Hey,

After reading several papers regarding the kelvin connection and the difference between 4 wires and 2 wires connection. I end-up by choosing to use a 2 pads current sense resistor for my PCB. The problem I am encountering right now is that when trying to use a kelvin connection for a 0603 shunt resistor, the tracks are too big to be connected inside the resistor, as it can be seen below :

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I tried to modify the nettie to achieve smaller dimensions but still in vain, the problem isn't with the nettie, it's within the dimensions of the resistor itself.

Ps: If you want to suggest to go for a bigger shunt resistor, it will be an overkill for my circuit. With 0.1 Ohms and (0.5A)



The datasheet recommends a shunt resistor of 0.01 ohm, which gives a voltage drop of 5 mV (with 500 mA and 4.2 V). However, the ADC operates with a range of -125 mV to 125 mV. That's why I opted for a 0.1 ohm resistor, the voltage drop will be 50 mV, which makes the ADC more sensitive to voltage variations. I don't want to go for a bigger resistor because the actual rated power of this resistor is 25 mW, which justifies the choice of a 0603 (0.1 ohm) resistor with a rated power of 100 mW. If I go for a bigger resistor it will has the power rating of 250mW which can be an overkill for my circuit.

Regards,

[MODERATOR: Fixed broken image link]

 

Would it be the only over-rated resistor in your circuit?
How about all the resistors in the 10k to 100k region? Are you using the tiniest resistor available for them, or are you using a sensible size?

Choose the size of resistor that makes it possible to make the best Kelvin connection. It won’t take up any more room because you won’t need the vias.

 

Would it be the only over-rated resistor in your circuit?
How about all the resistors in the 10k to 100k region? Are you using the tiniest resistor available for them, or are you using a sensible size?

Choose the size of resistor that makes it possible to make the best Kelvin connection. It won’t take up any more room because you won’t need the vias.

Actually shunt resistor are often very small, in the order of a few milliohms. The criteria I was basing on choosing this resistors are : the power rating and the resistance value.

The resistance value recommended by the datasheet is 0.01Ohm. However, using such a low resistor will generate a voltage drop of 5mV for an ADC that ranges from -125mv To +125mV. The question is, do you suggest using a higher resistance value such as 0.1 Ohm? this will lead to a voltage drop of 50mV which can allow the ADC to small current variation!

The second question is, when choosing the resistance rating value, what if we choose like a power rating value that is *100 times the dissipated power across the resistance ? can this be an overkill ? my resistance dissipates 2.5mWatt when using 0.01Ohm and 25mW when using 0.1Ohm.

 

I would suppose it’s about accuracy mainly, with size and power dissipation secondary.
If you make the worst shunt you could - simply a piece of track. The thickness isn’t accurately controlled and it drifts 0.4%/°C with temperature. There would be some applications, mainly where the measurement is just ratiometric, where that would be good enough.
If you do the job properly with a resistor, then the smaller the value of resistor, the worse the error from amplification, the greater the resistance of the solder, and the greater the difficulty of getting a Kelvin connection, but you have lower heat dissipation, and (maybe, if you don’t need vias) a smaller footprint.
A larger resistor will improve accuracy in amplification, make the tracking easier, reduce the effects of the solder resistance, but will make things warm, and take up more room; and as Bob Pease always said “heat is the enemy of precision”.
With a 2512 resistor, you can remove a section of the pad and eliminate the effects of the solder resistance.
Somewhere between those two extremes is a solution that is best suited to your application.