What is Kelvin Connection and its application in IC Design & Layout?

[tomsawyer3.14]

Hi,

I am doing IC layout and I saw on one schematic some comments like "Kelvin Connection" & "Kelvin Ground.

Is this the same with the Kelvin Connection on Instrumentation where you measure a resistor by passing current then measuring the voltage across?

I don't get its application/imporatance in the IC/ micro level.

Can you guys help me understand it?

Also hat are the layout consideration when there is a Kelvin Connection on the schematic?

Regards,
Tom

 

[KlausST]

Hi,

There are so many discussions and informations about it. Thus just in short:
It does not matter if wiring, PCB layout, or IC design....always the same.
You simply split one one wire into two:
* one carries the current (it has voltage drop)
* while the other senses the voltage at the point of interest..with about zero current thus avoiding errors caused by voltage drop.

For more informations please do an internet search.

Klaus

 

[danadakk]

This might help, attached. Keithley 7'th edition low level measurement handbook.


Regards, Dana.

 

[dick_freebird]

Products such as voltage references / regulators
will see degraded load rejection if the output
"force" (pass transistor) and "sense" (feedback)
are made to share a common bond wire and
package lead.

Designing it so the feedback comes back
from the pad, post or PCB by another route
lets the error amplifier "kill" the ohmic I*R error
by a bit, a bit more, altogether.

This also applies at die probe where probe tip
and contact resistance can be high and highly
variable.

 

[crutschow]

Products such as voltage references / regulators
will see degraded load rejection if the output
"force" (pass transistor) and "sense" (feedback)
are made to share a common bond wire and
package lead.

Say you have a voltage regulator or power supply and you want a stable voltage at the load independent of the load current.
For that you would have a separate set of wires from the regulator, besides the wires carrying the current, to measure the voltage directly at the load, so any resistive voltage drop to the load can be cancelled (ignored).
That is a 4-wire Kelvin connection.

You would do the same thing when designing a regulator at the chip level.
The IC traces that carry the load current to the output power and ground pins would be separate from the IC traces carrying the output voltage to the sense circuitry.

 

[timof]

The basic idea behind Kelvin (or four terminal) sensing is very simple, and is explained here (or in the posts above):

Four-terminal sensing - Wikipedia

en.wikipedia.org

The devil is in the detail, though.

When you have IC design, for example - power transistor, you want to measure the current or voltage on it.
Now, power FET usually uses multiple metal layers (from 1-2-3 in old technologies, to ~20 in latest technologies), and multiple "fingers".
Where you "connect" in this massive metal network, is very important.
The sense point should be selected so that it provides kind of an average of the whole device.
The information about where this average is, can be obtained from proper simulation.

Such methodology for selecting an optimal sense point - where you connect sense or replica (small) device - to measure the current in the large device - is very important for sensing accuracy.

 

[dick_freebird]

If you have a very cellular "big transistor" layout it can
be effective to distribute sense "cells" throughout.
Provided you have the high-enough metal stack to
do it.

But sometimes the real quantity of interest is some
external voltage, which needs to be brought back on
chip without accruing error. And you may not be
allowed the extra pin to do that cleanly, so have to
do the best you can with pads and wires and routing
(what you can control, as a designer).