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TPS7A47: The relation between TPS7A4700 output noise performance and its input

Mahmoud Zannerni62
Intellectual 655 points
Part Number: TPS7A47
Other Parts Discussed in Thread: ADS114S08, TPS7A39

Hello,

I am designing a power supply for an instrumentation system using ADS114s08 16 bit ADC, I would like to ask about using an isolated DC-DC infornt of the LDO. Will the DC-DC degrade the noise performance of the LDO? or the noise performance is guranteed regardless of the input noise?

The output of the DC-DC is 6 volts and the switching frequency is 100 KHz..  The TPS7A4700 is in 5V output mode.

Regards,

Mahmoud

  • 0
    TI__Expert 7035 points
    Hi Mahmoud,

    The noise specification shown in the datasheet is a typical number that is only valid for an ideal low noise input. The noise specification is only meant to show the intrinsic noise of the LDO. If you have a switching converter before the LDO there will be some spikes at the switching frequency and its harmonics. This can cause a degradation in the noise performance of the device at higher frequencies.

    You can see the example in the TPS7A39 datasheet on page 36 where there is a DC-DC converter being used before the LDO. You can compare that plot to the plots on pages 11 and 12 where it shows the noise with the ideal source, and the switching peaks are not present.

    Best Regards,
    Mark