RC4580

Hi,

Customer use RC4580-Q1 in automotive infotainment system, converting a single-ended audio signal to differential signal. According to their test, the differential output noise is more than 120uVrms. Test method: Short input to GND, and connect a 4kohm resistor between FL+ and FL-.

Could you let me know how to reduce the output noise?

Thanks

Dongbao

  • 0
    TI__Guru** 104215 points
    Hi Dongbao,

    Why not use the Single-Ended Input to Differential Output circuit from RC4580 data sheet?:

    There are some components/connections in the schematic that make it a bit confusing:
    - From my understanding, AMP_FL is the input signal for both channels. But there is 22k input resistor on +IN2 but not on +IN1, there is only 22k connecting input/+IN1 to +IN2.
    - -IN1 have 40.2k to GND, but -IN2 does not.
    - There is 33k connecting -IN2 to OUT1, why is this?
    - Resistors on feedback path seem to be different between channel 1 and channel 2, why not balancing the whole circuit?
    - Non-inverting input is used on both channels, so the output might be the same for both outputs, this wouldn't be differential output. One of the channels should invert the signal.

    Best regards,
    -Ivan Salazar
    Applications Engineer - Low Power Audio & Actuators
  • 0
    Guru 140200 points
    Hi Dongbao,

    the RC4580 is a low noise OPAmp. But your curstomer has wired it improperly. The feedback resistances are too high ohmic.

    The RC4580 generates a very low voltage noise but a rather high current noise. So, if you want to take the full advantage of low noise RC4580 you must have low ohmic feedback resistances and a low ohmic source impedance.

    Try the following values: C262=47µ, C66=47µ, C213=10µ, R178=10k, R171=0, R173=4k02, C263=22µ, R174=6k8, C266=68p, R175=6k8, R176=6k8, C285=68p. And you should add a RC low pass filter at +IN1 input (1k and 470p to GND, e.g.).

    C540...C543 are very probably not useful. The filtering effect is minimal and the capacitive loading to the Lineout outputs is impractical.

    If the RC4580 has to drive a cable at its output, it's wise to add some series resistance to FB18 and FB19. 47R is useful.

    Kai
  • 0 in reply to Ivan Salazar
    TI__Guru** 104215 points
    Dongbao,

    Have you tried the recommendations provided above? Is this working better now?
    We would like to follow up and close the thread accordingly.

    Best regards,
    -Ivan Salazar
    Applications Engineer - Low Power Audio & Actuators