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LP2985A: LP2985A output noise

Part Number: LP2985A
Other Parts Discussed in Thread: PCM4104, LP2985

Hello support team,

in our application we use LP2985A-50 as power supply regulator for the analog section of an audio DAC (PCM4104). On some board of our mass production we encountered an issue with the output noise of the DAC and, after some investigations, we conduct the origin at the regulator LP2985: some devices exibith an higher 1/f noise that extends up to the 1kHz band and thsu the noise floor exceed our audio specification.

In the LP2985 datasheet is stated the noise is 30uVrms (300Hz - 50kHz) typical: what you can say about the maximum noise that we can expect in the same condition?

We found that increasing the capacitor at the bypass pin up to 100nF, instead of the maximum recommended of 10nF, we can reduce the ouput noise. Is this a possible and reliable solution or we must not exceed the 10nF as reported in the datasheet?

Thank you in advance for the support,

Best Regards.


  • Hi Marco,

    The noise level does not typically vary much between devices, so only a few devices are tested for noise performance. It should not increase ten-fold, so I do not know exactly what is going on here. 

    The bypass capacitor forms a low-pass filter for the voltage reference, so in theory there should be no limit to how large you can make Cbypass. However, putting a very large capacitor on the voltage reference output slows down how quickly the reference can come up, so in practice the capacitor can't be "too large", based on the application. We normally recommend using 1nF - 100nF for Cbypass (in the datasheet for LP2985 it doesn't specify a maximum but rather a recommendation of 10nF), but it could be made larger if the startup time is not of concern. So for your application, 100nF should be just fine. 

    Best regards,


  • Thank you very much for the reply.

    Well, we will try to use a slightly bigger bypass capacitor as the startup time is not critical.

    Thank you again,



  • Glad to help!