Kelvin connection in Eagle

Question

Hey everyone!

I'm still learning eagle, so some obvious things are still difficult for me to wrap my head around, but I'm learning!

I have a question regarding Kelvin connections in Eagle. I have a board I need to design that performs current sensing. It has 2 input jacks, 2 output bnc jacks (that go to a differential amplifier) and a shunt resistor to measure the voltage drop across. My supervisor has asked me to implement the following pad layout for the kelvin connection with this shunt resistor: https://www.bourns.com/docs/Product-Datasheets/css2h-2512.pdf

To do this the only approach I could come up with was to design a new footprint for the shunt resistor in which both sides have 3 pads. So I came up with this footprint (Bourns_CSS2H-2512-A in this library: https://github.com/joshvandermeer/Eaglelibrary )

Then when I try to connect to the pads manually using the polygon tool in eagle, I get the following:

Where it looks like each pad has a maximum trace width for the trace connecting to it. This circuit will have a high current, so I want the traces to connect completely, not just by a thin trace at the terminals.

Is there anything I can do to improve this situation? I'm new to this, so I'm sure I'm overlooking something important, and I really appreciate any advice!

Thank you!

rachaelp · Accepted Answer

Hi Joshua,

It looks like there are more issues than just your connection widths. If you look you actually have the signal polygon fills shorting with your sense trace. You need to do your footprint and your schematic symbol to allow the 4 terminal connection. So on each side the two outer pad pieces connect to the corresponding main resistor pin and the central pad connects to a separate current sense pin with a separately named net. I'm at work at the moment so can't explain further but if you are still having issues I will try and explain better later.

With regards to the connection, turn off thermals for the polygon and then it will connect fully.

Best Regards,

Rachael

autodeskguest · Answer

On 15.10.2018 11:01, rachaelp wrote: wrote: I think you are way overdoing this circuit, and at the same time you weaken it. If the resistor is tightly dimensioned for your power, you need the full pad both for heatsink and current. Just route the kelvin probes out on the side of the SMD pads. Joshua does say in his first post "My supervisor has asked me to implement the following pad layout for the kelvin connection with this shunt resistor". It might not be what you would do, but it is what he was asked to implement by his supervisor. Best Regards, Rachael Fair enough Rachael If this was me, I would go back to my supervisor and demonstrate my skills and tell him about this implementations weakness but it may not be the right thing for Joshua this time. That's up to him. To add more feedback to his supervisor, Im not even sure if a manufacturer would like this implementation, due to it's potential bistable nature. When the lead melts, this got a bistable nature of not falling to rest on the large pads, but may be pulled to one side or the other.

rachaelp · Answer

autodeskguest wrote: I think you are way overdoing this circuit, and at the same time you weaken it. If the resistor is tightly dimensioned for your power, you need the full pad both for heatsink and current. Just route the kelvin probes out on the side of the SMD pads. Joshua does say in his first post " My supervisor has asked me to implement the following pad layout for the kelvin connection with this shunt resistor ". It might not be what you would do, but it is what he was asked to implement by his supervisor. Best Regards, Rachael

autodeskguest · Answer

On 08.10.2018 17:06, Joshua Vandermeer wrote: Hey everyone! I'm still learning eagle, so some obvious things are still difficult for me to wrap my head around, but I'm learning! I think you are way overdoing this circuit, and at the same time you weaken it. If the resistor is tightly dimensioned for your power, you need the full pad both for heatsink and current. Just route the kelvin probes out on the side of the SMD pads. The point of the kelvin probes are to avoid the voltage drop in the high current main path. If you just bring out some probes on the side of the smd's, there will be close to no current generating voltage drop on those tracks. If you want max noise immunity, route the two tracks differential back to the sensing circuit. For even more noise immunity route them through directly down a VIA to a different opposite layer. The implementation of the kelvin probes is done at the board layouyt, not the schematic, but if you need others to do the routing job based on your schematic drawings, draw it like you would route it and put a comment in the schematic at the info or guide layer.

autodeskguest · Answer

On 15.10.2018 17:19, Joshua Vandermeer wrote: Thank you both for the great advice. I agree that the implementation I was shooting for was very overcomplicated. I'm an intern right now and trying to take as much instruction as I can. I was given this document to follow, but there was a miscommunication and he just wanted me to learn from it, not actually implement it. Optimize High-Current Sensing Accuracy by Improving Pad Layout of Low-Value Shunt Resistors | Analog Devices ( http://www.analog.com/en/analog-dialogue/articles/optimize-high-current-sensing-accuracy.html ) Ended up just taking the kelvin connection from the inside of the pads. Interesting experiment there.. Using constant current source may not reveal all the weaknesses tho. Voltage applied to get to that current may vary to compensate higher R(solder) on the smaller area pads, and this may cause undesired effects. Also thermal effects may increase because of the same problem. Your conclusion seem to be something what I would do, but this is very dependant on how the resistor is actually built. Choosing the shortest path to the actual start of the resistor material would give best result. Because of the resistor physics, you could also analyze the result on different fine tuned positions of the resistor, but these are errors that are hard to control in mass production. If you really would need a high precision, high yield kelvin probe, some extra attention to all the side effects may be required, but usually you end up with some calibration process. This is why noise is more of a real problem than offset errors.

Kelvin connection in Eagle

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