Inductance of an SMD Shunt Resistor measured with the MFIA

The reliable and accurate measurement of current is critical to optimising any electronic system. The most commonly used method to measure current flows within a circuit is by using a low-value resistor mounted in series, and then measuring the voltage drop across this resistor. These current sensing resistors are commonly known as “shunt resistors” as they are used to guide the current flow down a specific path in the same way a train is shunted onto the correct track.

Although shunt resistors have very low resistance, and do not significantly affect the current flow, knowing the impedance of these components gives a fuller understanding how much energy will be dissipated as the working frequency increases. This blog post presents a measurement of a 5 mΩ shunt SMD resistor, where we confirm the parallel resistance and also measure the inductance at 100 kHz.

 

The Device Under Test

For this measurement, we chose a 5 mΩ SMD current sensing resistor from Wuerth Electronics with an 0805 form factor (Order number 580060720003). We solder-mounted it on an MFITF carrier in a four terminal configuration. It is important to use a four-terminal measurement to avoid errors due to the contact resistance of the solder joints.

Once mounted, we first measured the DUT using a micro-ohm meter to confirm its DC resistance value to be 5.051 mΩ. This allows us to judge the accuracy of subsequent AC measurements.

The AC measurements were carried out using an MFIA 5 MHz impedance analyzer, but can also be done on any MFLI with an MF-IA option. The first step of the AC measurements is to run a “short” compensation to set the baseline of the measurement and to avoid any offset coming from the carrier or fixture. This is done using the compensation advisor of LabOne, and once completed we then ran a measurement of the same short to confirm a flat baseline. This can be seen in figure 1 as the blue trace.

Figure 1: LabOne Sweeper Module showing real part of impedance of the short component used to compensate any offset (blue trace) and the DUT (red trace).

After confirming the instrument is correctly compensated, a sweep was then taken of the DUT, and the red trace in figure 1 shows the real part of the impedance, Real(Z), of the shunt resistor. We see a nice flat Real(Z) at frequencies below 10 kHz, which is expected as the impedance of the shunt is dominated by its resistance. Staying at lower frequencies, we run a second sweep to measure the Real(Z) more precisely, allowing us to compare to the DC resistance value. Figure 2 shows a frequency sweep where the standard deviation of Real(Z) is measured between 1 Hz and 10 kHz. We see in figure 2 that the value of Real(Z) averaged over this frequency range is 5.047 mΩ, which matches within 0.1% the DC value of 5.051 mΩ.

 

Figure 2: LabOne Sweeper Module showing real part of impedance Real(Z) of the shunt resistor over a frequency range from 1 Hz to 10 kHz (red trace). The “cursor area tool” measures the standard deviation over the this range to be 5.047 mΩ. 

 

Impedance Measurements at Fixed Frequency

Using the Plotter Module of LabOne, we can measure the Real(Z) of the shunt at a fixed frequency. Figure 3 shows such a fixed frequency measurement at 1 kHz. The Real(Z) measurement (Purple trace) is averaged over 5 seconds to show the value of Real(Z) to be 5.058 mΩ, with a standard deviation of just 15 microΩ. This value matches the measured DC resistance of 5.051 mΩ to 0.15 %.

 

Figure 3: LabOne Plotter Module showing real part of impedance Real(Z) of the shunt resistor at a fixed frequency of 1 kHz (purple trace). The “cursor area tool” measures the standard deviation over a five second period to be 5.058 mΩ with a standard deviation of 15 microΩ. 

 

Inductance of the Shunt Resistor

Figure 1 shows us that as the frequency goes above 100 kHz, the impedance increases. This is due to the onset of the inductance of the shunt, and should be considered in case it affects the performance of the circuit at higher frequencies. To measure the inductance, we select series inductance to the vertical axis group of the LabOne Sweeper and retake the sweep. Figure 3 shows the series inductance of the shunt as an orange trace, while the blue trace is the inductance of the short component to give us an idea of the measurement baseline. The sweep in figure 4 starts at 3 kHz, as the inductance below this frequency is strongly dominated by the resistance. The upper frequency of the sweep is 5 MHz, where the inductance can be read off as 923 pH. At 100 kHz, the inductance is 1.84 nH.

 

Figure 4: LabOne Sweeper Module showing the series inductance of the shunt resistor over a frequency range from 3 kHz to 5 MHz (orange trace). The blue trace shows the series inductance of the short component used to compensate the fixture. Already at 3 kHz, the inductance is dominated by the resistance of the shunt and is noisy. 

 

Inductance Measurements at Fixed Frequency

Turning again to the LabOne Plotter Module, we measure the series inductance (Ls) of the shunt at a fixed frequency of 100 kHz. Figure 5 shows such a fixed frequency measurement at 1 kHz. The Ls measurement (Purple trace) is averaged over 5 seconds to show the value of Ls to be 1.86 nH, with a standard deviation of just 19 pH.

Using the plotter module allows us to measure more precisely at a fixed frequency, and for a user-selectable averaging window.

 

Figure 5: LabOne Plotter Module showing series inductance (Ls) of the shunt resistor at a fixed frequency of 100 kHz (red trace). The “cursor area tool” measures the average value over a five second period to be 1.86 nH with a standard deviation of 19 pH. 

 

Measuring inductance and real(Z) simultaneously using the second IA unit

Using the optional second IA unit of the MFIA (requires MF-MD option), you can measure both Real(Z) at 1 kHz and Ls at 100 kHz simultaneously. Figure 6 shows an animated gif of the LabOne Plotter module displaying Ls at 100 kHz and Real(Z) at 1 kHz.

Figure 6: LabOne Plotter Module showing series inductance (Ls) of the shunt resistor at a fixed frequency of 100 kHz (orange trace), and simultaneoulsy the Real(Z) of the shunt at 1 kHz (green trace). The “cursor area tool” measures the average value over a five second period with the corresponding standard deviation. 

 

Wrap up

Measuring the low impedance of SMD components, both precisely and reproducibly, is a key strength of the MFIA. Thanks to the compensation advisor, the parasitics of the fixture and carrier can be compensated allowing for low impedance measurements. The Sweeper module allows measurements as a function of frequency which thus characterises the impedance of the component at the working frequency or over a frequency range of interest. This blog post presented measurements taken of a 5 mΩ shunt resistor to demonstrate the accuracy and precision of the MFIA. The results for real(Z) match the expected DC resistance values taken from the same component. The series inductance of the shunt was measured at 100 kHz to be 1.86 nH.

If you would like to know more about low impedance measurements with the MFIA, please get in touch.