Displaying Nyquist Plots on the MFIA Impedance Analyzer

A short guide to displaying impedance data as Nyquist plots on the LabOne Sweeper of the MFIA.

Nyquist plots are a key tool when displaying complex impedance data. They display the real part of the signal on the X-axis against the imaginary part on the Y-axis. Each point on the plot represents a different frequency.

The MFIA (and MFLI with the MF-IA option) can display impedance data in real time as a Nyquist plot. This blog-post is a step-by-step guide to displaying impedance data in the sweeper module of LabOne.

The sample-circuit used in this blog-post is shown in figure 1, mounted onto an MFITF carrier. It was measured in 4-Terminal configuration with a test signal of 300 mV. The frequency was swept logarithmically from 1 Hz to 5 MHz and 200 points were acquired.

Figure 1. A compound RC circuit including a capacitor, a parallel resistor and a series resistor.

 

Annotated steps to Nyquist Plots:

1. Open the sweeper module and select the desired start frequency and stop frequency in the control tab.

2. Within the control tab, enable the XY mode from the drop down menu circled as step 2 in figure 2. Select Real(Z) as the X signal from the selection tree. If ON-invert is selected, the Y-axis will be inverted to display the nyquist plot in the conventional way.

3. In the Vertical Axis Groups field, select the imaginary part of Z with “IA Imag(Z)” and add to the vertical axis group.

4. Start the sweep by clicking the single or run/stop button

5. Scale the Y-axis using the manual or auto button. The X-axis can be fixed to match the scale of the Y-axis by using the track button. Fixing the scale enables the user to see if the resulting plot deviates from a circle.

6. Save the data in .txt, .csv, ZView or MatLab format. Or save the Nyquist plot as vector graphics using the button circled “6” in Figure 2

 

Figure 2. Screenshot of the LabOne showing the impedance tab and the Sweeper tab. The orange ovals show the six steps to displaying impedance data as a Nyquist plot. Click on the image for full resolution.

 

The frequency dependance of this circuit means the capactitance can be considered a short at high frequency, and open at low frequency. The Nyquist plot in Figure 3 is overlaid with a curved arrow showing the direction of the frequency progression. The value of Real (Z) at high frequency gives us the value of Rs (2.2 kΩ), and the value at low frequency gives us Rp + Rs (6.8 kΩ + 2.2 kΩ = 9kΩ). For more information on interpreting Nyquist plots, see the blog-post of Mehdi Alem here.

 

Figure 3. Screenshot of the LabOne showing the impedance tab and the Sweeper tab. The orange arrow show the direction of increasing frequency of the Nyquist plot. The value of Rs and Rp can be read off the chart using the cursor functions, as highlighted in the orange oval. Click on the image for full resolution.

 

Conclusion
Realtime Nyquist plots are quick and easy to display on the MFIA thanks to the new XY mode function in LabOne’s Sweeper module.