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关于LDO内部噪声的问题

ping zhang2
Prodigy 80 points
Other Parts Discussed in Thread: TPS7A8101

用LDO给运放或者ADC供电，运放和ADC都有PSRR，而且在低频端PSRR都高达80dB，LDO的内部噪声刚好也是低频的，按理说，运放和ADC的PSRR可以把LDO输出的内部噪声抑制的干干净净，但我实测不是这样的。

我的测试方法是这样的，（1）、用LDO输出给数据采集系统中的运放和ADC供电，采集数据。（2）、加了前馈电容的LDO给数据采集系统中的运放和ADC供电，采集数据，说明：前馈电容专门用来降低LDO的内部噪声，和反馈电阻R1并联。

第二种方法测到的数据波动明显小，说明前馈电容降低LDO内部噪声比较明显，但是问题来了，为什么运放和ADC的PSRR不能把LDO输出的内部噪声抑制的干干净净呢？

  • 0
    Prodigy 100 points

    你说的“前馈电容”是Cbypass吧，本来就是有它才能达到指标PSRR的。

    a bypass capacitor (CBYPASS) bypass noise generated by the bandgap reference to improve PSRR

    In most LDOs, the bandgap is the dominant noise source. If a noise reduction capacitor (CNR) is used with the TPS7A8101, the bandgap does not contribute significantly to noise. Instead, noise is dominated by the output resistor divider and the error amplifier input. If a bypass capacitor (CBYPASS) across the high-side feedback resistor (R1 ) is used with the TPS7A8101 in addition to CNR, noise from these other sources can also be significantly reduced. To maximize noise performance in a given application, use a 0.47-μF noise-reduction capacitor plus a 0.47-μF bypass capacitor. 

  • 0 回复 River Liu
    Prodigy 80 points

    看了一下TPS7A8101的规格书，Cbypass确实是前馈电容，前馈电容是用来降低，LDO内部REF在LDO输出端产生的噪声，跟LDO的PSRR没什么关系，PSRR针对LDO的外部噪声，前馈电容是针对LDO的内部噪声，两者没什么关系。