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 ?

  • 9
    \$\begingroup\$ I would think the wires/traces length \$\endgroup\$
    – Eugene Sh.
    Oct 10 '19 at 19:20
  • 6
    \$\begingroup\$ I had to suspect inches honoring bends in your tracks. Not just a radius. \$\endgroup\$
    – Janka
    Oct 10 '19 at 19:21
  • 6
    \$\begingroup\$ I wonder if this wire distance needs to be reduced according to the dielectric velocity factor? \$\endgroup\$
    – hotpaw2
    Oct 10 '19 at 21:17

"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.


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).


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.