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I was reading a PCB design notes and guidelines from Xilinx for their 7 series FPGAs and came across this sentence:

The capacitors should be placed as close as possible to the FPGA. Any placement within two electrical inches of the device’s point of load is acceptable.

What is the meaning of electrical inches ?

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  • 9
    \$\begingroup\$ I would think the wires/traces length \$\endgroup\$ – Eugene Sh. Oct 10 at 19:20
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    \$\begingroup\$ I had to suspect inches honoring bends in your tracks. Not just a radius. \$\endgroup\$ – Janka Oct 10 at 19:21
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    \$\begingroup\$ I wonder if this wire distance needs to be reduced according to the dielectric velocity factor? \$\endgroup\$ – hotpaw2 Oct 10 at 21:17
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"Electrical inches" is shorthand for "inches of trace along which the electrons must flow". In other words, it's the length of the wires or traces, following any bends, not the straight-line distance.

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An electrical inch is the actual distance along an electrical path.

Source: UltraScale Architecture PCB Design from xilinx.com

Footnote on page 19
Referred to as "Added footnote with definition of electrical inch to 0805 Ceramic Capacitor" in revision history (version 1.6).

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