Electronic – Kelvin “4 Wire” Resistance PCB Design Questions

pcbresistance

I am designing a PCB that will utilize 4 wire measurement for 40 or so test points.

A little background here – day to day I troubleshoot PCB's to component level, I'm an electronics technician and not an engineer and I'm okay with that. 95% of the time I can find an issue with a circuit based off of how the DVM measures resistance and it's behavior. My boss wants me to automate my process using LabVIEW; so I need to make a PCB that does just 4 wire resistance testing. I have an existing pogo pin block and a PCB that mounts to it, I just need to re-spin the PCB board and write the LabView program.

Are there any special PCB design details I need to consider when doing this very basic design?

Please be gentle; this is my first engineering project.

Best Answer

The nature of the Kelvin measurement is that you have a separate current path and measurement path so that no current (except leakage and bias currents) flow through the measurement conductors.

Thus, the PCB layout (unless the design itself is faulty) is remarkably easy, and I don't expect you'll have any troubles, since series resistance hardly matters.

schematic

simulate this circuit – Schematic created using CircuitLab

In the above schematic, the exact value of the resistors R1, R2, R3, and R4 hardly matter, provided they are reasonably low. R4 affects the common-mode voltage the instrumentation amp sees, R1 and R2 affect errors due to input offset current (and noise) a bit, but really the PCB layout is not very important until currents and voltage drops start to become significant wrt the common mode range of U1. So you want to keep the voltage drop across R4 reasonably low (not a problem typically unless you use very thin traces and/or very high currents).