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    PSRR of Error Amplifier

    Hi,

    I am designing an error amplifier as a part of LDO circuit and I needed to get its PSRR. I placed an AC source as a power supply with a swing=1v and freq=1KHz. When I put DC voltage=3.5v in the AC source, the PSRR graph was completely wrong, it was improving as freq. increases!!!! When I put DC voltage=zero, the graph was right, PSRR value was getting worse as freq. increases.

    The question is, is it right to put DC voltage=zero? because I didn't know that and I was just trying different ways to fix the problem and I thought that the DC value of power supply should be kept during this analysis.

    Thanks,

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    Re: PSRR of Error Amplifier

    is it right to put DC voltage=zero?
    NO
    It must be in the normal operating DC range. Ripple is assumed to be low impedance source like the DC source, which was not mentioned and may be your issue.

    The recommended approach uses both sources in parallel with impedance isolation. Note the AC or RF source must be zero ohms or buffered somehow to prevent loading by any connection. A Spectrum Analyzer or Network Analyzer is required.

    Below is an alternative method showing both AC & DC combined into a suitable low Zout buffer.
    Click image for larger version. 

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    A best design is easily achieved with good test specs™
    A better question deserves a better answer. ™
    ... so include all your acceptance criteria ( values, % tolerance) and assumptions in your question or any design.

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    Re: PSRR of Error Amplifier

    You should show your test circuit, which is probably somehow wrong. PSRR of an amplifier is defined as the input referred suppression of power supply variation, your test circuit must implement frequency independent gain to visualize the PSRR directly.

    PSRR of the complete LDO, including bypass capacitors, isn't necessarily monotonously decreasing versus frequency.



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    Re: PSRR of Error Amplifier

    This is the EA circuit. I put an AC source in series with the DC source to represent the ripples of the power supply.

    Click image for larger version. 

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    Re: PSRR of Error Amplifier

    I figured out what was wrong. It finally worked.

    Thanks for your help.



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