Current Sense Amplifier: Achieve Accurate Shunt Resistor Connections

This article is the subject of achieving accurate shunt resistor connections. We will talk today about the shunt resistor design architecture and the shunt resistor manufacturer's typical recommended guidelines for connecting to their shunt resistors. There are a lot of connections that are wrong, and only follow the recommended guidelines of the shunt resistor manufacturer.

 

In Figure 1 below, look at the shunt resistor connection labeled "Ideal" on the left. The ideal connection uses traces of the same length and size; these traces are connected to the shunts typically recommended by the shunt manufacturer, and finally the voltage measured or sensed by the amplifier corresponds exactly to the voltage drop of the active portion of the shunt. Now, take a moment to compare the ideal connections shown in the figure with the "non-ideal" connections.

 

Ideal VS. Non-ideal Shunt Resistor Connection

Figure 1: Ideal vs non-ideal shunt resistor connection

 

Shunt resistor design

 

For maximum energy efficiency, it is necessary to understand the structure of the shunt. The terminals of the shunt that are soldered are typically copper material, which is a different material than the shunt body, such as manganese copper; the shunt manufacturer is adjusting this intermediate material to an exact value. The aim is to accurately obtain the pressure drop of the resistive material itself and there is no pressure drop at the end of the joint. Figure 2 shows the connection point of the two-terminal shunt resistor that is usually recommended by the manufacturer. The test line is connected in the middle of either side, on the plane of the shunt resistor material and the copper lead interface

 

Connection point for a two-terminal shunt resistor that is usually recommended by the manufacturer

Figure 2: Connection point for a two-terminal shunt resistor that is usually recommended by the manufacturer

 

If they differ from the one highlighted in Figure 2, the shunt manufacturer's recommendations for connecting to the shunt resistor should always be followed. Note that Figure 3 shows the stray lead resistance and the spurious detection trace resistance - an improper connection will add these unnecessary stray resistance and increase the error to the measurement.

 

Stray resistance (RLead and RSense) connected to a two-terminal shunt resistor

Figure 3: Stray resistance (RLead and RSense) connected to a two-terminal shunt resistor

 

The four-terminal shunt resistor shown in Figure 4 provides a more obvious way to connect the Kelvin line to the shunt while maintaining high accuracy. However, it should be noted that the four-terminal shunt resistor has no advantage in terms of cost.

 

Connection point for a four-terminal shunt resistor as recommended by the manufacturer

Figure 4: Connection point for a four-terminal shunt resistor as recommended by the manufacturer

 

Achieving accurate shunt resistor connections is not difficult, but it requires an understanding of the shunt manufacturer's connection guidelines and proper PCB design to make the correct connections.