An LDO's output noise and PSRR are largely set by the feedback divider network. A tiny capacitor placed across the top feedback resistor — the feedforward capacitor (C_FF) — is one of the highest-leverage tricks in linear regulator design. It costs pennies, occupies one 0402 footprint, and can improve noise, PSRR, and transient response simultaneously.

Here's the mechanism: the LDO's error amplifier sees a divided-down version of V_OUT through R1 (top) and R2 (bottom). At DC, the divider ratio sets V_OUT. But if you shunt R1 with C_FF, at high frequencies the divider ratio approaches unity — the error amplifier sees the full V_OUT, not the attenuated version. This means the loop gain at those frequencies effectively increases by (1 + R1/R2), the same factor as the DC gain. More loop gain means better PSRR and lower noise in that band.

The feedforward network introduces a zero at f_z = 1/(2π·R1·C_FF) and a pole at f_p = 1/(2π·(R1||R2)·C_FF). You want the zero placed inside the loop bandwidth to buy back gain that the divider stole, while keeping the pole well below the unity-gain crossover to avoid stability trouble.

Real-world example: The TI TPS7A47 (a low-noise LDO popular for driving ADC references and RF PLLs) spec sheet actually plots noise and PSRR versus C_FF. On a 3.3 V output with R1 = 40.2 kΩ, R2 = 10 kΩ, adding a 10 nF C_FF drops integrated output noise from ~15 μV_RMS to ~4 μV_RMS (10 Hz–100 kHz), and improves PSRR at 1 kHz from ~55 dB to ~75 dB. That 20 dB is the difference between a barely-usable rail and a genuinely quiet one for a 16-bit ADC.

Rule of thumb: Place the zero roughly a decade below the LDO's loop crossover frequency. If your LDO has ~100 kHz bandwidth, target f_z ≈ 10 kHz. With a typical R1 of 40 kΩ:

• C_FF = 1/(2π · 40 kΩ · 10 kHz) ≈ 400 pF — round to 470 pF or 1 nF.

Caveats:

• C_FF slows startup — the soft-start ramp charges C_FF, so V_OUT overshoots the target briefly. Some regulators (like the LT3042) integrate C_FF internally and precharge it to avoid this.
• Use C0G/NP0 dielectrics. X7R's voltage coefficient will detune your zero.
• Don't use C_FF with internally-compensated LDOs that already include feedforward — check the datasheet.
• C_FF interacts with the output cap ESR — verify phase margin on the bench with a load step.