Uwharrie Test Solutions


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Kelvin Testing, not always so good


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.


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