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I am making a power management circuit for multiple sensitive ADCs that will get power from it.

The basic idea is kind of like this:

Where this circuit I'm about to make will be a power hub of 8 power sensitive ADC. The ADC circuit already has its own filters designed as recommended by it datasheet (PSRR 60dB@100kHz and 90dB@100Hz) which should be theoretically enough, but since I don't have much control and information on how clean the main supply is, it maybe total trash or really good.

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Hence I would like to place something on the middle. The ADC circuits need 5V, hence I need a regulator and I found this which is advertised to be good at noise sensitive application TPS7A47. Is there something I can do to further give better protection against noise from the main supply? Also for inputs above 5V there is nothing to worry since the PSRR would kick in but for inputs that are exactly 5V (and output 5V) from what I know an LDO does not have PSRR in these situations so how can solve this issue?

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  • \$\begingroup\$ "PSRR 60dB@100kHz and 90dB@100Hz" is that what the ADCs already offer? Do you have better specifications and maybe a schematic? I'm really not sure what you're asking and what you actually need. \$\endgroup\$
    – michi7x7
    Sep 27 '20 at 20:17
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Also for inputs above 5v there is nothing to worry since the PSRR would kick in

This statement is untrue. LDOs take time to react to transients/noise at their input. Frequency/bandwidth/transition times is what is important here, not amplitude.

The PSRR of linear regulators and especially SMPS are not going to be effective against switching harmonics (such as those an SMPS).

Most definitely use a good ol' series resistor (probably 1 or 2 Ohms) before the decoupling cap for the regulator and the voltage reference where voltage is going to be dropped anyways. But you may also want them for each ADC as well. You could use ferrites for the ADC if voltage drop is a concern but selection is pickier than resistors and could make noise worse due to noise peaking.

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  • \$\begingroup\$ There is actually an RC filter of each ADC power, that is why i mentioned that they need 5v even though the adc runs on 3.3v. So there is no merrit for me in choosing this low noise LDO? Would there be a "level-up" solution to RC filter against SMPS noise? \$\endgroup\$
    – DrakeJest
    Sep 27 '20 at 18:54
  • \$\begingroup\$ Sure there's merit in that it would help with low frequency noise and noise from the LDO itself. RC is pretty heavy handed to begin with. Ferrite and resistor in series? The resistor might provide damping and make gain peaking less severe. Easy enough to replace with jumpers instead if not needed. \$\endgroup\$
    – DKNguyen
    Sep 27 '20 at 18:56
  • \$\begingroup\$ when you said heavy handed did you mean it in a good or bad way? Good that its over the top solution, or bad (i do not know any demerits which could result in worse noise) \$\endgroup\$
    – DrakeJest
    Sep 27 '20 at 19:30
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    \$\begingroup\$ @DrakeJest To see how useless a LDO is when you have high frequency switching noise and its solution, read Jim Williams application note AN101 - Minimizing Switching Regulator Residue in Linear Regulator Outputs. \$\endgroup\$ Sep 27 '20 at 21:59
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    \$\begingroup\$ You can also just look at the PSRR graphs for an LDO and note that a SMPS with a 1MHz switching frequency can have harmonics past 100MHz, at which even capacitors can have trouble dealing with due to their parasitic inductance. \$\endgroup\$
    – DKNguyen
    Sep 27 '20 at 23:54

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