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TPS77401-EP: TPS774 Output Noise Characterization below 300-Hz

Sean McCarthy
Prodigy 140 points
Part Number: TPS77401-EP
Other Parts Discussed in Thread: ADC128S102, TPS774

Good Morning,

I am considering the use of this part to supply power to a TI 12-Bit ADC (ADC128S102 series, both VD and VA supplies).

From first analysis it appears that both line regulation variations and 300-Hz to 100-KHz noise characteristics will be below the minimum resolution I will use for my application.

But as I will be measuring fairly low frequency signals down to DC I am looking for the TPS774 series 1/f noise characteristics. 

Is it possible to obtain this data ?  or is there a spice macromodel that would characterize this accurately ?

Regards,

Sean

  • 0
    TI__Mastermind 23361 points

    Hi Sean, 

    You could find noise data down to 100Hz from Figure 8. We don't have any noise data beyond what we already listed in the datasheet. If you are flexible on the other LDOs, we may recommend you with other LDOs with much less noise (<5uVrms). 

    Regards, 
    Jason Song

  • 0 in reply to Jason Song
    Prodigy 140 points

    Hi Jason,

    Thank you for the quick follow up.  I did observe the existing noise spec in the DS down to 100-Hz.  I have no issue with this noise performance and meets my requirements

    I was hoping to get some feel for the noise below 100-Hz without having to spend time doing these measurements,  I'm in a time crunch at the moment and this part is already spec by other group in other parts of the project.  Basically, I have an ADC with minimum resolution of 800-uV.  Can I get some idea on the magnitude of the TPS774 1/f noise relative to this minimum resolution ?  I can tolerate some drop in ADC dynamic range.  I would appreciate if TI could provide some sense of the 1/f magnitude relative to my case...don't need exact data short term...Just need a sense of the ballpark values ?

    Additionally, I would have the following questions:

    The first page of the datasheet has the bullet point "2% Tolerance Over Specified Conditions For Fixed-Output Versions", yet on page 7 of 25 the first table entry indicates +/-2% for adjustable version.  Is this correct considering the first page bullet point ?  I assume the adjustable version is also +/-2% over temperature but will now have additional error based on external feedback resistor performance ?

    It would be interesting to know how the line regulation performance degrades when Vin drops below Vo + 1V towards dropout.

    Regards,

    Sean

  • 0 in reply to Sean McCarthy
    TI__Mastermind 23361 points

    Hi Sean, 

    For <100Hz, the 1/f noise will dominate, and you are likely to see the noise moving up linearly when lowing the frequency to 100Hz. You may find some examples of the noise density down to 10Hz in this app note. If you are using the adjustable version, you may also consider adding a feedforward capacitor to lower the noise as well. 

    For your question about the 2% accuracy, yes, when using an external resistor divider, the output will have an additional error term from the feedback resistor tolerance.

     

    Regards, 
    Jason Song

  • 0 in reply to Jason Song
    Prodigy 140 points

    Hi Jason,

    Thank you for the reference to the app note.  I will provision for the feed forward capacitor

    Regards,

    Sean