1. Name the disturbance that needs attenuation
Begin with the disturbance reaching the LDO input. Power-supply rejection concerns how much of that disturbance appears at the regulated output. The disturbance's frequency therefore belongs in the requirement. A request for a low-noise regulator can leave this question unanswered if it does not distinguish unwanted input ripple from noise generated within the regulator itself.
The distinction changes which evidence should be compared. An output-noise description addresses a different contribution from the transfer of an input disturbance. Neither heading should replace the other merely because both affect the supply seen by a sensitive load. Identify whether the present question concerns input ripple, generated output noise or separate requirements for both.
For example, an application concerned about a particular switching-related disturbance needs rejection evidence applicable to that frequency context. A favorable description at another frequency does not answer the same question. This frames a useful comparison without selecting a numerical attenuation requirement or assuming that the upstream supply contains only the disturbance the application has identified.
2. Compare rejection at the same frequency
Analog Devices explains that PSRR varies with frequency, with the LDO control loop playing an important role. The regulator's error-amplifier response is therefore part of the rejection context. A single detached PSRR figure cannot describe that response across the spectrum. Two figures taken at different frequencies should not be ranked as answers to one identical requirement.
The article also distinguishes the region in which the output capacitor and parasitic paths become important as feedback influence diminishes. This explains why the frequency coordinate matters technically: the relative contributions within the circuit change. It does not supply a universal boundary frequency or show how a particular customer layout behaves. Use the exact regulator's applicable evidence for the proposed operating region.
Capacitor discussion must retain that purpose. A capacitor change relevant to high-frequency rejection still has to satisfy the regulator's separate stability requirement. The existing ESR guide addresses that different decision. Do not convert a general observation about output-network effects into permission to substitute the lowest-ESR capacitor in every LDO circuit.
3. Preserve load and input-output headroom
Matching frequency is necessary for a meaningful PSRR comparison, but it does not make every other condition equivalent. Analog Devices identifies load current as another influence on the feedback response. A specification or characteristic associated with one load context therefore needs that context retained when it is compared with the application's requirement.
Input-output headroom is another independent coordinate. The article explains how reduced headroom can reduce the pass stage's contribution to loop gain and consequently affect rejection. The useful question is whether the candidate's rejection evidence covers the intended headroom and load together. It is insufficient to name the input voltage while leaving the required output or relevant load condition ambiguous. Two descriptions at the same disturbance frequency can still refer to different headroom and load combinations. Before ranking them, determine whether each describes the required operating point. Where one does not, the difference is a specific applicability question, rather than evidence that the regulator with the more favorable detached figure will reject the actual disturbance better.
This is different from the dropout decision. Dropout review asks about the voltage condition needed for regulation under the device's definition. PSRR asks how an input disturbance transfers to the output at the stated operating point. Satisfying a dropout requirement does not automatically preserve the rejection described under a more generous headroom condition.
4. Keep component rejection separate from rail acceptance
A useful candidate comparison begins with one disturbance-frequency question and the associated load and headroom. It then identifies which evidence answers that combination. If the evidence uses another combination, explain the difference rather than comparing the headline as though its conditions matched. This keeps the remaining work tied to an actual operating point.
Next distinguish the regulator contribution from the completed rail's acceptance. A component PSRR characteristic does not report the disturbance present on the customer's input, the implemented output network or the resulting supply at the sensitive load. Those are application inputs and results. The comparison can identify their relevance without claiming that any rail has been measured or accepted.
The handover to engineering should therefore ask whether the available rejection evidence is applicable to the disturbance and operating conditions that matter. Generated output noise and capacitor stability remain separately identifiable questions. This gives the sourcing team a concrete reason for requesting particular evidence, without inventing a PSRR value, ranking a candidate or promising attenuation on the finished board.
Sources and further reading
Analog Devices' saved Power Supply Rejection, frequency, load-current and headroom prose was read on 15 October 2026. No plotted values, equations, numerical PSRR recommendation, capacitor substitution or completed-rail result is supplied.