What is a Kelvin Connection and a Shunt Resistor?

What is a Kelvin Connection and a Shunt Resistor?

Have you ever tried to measure a current through a resistor? Probably you did. And you know it is very difficult sometimes to get an accurate result. Measuring a current through a resistor is called Shunt Current Measurement. And the resistor gets an application specific name Shunt Resistor.
If you have never tried to measure a current, especially μA (micro ampere) or nA (nano ampere) ranges, I can say that it is difficult. It is because of the probes (or wires)(assuming when you use a multimeter) crates unwanted impedance which affects the measurement accuracy in a bad way. Here the Kelvin Connection saves our life!

Shunt Resistor

Shunt Resistor: WSBS8518L5000JK35 from Vishay

Shunt resistor usually have 4 leads (terminals). 2 leads are for the current to pass through, the other 2 leads are for the measurement. On the right image a shunt resistor from Vishay can be seen. This is a special high current shunt resistor. It is aimed for the automotive market. Although 4 leads are better for measurement, there are 2-lead shunt resistors as well. In that case, both the power and the measurement leads are sharing the same terminal.

Kelvin Connection

if only a 2-terminal connection is employed (a 2 terminal resistor), in the applications where you want to measure high accuracy and low values of current, the contact resistance and the lead resistance may be greater than the element resistance itself, so it can lead connection errors and the temperature coefficient of resistance errors can be significant. The following figure shows a resistor in series with an inductor in parallel with a capacitor including the resistance and inductance of the leads. This is the equivalent circuit of a 2-terminal resistor.

Shunt Equivalent Circuit of a 2 terminal resistor

Ignoring the inductance and capacitance for now, the 2-terminal resistor is shown below. If r1 = r2 = r, then the total resistance RT = R + 2r. The lead resistance r is uncertain because there is no user assured connection to the lead. Thus, if we allow r to be significant compared to R, small inaccuracies in lead connections become large inaccuracies in readings.

Shunt Equivalent Circuit Simplified

In the 19th century, Lord Kelvin developed, the 4-terminal method of measurement, which eliminated both the uncertainty of lead resistance and lead response to temperature.

Below figure is the Kelvin solution. If the voltage measuring system used here is of a high impedance, then r5 approaches infinity and the measurement current lm approaches zero. With zero lm there is zero IR drop through r3 and r4, and therefore, it does not matter whether the contact resistance is large or small. It also does not matter if the contacts have a high TCR (Temperature Coefficient of Resistance). Similarly, the TCR of the current leads is no longer important because the voltage connections are fixed inside the lead resistance, and, the resistance and TCR of the element only are sensed.

Kelvin Connection

Any errors associated with the lead resistance, contact resistance, and/or lead TCR are thus eliminated. A Kelvin connection to a four-terminal resistor is essential for precise current sensing.

Kelvin Connection on Schematics

An example schematics is shown. I have used this circuit in my MSc thesis. R15 is the Shunt Resistor. Of course shunt resistor and Kelvin connection aren’t the only things to do an accurate measurement. The complete system shall be designed with a care. We will discuss the rest of the circuit in the following articles.

Kelvin Connection on PCB

In the following figures, Kelvin connection can be seen on the PCB layout. The trace lengths are identical in length. This is also guaranteed by using a DIFFPAIR feature in Altium Designer.

Kelvin connection eliminates unbalanced trace impedances, hence improves the measurement accuracy. It can lead to less noise measurements.

  • Increasing the shunt resistance value increases the voltage drop on the shunt, which helps to lower the requirements on the voltage offset (VOS) and input bias current offset (OSI) of the back-end amplifier. The trade-off, however, is that a larger value of shunt resistance can produce self-heating resulting from increased power dissipation. The temperature drift changes the nominal resistance of the shunt, which then affects the measurement accuracy.
  • Using a smaller value of shunt resistance requires a larger gain configuration on the amplifier to match the full dynamic range of the ADC, which results in higher noise and affects overall system accuracy. Another critical requirement is to select a shunt resistor with a low temperature coefficient and tolerance because these parameters have a direct impact on the measurement accuracy.

Stay tuned for more articles! :)
Source: Texas Instruments, Vishay, Wikipedia.

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