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What is the gain settings and gain value of LM4911, LM48861 and LM4928 to obtain PSRR specified in the datasheet?

Tsz Kwok
Prodigy 30 points
Other Parts Discussed in Thread: LM4928, LM4911, LM48861

Hello,

May I know the value of gain settings and gain value of LM4911, LM48861 and LM4928 to obtain PSRR specified in the datasheet?
If the PSRR is not enough, what is the largest value of bypass capacitor used in LM4911 & LM4928?

Thank all expertises very much for their comments and feedback.

Best regards

  • 0
    TI__Expert 3350 points

    Tsz,

    The gain of the LM4911 and LM48861 were set to 0dB for the PSRR measurement.  The gain of the LM4928 was set to 0dB, assuming a fully differential source.  For bypassing, while you can use a large cap value, I would suggest using mutliple, smaller valued caps.  If you know you will have power supply noise at a particular frequency, choose a bypass cap specifically to attenuate that frequency. 

    Regards,

    royce

  • 0 in reply to Royce Higashi
    Prodigy 30 points

    Thanks Royce very much for your information.


    Since the supply noise is rectangular pulse with irregular duty cycle(sometimes ~200Hz, sometimes ~hundreds of kHz or few MHz(measured by oscilloscope)) and irregular amplitude, could you suggest what bypass capacitor value for this type of supply noise?

    If the required inverting feedback gain is close to 100 and the supply-ground decoupling capacitor is about 47uF, what further action can be used to improve these amplifier supply ripple rejection?

    Thanks in advance for any expertise comments/advices.

    Regards,

    Tsz

  • 0 in reply to Tsz Kwok
    TI__Expert 3350 points

    Taz,

    You can ad large bulk bypassing at your supply to handle the 200Hz, then smaller caps near the device for the higher frequencies.  You can calcuilate the capacitance needed to attenuate certain frequency ranges.  1-10uFs will attenuate 100s of kHz, 110s of nFs will attenuate MHz.  You can also add a ferrite bead in series with the supply to further attenuate noise.  Proper grounding and signal routing will also help with PSRR.

    Regards,

    royce

  • 0 in reply to Royce Higashi
    Prodigy 30 points

    Royce,

    Thanks so much for your reply.
    May I know the exact circuit configuration of 0dB for PSRR measurement? Should LM4911, LM48661 and LM4928 be connected as unity-gain buffer configuration OR inverting-gain (G=-1) configuration?

    Regards,

    Tsz

  • 0 in reply to Tsz Kwok
    TI__Expert 3350 points

    Tsz,

    The LM4911 and LM48861 can only be configured as an inverting amplifier, so setting Rin = Rf will give you a gain of -1V/V.  The LM4928 is a differential amp, set the Rin = Rf, and you will get a differential gain of 1V/V

    Regards,

    royce

  • 0 in reply to Royce Higashi
    Prodigy 130 points

    Thank you very much for Royce's reply.