Compensating Cables for Accurate Impedance Measurements

Measuring impedance accurately can be a challenge, especially when the device or material under test is located some distance away from the impedance analyzer at the end of cables. The phase delay and parasitic impedance of the cables can significantly reduce the accuracy of a measurement. This blog post seeks to address this challenge by taking measurements of a well defined load (a nominal 100 pF SMD capacitor) at the end of 2 m long cables, both with and without User Compensation to show the positive effect of cable compensation. We present measurements of the capacitor across a frequency range of 1 Hz to 5 MHz taken with the MFIA impedance analyzer (or the MFLI with MF-IA option), in 4-terminal configuration. The results show that with careful compensation, you can get similar impedance accuracy at the end of cables as you can with no cables.

 

Figure 1: Photo showing the setup used in this blog post. MFIA with MFITF attached in 4-terminal configuration via 4 x 2m BNC RG58 cables. The device under test (DUT) is a 100 pF SMD mounted on an MFITF carrier and inserted into the MFITF.

 

Determining the value of of the reference sample

To be able to judge the accuracy of our measurements, we first need to determine the “true” value of this capacitor. We already know from the manufacturer that is is a nominal 100 pF with a 2% tolerance (Digikey part number 712-1309-1-ND), but we don’t know the actual value, so this needs to be measured. We mount the device under test (DUT) on a 4-terminal carrier and insert into the MFITF fixture. We then measure the DUT with two third-party impedance analyzers from Keysight, along with an MFIA (different to the one used for the subsequent measurements). The following table shows the value of the capacitance of the DUT taken from three different impedance analyzers. They have an average value of 100.443 +/- 0.0095 pF. We will take this value to be the true or expected capacitance and assume it remains constant over the full frequency range for the purposes of this cable compensation blog post.

 

Figure 2: Table showing the measured values of the 100 pF (nominal) DUT used for this blog post, taken by three different impedance analyzers

 

Reference measurement without cables

Now we have established the expected value of our DUT, let’s measure our test DUT without cables directly on the front panel BNC connectors of the MFIA.¬†This will confirm that the measured value agrees with the true value of 100.443 pF. At this point, we should consider the accuracy chart of the MFIA when measuring a 100 pF DUT; the basic accuracy region along the 100 pF path is from 1 kHz to 500 kHz, as shown in Figure 3.

 

Figure 3: Extract from the MFIA accuracy chart showing the expected accuracy for a measurement of 100 pF (highlighted with an orange arrow). The full accuracy chart can be found here.

 

We sweep the frequency using the LabOne Sweeper module and measure the capacitance and phase of the DUT, as shown ion Figure 4. We use the Math tools of LabOne to measure the average capacitance of the DUT between 1 kHz and 500 kHz (see the vertical cursors in figure 4). The result is 100.424 pF, which matches the expected value of 100.443 within the basic accuracy of 0.05%.

 

Figure 4: Screenshot of the LabOne Sweeper Module showing an impedance sweep of the 100 pF DUT from 1 Hz to 5 MHz without any connecting cables. The capacitance is shown as the purple trace in the upper chart and the phase in red in the lower chart. Click on the figure to enlarge.

 

Attach the cables and re-measure

We see in figure 4 that the MFIA measures the capacitance accurately to within the basic accuracy of 0.05%. Now let’s attach the cables, again in 4-terminal configuration, and re-measure. Figure 5 again shows the LabOne Sweeper with two plots each with two traces. The first trace in blue shows the measurement at the front panel without cables. The second, orange trace shows the measurement taken with 4 x 2m long cables (RG58) without any user compensation. The measurement matches the expected value for capacitance between the vertical cursors (1 kHz to 500 kHz), but deviates strongly above 500 kHz. To mitigate for this, we will run a user compensation routine as follows.

 

Figure 5: Screenshot of the LabOne Sweeper Module showing an impedance sweep of the 100 pF DUT from 1 Hz to 5 MHz with (orange trace) and without (blue trace) 2m long BNC cables in 4-terminal configuration. The measurement with cables is taken without user compensation. The capacitance is shown in the upper chart and the phase in the low chart. Click on the figure to enlarge.

 

User Compensation saves the day

To compensate for this deviation from the expected value of capacitance which we observe above 500 kHz, we now run a User Compensation routine in LabOne. The User Compensation tab is found in the Impedance Module as a sub-tab named “Cal” as shown in Figure 6 below.

 

Figure 6: Screenshot of the User Compensation tab within the LabOne Impedance Module. Here, you can select a suitable user compensation and adjust to the frequency range which suits your needs. Click on the figure to enlarge.

 

We choose a short-load compensation, using a low inductance short and a 1 kOhm load for our reference value. The load used is an 0805 SMD which has low capacitance (circa 18 fF) and therefore can be considered very close to a pure resistance. We run the routine over a frequency range of 10 Hz to 5 MHz in 100 points. To start the routine, we insert the short carrier and click on “Compensate”. The MFIA runs through the frequency range and when it finishes the “short” stage, pauses for the user to change from the short to the load carrier and click to continue. Once the User Compensation routine has finished, a message appears in the log window that the compensation has been transferred to the MFIA. At this point you can opt to save the compensation for recall at a later time.

Now let’s rerun the measurement of the 100 pF DUT with the User Compensation enabled. Figure 7 shows the compensated measurement as a purple trace. It matches the impedance measurement taken without cables very well over the full frequency range. Even at 5 MHz, the value of 99.817 pF matches the expected value of 100.443 pF within the instrumental accuracy band of 1%. This shows the power of a good compensation, and how easy it is to do this in LabOne.

 

Figure 7: Screenshot of the LabOne Sweeper Module showing the result after applying the User Compensation to the measurement. The purple trace shows the compensated measurement matches that taken without cables (blue trace, hidden). The average value of the capacitance from 1 kHz to 500 kHz is 100.429 pF, which agrees with the expected value of 100.443 pF within the basic accuracy of 0.05%.  Click on the figure to enlarge.

 

Conclusion

This blog post shows the importance of compensating the cables which connect your device or material to the impedance analyzer. We see that with careful compensation, the impedance accuracy achieved without cables can also be achieved at the end of 2 m long cables. And if you are measuring below 500 kHz, we see that the MFIA returns accurate impedance measurements even without compensation in 4-terminal. If you would like to know more about User Compensation, please get in touch.