Webinar Q&A- Impedance Analysis; Measuring Fast and Low

This blog post answers the many welcome questions from the audience during the recent webinar “Impedance Analysis; Measuring Fast and Low”. This webinar covered two distinct and equally important impedance measurement challenges. The first involves measuring the low equivalent series inductance and resistance of a DC-Link capacitor; the second encompasses fast impedance measurements for transient analysis. Both measurements were carried out with the MFIA Impedance Analyzer, which offers powerful functionalities particularly suited to these applications. The webinar was part of the IEEE Tech Insiders Webinar series, and you can replay it here. Many thanks for making the webinar such a success and thanks for all your great questions.

Here’s a selection of questions posed during the webinar, which we have answered retrospectively. As always, if you have any follow up questions, please get in touch.

Enjoy browsing the questions below…

How can you export the data in LabOne from the MFIA?

Once you have taken your data in any of the measurement modules, (for example, the Sweeper, Plotter, DAQ), you can export the chart as vector graphics, PNG graphics, or as CSV text. Alternatively, you can also export the full data set in MatLab format, HDF5 or ZView format.

Figure 1. Sweeper tab of LabOne showing an inductance sweep taken on the MFIA or (MFLI with MF-IA option). The three methods of exporting data are annotated in orange; 1. Export as graphics. 2. Export as text. 3. Export as Matlab, HDF5, CSV or ZView. Click to enlarge this figure.

How accurate can the MFIA measure?

The MFIA offers an accuracy among the highest available in a commercial IA, or 0.05% basic accuracy. The working accuracy varies with frequency, impedance and on the level of fixture compensation. Take a look at the accuracy chart of the MFIA in fig. 2 below.

Figure 2. Accuracy chart of the MFIA. This chart shows the accuracy expected on the absolute impedance measurement at the front panel of the MFIA. The frequency can be seen on the horizontal axis and the impedance on the vertical. The diagonals correspond to capacitance and inductance as labelled. Click to enlarge.

You discussed measurement precision.  What about measurement accuracy?  What frequency range are these accuracies valid?

Thanks for pointing this out! Indeed, this webinar covered precision and reproducibility rather than accuracy. The accuracy of the MFIA is very high, just 0.05% basic accuracy (see accuracy chart above in fig. 2). This chart gives you an idea of the accuracy when measuring with the MFITF fixture. However, with careful fixture compensation, the MFIA can measure outside this accuracy zone, as we saw in the DC-Link capacitor demo. The key point was to give a well defined short, and then the MFIA can measure accurately outside the spec of the accuracy chart.

What is the highest frequency at which the capacitance can be measured ?

The maximum test signal frequency is 5 MHz. At this frequency, you can measure capacitance from 1 pF to 10 nF. See the accuracy chart in fig. 2 (above) for more details.

Which is the lowest possible frequency to make impedance measurements with enough accuracy?

The MFIA can measure down to 1 mHz while maintaining the accuracy defined in the reactance chart. For example, let’s look at the accuracy chart in fig. 2 (above). If we want to measure a 1 MΩ resistor at a frequency of 1 mHz, you can expect a maximum accuracy of 0.05%. For a 100 pF component at 1 mHz, you can expect a maximum accuracy of 10%.

Can I measure the impedance at low frequency, such as the impedance of a acoustic driver?

Yes. You can measure down to 1 mHz with the MFIA, and even lower if you have the patience! At this low frequency, the impedance range is at its widest, so you can measure from mΩ to TΩ. See the accuracy chart in fig. 2 (above) for more details.

Is it possible to use this instrument to measure the impedance of an underground transmission cable system?

Alas, no. The measurement of the impedance of underground cables would require some reflectometry techniques on the ns timescale. There are dedicated tools such as Time-Domain Reflectometers which would be a better choice.

Can we use this instrument for distance protection relays that work on principle of impedance?

As the MFIA is a small-signal instrument, the compliance voltage and current of 10 V and 10 mA would make it unsuitable for characterizing distance protection relays on high voltage lines.

What is the uncertainty of the measured D and C values?

The accuracy of the MFIA given in fig. 2 takes into account the systematic error of the instrument as well as random errors such as temperature drift, so we can consider this as the uncertainty of the MFIA. However, the values given in this chart are for absolute impedance and therefore do not give the accuracy for both D and C. In Fig. 3a, the accuracy of the phase measurement gives a useful proxy to the uncertainty of D measurements, and the chart in fig. 3b gives an idea of the maximum Q (reciprocal of D) which can be measured without fixture compensation.

Figure 3 (a) Phase accuracy of the MFIA. This chart shows the accuracy expected on the phase measurement at the front panel of the MFIA. (b) Maximum Q-Factor which can be measured on the MFIA without further fixture compensation. The frequency can be seen on the horizontal axis and the impedance on the vertical. Click to enlarge.

Is it possible to measure the dissipation factor of a capacitor?

Yes. Thanks to the excellent phase resolution of the MFIA of just 2 millidegrees, it is possible to measure Dissipation values of capacitors down to 10-6 (see Q value chart in fig. 3b above).

Can we measure quality factor?

Yes. Thanks to the excellent phase resolution of the MFIA of just 2 millidegrees, it is possible to measure Q values of capacitors up to 100k  (see Q value chart in fig. 3b above).

How can I extract a capacitor’s dielectric absorption from these measurements? Can the instrument compare film dielectrics such as teflon or polystyrene to C0G ceramic?

The dielectric absorption (DA, also known as dielectric relaxation, soakage) gives information on the ability of the capacitor to discharge.  It is empirically determined by charging the capacitor for 10 minutes at its specified DC voltage, discharging for a 1 minute and then measuring the remaining voltage after a further 10 minutes. The DA is given by the ratio of these voltages.

This relatively long measurement time does not give a full picture of how the capacitor performs at the shorter timescales which are typical in modern circuits. An alternative parameter to consider would be the dissipation factor (D) of the capacitor, or the equivalent series resistance (ESR). Both of which can be measured by the MFIA over a wide frequency range. The measurement of D gives a measure of the ratio of the energy lost per cycle to the energy stored in the capacitor, and it therefore related to DA. The ESR at the frequency range of the MFIA is typically dominated by internal loss with the dielectric and therefore a good proxy for DA.

These parameters can be measured by the MFIA for various dielectrics to allow comparison across dielectric types including teflon, polystyrene and ceramic.

In the demo you measure a part with plan parallel connections – do you have other jigs/fixtures to measure other component form factors such as overlapping connections or top-accessible?

The fixture we used to measure the DC-Link was a custom made fixture to accommodate this specific type of component with offset plane connecters. For other component form factors, we would recommend making a PCB surrogate (short with the same form factor as the DUT) . This additional effort is worth while, as it allows you to define the short very well. Please get in touch to discuss further.

Please could you say more about the connectors boxes, their properties and options offered?

The MFIA ships with the included MFITF fixture. The MFITF allows you to mount small DUTs on carriers and insert them into the header socket for accurate measurements.The MFITF has been designed to have very low parasitics, just 2 fF for the fixture and a further 8 fF for the carriers.  For larger DUTs which would not fit into the MFITF, these can be connected directly to the front panel of the MFIA using third-party fixtures based on the standard 22 mm BNC spacing. Whichever fixture you choose, Please get in touch to discuss your fixturing requirements.

Can we extract the model from unknown impedance vs frequency data?

The MFIA excels at the acquisition of the impedance spectra, and offers a list of two-component models to calculate impedance parameters. For more complex models, the data taken on the MFIA can easily be exported in formats which support post-processing such as MatLab, HDF5, ZView and CSV. The data can then be modelled in third-party software.

Does the LabOne measurement software create models for components?  Can LabOne generate a SPICE model for the DUT?

LabOne uses a two component model to calculate the impedance parameters. For more advance modelling, LabOne can export the measured impedance data in Matlab, HDF5 or ZView for use with third-party modelling packages.

What is the maximum impedance that can be measured?

The MFIA can measure up to 1 TΩ in absolute impedance at frequencies below 1 Hz.

Is there a way to measure the insulation resistance / leakage current of a capacitor?

Insulation resistance (IR) of a capacitor is typically measured by a DC measurement, so outside the scope of the MFIA which is an AC measurement instrument. However, the insulation resistance corresponds to the parallel resistance (Rp) of the capacitor, and so can be measured with the MFIA in AC mode. If we measure at low frequency, where the ESL can be neglected, and assuming the ESR to be close to zero (a reasonable assumption when measuring the high impedance of Rp), the MFIA can measure the Cp and Rp of a capacitor based on a two component model. Measuring the Rp in this way would provide information on the behaviour of the resistance at different timescales.

Is it possible to perform multifrequency measurements?

Yes, you can add the MF-MD option to the MFIA which enables a second impedance measurement module. This can be used to measure impedance at a second, different frequency simultaneously.

Can the MFIA obtain impedance spectra (not a single frequency point) as a function of time (multiple frequencies at the same time, dynamically)?

The MFIA can measure impedance at a single frequency, or two frequencies simultaneously with the MF-MD option, as a function of time. It would not be possible to measure a full spectrum simultaneously as a function of time. As an alternative, the sweeper module can measure impedance spectra repeatedly to allow for the time domain of the spectrum to be visualised, and measurements from each spectrum can be taken to capture the trend. Keep in mind, each spectra requires 20 ms per point.

How can the MFIA be connected to a device under test in a low temperature chamber?

The MFIA has four BNC connectors for measuring in four-terminal configuration. You can connect these directly to your low-temperature chamber to enable both four- and two-terminal measurements.  It’s important that the cables are shielded as far as possible, up to the contacts. A “short” or “load” compensation would allow for the parasitic effects of the cables to be mitigated.

Can you connect the instrument to a probe station for on-wafer transient time test?

Yes. The MFIA has four BNC connectors for measuring in four-terminal configuration. You can connect these directly to your probe station to enable both four- and two-terminal measurements.  It’s important that the cables are shielded as far as possible, up to the contacts.

What is the highest offset voltage the impedance analyzer can provide?

The MFIA can provide a voltage offset of between -10 V and +10 V when measuring in two terminal, and between -3 V and + 3 V when measuring in four terminal.

The capacitance transient measurement is quite impressive. Is it also possible to drive the bias voltage in closed-loop i.e. PID so that it can be locked to a set value?

The MFIA also offers a quad PID/PLL controller option for closed-loop experiments. This controller can be configured to compare the measured current at Lcur with a set value and drive the bias voltage to minimize the error.  This feedback loop is fast, with a maximum feedback bandwidth of 50 kHz. In addition, an auto-tuner is provided to conveniently determine PID parameters and corresponding closed-loop transfer function for a number of DUT models.

If I understand correctly, you measure the change of capacitance by fitting it?

The MFIA directly measures voltage, current and phase between the two to a very high precision. Taking these fundamental signals, automatically applying a calibration to compensate for the internal parasitics, and then applying a two component model, the impedance parameters are then displayed. The MFIA does all this on a very short timescale, which is how it can measure capacitance in just 10 microseconds at 1 MHz. The change in capacitance that you saw in the second demo of this webinar can therefore be measured without further fitting.

Can these fast measurements also be applied to resistance or inductance?

Yes, the demonstration in the webinar showed measurements of capacitance, but this can also be used for inductance resistance and any other parameters which the MFIA can model. The model pairs are as follows: Rp||Cp, Rs+Cs, Rs+Ls, G-B, D-Cs, Q-Cs, D-Ls, Q-Ls.

In addition, fundamentals such as absZ, ReZ, ImagZ, phase and current can be measured in this fast measurement mode.

What are the other parameters that can be measured accurately?

The MFIA can measure many impedance parameters accurately, with a basic accuracy of 0.05% on the absolute Z. The MFIA applies a two component model to calculate impedance parameters; the model pairs are as follows: Rp||Cp, Rs+Cs, Rs+Ls, G-B, D-Cs, Q-Cs, D-Ls, Q-Ls.  In addition, fundamentals such as absZ, ReZ, ImagZ, phase can also be measured accurately.

Is the excitation signal a Voltage or current output?

The test/excitation signal is a voltage output, ranging from 1 µV to 10 V.

Can you use the various modules simultaneously?

Yes, you can open each LabOne tool in a new tab and view them simultaneously. The there is no need to close one module before opening the next. This is especially useful when optimising the measurement parameters, as you can see the changes happening in realtime in the plotter module or the numeric tab.

You set up a relatively complex measurement. Is it possible to record this measurement settings for reproducibility?

Yes. We understand that most instruments in labs are used by multiple users. You can save a snapshot of the instrument settings and export for safe keeping. Just reload it, and the instrument will be configured in the same way as you left it.

Where can I find application notes and further information to learn more about what has been presented?

Below is a list of resources to learn more about this webinar. If these don’t answer your questions, please get in touch directly at [email protected]

This blog gives further information on the DC-Link measurements
This Application Note outlines how the MFIA can be used for Laplace DLTS
This Application Brief Summarises the benefits of the MFIA for DLTS
This blog outlines how to set up the transient measurements
This blog will help you set up gated transfer for fast transient measurements