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Target Impedance Calculation for Multiple Devices on a Shared Power Rail

A common power integrity question is whether nearby devices on the same power rail should be treated as one load or as separate loads when calculating target impedance.

Target impedance is generally calculated as:

Ztarget = ΔV allow / ΔI transient

If multiple devices share the same rail, local PDN path, and decoupling network, the PDN sees the combined current demand. Therefore, the rail-level target impedance should normally be based on the summed worst-case transient current.

Examples

Scenario	Current Used	Example Ripple	Target Impedance
1 A device + 0.1 A device	1.1 A total	50 mV	50 mV / 1.1 A = 45.5 mΩ
Two nearby 1 A devices	2 A total	50 mV	50 mV / 2 A = 25 mΩ
FPGA 4 A + DDR 2 A on same rail	6 A total	27 mV	27 mV / 6 A = 4.5 mΩ

Practical Guideline

Nearby devices on the same rail and decoupling island: use the combined worst-case transient current.
Devices far apart: still use the aggregate rail current, but check local impedance and ripple at each load.
Mutually exclusive activity: use the worst active current only if the assumption is validated.

Key Takeaway

Target impedance is primarily a property of the shared PDN/rail, not of an individual IC. Use the sum of simultaneous transient currents for the rail target, then verify each device locally for acceptable voltage ripple and impedance.

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