Faster Impedance Measurements at Low Frequency

March 1, 2022 by Tim Ashworth

Impedance measurements at frequencies as low as 1 mHz can take a significant amount of time for a multi-point sweep. The MFIA Impedance Analyzer allows for sweeps from 1 mHz to 5 MHz, with a freely selectable number of points up to 100000. In standard mode on the MFIA, a sweep of 200 points from 5 MHz to 1 mHz typically takes 20 hours. Such valuable data is worth the wait and is critical for applications requiring low frequencies such as the characterization of supercapacitors. To enable faster sweeps while maintaining the measurement accuracy at an acceptable level, we have introduced a new feature on the MFIA (and on the MFLI with the MF-IA option): each impedance data point below a user-selectable frequency threshold can now be measured in just one period, thus significantly reducing the acquisition time of a sweep. This technique works well for devices under test that display a linear behaviour over the full sine period. 

To enable one-period averaging in LabOne®, check the box in the Impedance Analyzer module as indicated in Figure 1 (see the orange oval). One-period averaging will be active for frequencies below a threshold frequency: for the default data transfer rate of 13.3 kSa/s, the switching threshold is 13 Hz. Changing the data transfer rate will modify the one-period threshold accordingly. When the frequency falls below this threshold, a green indicator will light up next to the enable switch. 

Figure 1: Screenshot of the LabOne Impedance Analyzer Module showing the one-period averaging feature enable switch and indicator (the indicator is in the off state, grey). 

Figure 1: Screenshot of the LabOne Impedance Analyzer module showing the one-period averaging feature enable switch and indicator (the indicator is in the off state, grey).

Once enabled, the impedance measurement mode will switch to one-period averaging for frequencies below the threshold frequency. This allows you to use one-period averaging in the Sweeper, Plotter and DAQ tools. Figure 2 shows a comparison sweep of the one-period mode versus the standard measurement mode. The sample under test is an air-gap capacitor with a value of 10 pF. The sweep starts from 5 MHz and goes down to 1 mHz in 200 data points. The current input ranging is handled by the current zone ranging, a functionality explained in further detail here. Please note that the sweep shown in Figure 2 is sped up by a factor of 1000 for ease of comparison. In standard mode the sweep requires 1200 minutes, whereas with one-period averaging the same sweep is achieved in just 300 minutes.

Figure 2: Dynamic screenshots of the LabOne Sweeper Module montaged together for ease of comparison. The top sweep has one-period averaging enabled. The Bottom sweep is taken in standard mode. The transition from standard mode to one-period averaging takes place at 13 Hz. 

Figure 2: Dynamic screenshots of the LabOne Sweeper module montaged together for ease of comparison. The top sweep has one-period averaging enabled. The bottom sweep is taken in standard mode. The transition from standard mode to one-period averaging takes place at 13 Hz.

Looking at the data above the threshold frequency, acquisition time and signal noise are identical as the standard mode is used in this case. Below the threshold frequency, we see how the one-period averaging mode is much faster, with a shorter dwell time per data point. The phase signal is somewhat noisier, but is still of high quality and gives an accurate capacitance measurement. The noise on the standard mode phase signal is slightly better, but this needs to be balanced against the one-period averaging mode reducing the measurement time by a factor of four. This feature gives you the freedom to pick your trade-off between measurement time and signal-to-noise ratio when taking low-frequency impedance data.

If you're curious to learn more about this feature or other aspects of low-frequency impedance measurements, please get in touch and we'll be happy to talk about your specific application.