Editor’s note: This two-part series looks at the underestimated nuances of current sensing. Part 1 (here) will discuss the general setup, selection, and implementation of a current sense resistor. Part 2 will discuss associated circuitry such as the critical analog front-end (AFE) and instrumentation amplifier.
The basics of current measurement
Current flow is one of the most common parameters used for evaluating, controlling, and diagnosing the operational effectiveness of electronic systems. Because it’s such a common measurement, designers often get into trouble if they underestimate the nuances of accurate current measurement.
The most common sensing element used to detect current flow is a low value, precision resistor placed in the current path. This resistor, usually called a shunt, develops a voltage across it that is proportional to the current passing through it. Because the shunt resistor should not affect the current flow significantly, it is often quite small, on the order of milliohms or fractions of a milliohm (mΩ). As a result, the voltage developed across the shunt resistor is also quite small, and often requires amplification before being converted by an ADC.
As such, a common signal chain configuration for monitoring current involves an analog front-end to amplify the voltage developed across the shunt resistor, an ADC to convert the amplified voltage into a digital representation, and a system controller (Figure 1).
Figure 1: The easiest way to measure current flow is with a current shunt resistor (far left), across which a voltage is developed that’s proportional to the current flowing through it. An AFE amplifies the low voltage across the shunt resistor in order to use the ADC’s full measurement range. (Image source: Texas Instruments)
An AFE, usually implemented with an operational amplifier or dedicated current sense amplifier, converts the small differential voltage developed across the shunt resistor to a larger output voltage that uses more of the ADC’s full measurement range. The ADC, which can be a standalone device or an on-chip block within a microcontroller or system-on-chip (SoC), digitizes the voltage signal and provides the resulting information to the control processor. The system controller uses the digitized measurement of current flow to optimize system performance or to implement safety protocols to prevent damage to the system if the current flow exceeds a preset limit.
As the sensor component in the chain used to convert current to voltage, the resistor’s physical characteristics (resistance, tolerance, power capacity, thermal coefficient, and thermal EMF) all affect accuracy. Consequently, choosing the proper shunt resistor is critical to optimizing the current measurement.
The shunt resistance value and corresponding voltage developed across the shunt resistor perturbs the system. For example, a shunt resistor with too large a resistance can reduce the voltage available to drive the load and cause unnecessary losses.
For example, when measuring the current supplied to a motor winding, a reduced voltage decreases the electrical power available to the motor, affecting its efficiency and/or torque. In addition, large currents flowing through the shunt resistor (tens or hundreds of amps) will cause the resistor to dissipate a significant amount of power as waste heat, making the measurement less accurate as well as less efficient. For these reasons, the shunt resistance should be as small as possible.
Picking a shunt resistor for measuring current
The fact that shunt resistors dissipate power as a result of the load current flowing through them requires that they have very low resistance values. Additionally, for measurement stability, current sense resistors should also have a very low temperature coefficient of resistance (TCR). A low TCR will result in high measurement accuracy with a low temperature dependency.
The current sense resistor’s thermal EMF is another important characteristic. Current shunt resistors must operate over a wide range of currents. When the current is low, for example in a battery application during sleep or standby mode, the shunt’s thermal EMF adds a measurable error voltage to the voltage generated by the current flowing through the resistor. This error voltage should be significantly lower than the lowest expected voltage generated by the current of interest flowing through the shunt resistor, minimizing measurement error.
Shunt resistors for current sensing applications are available with two or four terminals. A shunt resistor with two terminals is the simplest to understand because it works the same way that any two-terminal resistor works. Passing a current through the two-terminal shunt resistor develops a voltage across its terminals that’s proportional to the current passing through it.
Examples of two-terminal shunt resistors include the Bourns CSS2 shunt resistor series and the Vishay WSLP shunt resistor series. The Bourns CSS2 series includes shunt resistors with power ratings of 2 to 15 watts, resistances of 0.2 to 5 mΩ, and maximum current ratings of 140 to 273 amps. A typical device in the series, the CSS2H-2512R-L500F, comes in a 2512 surface mount package and has a resistance of 0.5 mΩ and a power rating of 6 watts.
Vishay’s WSLP shunt resistor family includes devices in several surface mount package styles ranging in footprint size from 0603 to 2512 with power ratings of 0.4 to 3 watts, resistances from 0.5 mΩ to 0.1 Ω, and resistance tolerances of 0.5 or 1%. A typical Vishay current shunt resistor is the WSLP1206R0150FEA, which comes in a 1206 package with a resistance of 15 mΩ, a 1% tolerance, and a power rating of 1 watt.
Note that these surface mount technology (SMT) current shunt resistors are small and require very little board space, but because they can dissipate significant amounts of heat they should be placed well away from heat sensitive components.
Three resistances in one shunt resistor
Despite appearances, current shunt resistors are not as simple as they seem. In particular, a shunt resistor’s resistance actually consists of three resistances (Figure 2). First, there’s the resistance of the shunt resistor itself. Then there are the resistances of the shunt resistor’s leads and the leads on the printed circuit board connected to the shunt resistor. Normally these lead resistances are insignificant, but current shunt resistors usually have very low values. In high current measurements, even small lead resistances introduce measurement errors because they’re not part of the manufacturer’s resistance specifications of the shunt resistor.
Figure 2: A two-terminal current shunt resistor has three series resistances: the resistance of the actual shunt resistor, the resistance of the resistor’s two leads, and the resistance of the leads or traces on the pc board connecting to the resistor (not shown). The lead resistances can cause measurement error in high current measurements. (Image source: Bourns)
One way to avoid the measurement errors introduced by the extraneous lead resistances is to create a Kelvin connection by running separate sense traces to the two-terminal shunt resistor (Figure 3).
Figure 3: A Kelvin connection to a two-terminal current sensing resistor reduces the measurement error caused by the resistor’s and circuit board’s lead resistances. Example images of two-terminal current shunt resistors appear on the right. (Image source: Bourns)
In this configuration, large circuit board traces carry current in and out of the current shunt resistor. Much smaller traces that are not in the main current flow but positioned as close as possible to the shunt resistor’s resistance element, pick off the voltage across the shunt resistor and convey that voltage to the AFE. Separating the current carrying terminals from the sensing terminals defines the Kelvin connection.
The resulting schematic representation of a Kelvin connection using a two-terminal shunt resistor is shown in Figure 4.
Figure 4: Using a Kelvin connection to a two-terminal shunt resistor takes the voltage sensing lines out of the main current path, resulting in a more accurate voltage measurement across the shunt resistor. (Image source: Bourns)
Very little current flows through the two sense resistances shown in Figure 4 because they are connected to the high impedance inputs of either an amplifier or an ADC, making their resistance much less critical than the resistance values of the leads carrying the high currents in and out of the shunt resistor. Consequently, the voltage drops across the sense resistances are quite small and not a significant source of error for the current measurement.
Two terminals or four?
As can be seen in the pc board layout diagram in Figure 3, it’s not possible to entirely eliminate the lead resistances in a two-terminal shunt resistor even when using a Kelvin connection. There needs to be some pad layout tolerance to accommodate positioning error when the shunt resistor is placed and soldered onto the circuit board.
Additionally, the TCR of the pc board’s copper traces (3900 ppm/ ̊C) is much higher than the TCR of the shunt resistor’s resistive element (often less than 50 ppm/ ̊C). These parametric differences cause the resistance change in the circuit board traces to be much higher than the change of the current sense resistor, making the temperature dependency of the sensing circuit to be high.
When using a two-terminal shunt resistor with a Kelvin connection, the level of precision may not be adequate for many current sensing applications where very high currents are involved. For such applications, manufacturers offer shunt resistors with four terminals that implement the Kelvin connection within the resistor. By incorporating it, the manufacturer can completely control all of the tolerances and temperature coefficients relating to the Kelvin connection (Figure 5).
Figure 5: A four-terminal shunt resistor implements a high precision Kelvin connection with the sense connections located very near the shunt resistor. An example image of a four-terminal current shunt resistor appears on the right. (Image source: Bourns)
A four-terminal current-sense resistor using a Kelvin connection has separate terminations for high current flow through the resistor and for the voltage measurement, helping improve measurement accuracy. Additionally, using a four-terminal shunt resistor with a built-in Kelvin connection reduces TCR effects by providing improved temperature stability when compared to a two-terminal shunt resistor using a circuit board layout to implement the Kelvin connection.
Bourns offers several four-terminal shunt resistors in its CSS4 series of surface mount devices (Figure 6).
Figure 6: Bourns’ CSS4 surface mount shunt resistors use a four-terminal Kelvin connection to maximize current measurement accuracy. (Image source: Bourns)
Representative members of the Bourns CSS4 series include the 1%, 5 watt, 0.5 milliohm CSS4J-4026R-L500F and the 1%, 4 watt, 2 mΩ CSS4J-4026K-2L00F shunt resistors. Both of these devices feature low TCR, low thermal EMF, and a physical footprint that’s smaller than 10 mm by 7 mm.
The first step in measuring current flow is to convert the electrical current into a more easily measured voltage parameter. Current shunt resistors are inexpensive components that accomplish this task. However, as shown, the value of a shunt resistor should be low to minimize its impact on the circuit, and to minimize power dissipation in the resistor itself.
Other significant parameters for current shunt resistors include TCR and thermal EMF, both of which can significantly affect the current measurement accuracy.
Finally, to maximize measurement accuracy, it’s critically important to keep the high current flowing through the current sense resistor out of the sensing path, either by using a special printed circuit layout that creates a Kelvin connection for a two-terminal current sense resistor, or by using a four-terminal current sense resistor.
As a low resistance value means that the voltage developed across the current sense resistor will be small, Part 2 of this article series will discuss the considerations for designing an AFE that amplifies the low voltage into a larger one, allowing it to more easily be measured by an ADC.
- Pini, A. (2018). Select and Apply Current Sense Amplifiers Effectively to Better Manage Power. Digi-Key Article Library.