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Original question:

RC4580-Q1: low SNR issue

RC4580-Q1: schematic review

Dongbao Zhou
Expert 4000 points
Part Number: RC4580-Q1
Other Parts Discussed in Thread: RC4580, OPA1678

Hi team,

Could you help review below schematic of 4580 used in SE to DM conversion?

For C265, if i remove this capacitor, it will cause self-excitation. If i keep this capacitor, there will have phase delay and this circuit become very sensitive to noise.

Could you give some comments, thanks.

Dongbao

  • 0
    TI__Guru** 102355 points
    Hi Dongbao,

    I'll check the schematic and provide comments, but in the meantime: have you considered on swapping the places of output capacitor and resistor? I mean to place EC15 on R545 and viceversa, and also EC27 on R546 and viceversa. Capacitive load at the output of opamps tend to lead them into instability.

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

    I would increase C66 from 4µ7 to 47µ. C262 can also be increased from 4µ7 to 47µ. The rest is fine.

    Kai
  • 0 in reply to kai klaas69
    TI__Expert 4000 points

    Hi Kai,

    Thanks for your advice, could you let me know the reason, do you think the input voltage may not stable with 4.7uF?

    Dongbao

  • 0 in reply to Ivan Salazar
    TI__Expert 4000 points

    Hi Ivan,
    Thanks for your feedback, I will ask the customer to try your advice. Could you let me the reason for swapping the capacitor and resistor?


    Dongbao

  • 0 in reply to Dongbao Zhou
    Guru 140200 points
    Hi Donbao,

    following a rule of thumb the impedance of C66 should be less than 1/100 of the impedance of R178 at the lowest frequency of interest which is 20Hz here. The Impedance of a 4µ7 cap is 1700R at 20Hz which is a bit high. A 47µ cap has only 170R and is less than 22k/100=220.

    There's still another reason why C66 should be 47µ and not 4µ7. The power supply of OPAmps ("Amp_+8V5") might be contaminated with noise. Keep in mind, that all the noise that is not suppressed by the voltage divider R172, R179 and C66 is directly injected into the signal path. Increasing C66 by a factor of ten adds 20dB more noise suppression.

    Refering to the recommendation of swapping the capacitor and resistor mentioned by Ivan: I guess Ivan was in a hurry and has overlooked that the cap and the resistor are in series and that it does not play a role which one comes first.

    Kai
  • 0 in reply to Dongbao Zhou
    TI__Guru** 102355 points
    Dongbao,

    It's been a while since last communication. Can we consider this thread closed?

    Best regards,
    -Ivan Salazar
    Applications Engineer - Low Power Audio & Actuators
  • 0 in reply to Ivan Salazar
    TI__Expert 4000 points
    Hi Ivan,

    Thanks for following up.
    This noise issue is solve by replace RC4580 with a rail-to-rail output OPA, i choose OPA1678.

    Dongbao