High speed digital lines often have low value series resistors that
are used to improve signal integrity and reduce line reflection. This is often done with
clocks, address lines, control lines etc... Test engineers often double probe and/or
double wire these signals in their ICT fixture, on both sides of the series resistor. That
will enable them do a 4-wire Kelvin Measurement. Fixture CAD software such as D2B and
CBTest have resistor value threshold you can set that will automatically have extra probes
and wires added to these nets.
This is fine for low speed analog, current sense resistors and such. However,
this is a really bad idea for high speed digital signals. Especially the
ones that are connected to devices that you want to test or ISP. You should have only one
probe and one wire connected to your high speed signal, but now you have four. It is much
better to sacrifice a few ohms of tolerance for these resistor tests than to double probe
or double wire their signals.
These signals should not be double probed or double wired. It may
also be necessary to remove one of the probes on one side of the series resistor, then
test your series resistor and your digital device as a cluster.