Calculators / 07
PDN Target Impedance Calculator
If the load's current can jump by ΔI, the rail's impedance at the frequencies in that jump turns it into a voltage error. Keep that impedance below V·ripple/ΔI and the error stays inside the ripple budget. It is the first number in any PDN design — and, as the topic page explains, only the first.
Reading the numbers
- Halve the voltage or the ripple, or double the step, and the target halves: modern low-voltage, high-current rails reach single-digit milliohms.
- The flat line assumes the worst-case step can contain any frequency. Real loads have a spectrum, and the target can rise where they have little energy.
- Staying under the line is sufficient, not necessary: a narrow peak above it may never be excited.
Every value is in the address bar as you change it, so a link reproduces the calculation exactly.
Type values with SI prefixes — 100n, 2.2p, 16G — or
drag the sliders; a bare number keeps the prefix already shown.
The formulas
Go deeper — assumptions, how it is checked, and sources
Assumptions. A current step with a flat spectrum, a linear network, and one ripple figure for all frequencies. Where the load spectrum is known, a frequency-dependent target is tighter where it matters and looser where it does not.
How it is checked. Units reduce to ohms; the target scales linearly with voltage and ripple and inversely with the step; and the ripple the target implies, ΔI·Z, returns the budget that went in.
Source. L. D. Smith et al., “Power distribution system design methodology and capacitor selection for modern CMOS technology,” IEEE Trans. Advanced Packaging, vol. 22, no. 3, 1999.