5-tips to Improve Your Impedance Measurement

Introduction

Impedance applications often have different requirements, but the common goals are similar; get the best possible accuracy, precision, and repeatability.

This blog post will guide you to the best measurement settings to improve your impedance measurements. It will help you to get the best data and save your valuable time when measuring with the Zurich Instruments MFIA impedance analyzer (and MFLI lock-in amplifier with MF-IA option). MFLI users without the IA option are also welcome to have a look at the last section ‘How to measure impedance without the IA option?‘ for a quick comparison.

Here is a quick overview of the 5 tips to be discussed next: 

  1. Make the best possible connection to your device under test
  2. Improve measurement accuracy by user compensation
  3. Choose the correct terminal configuration
  4. Get the most out of auto-ranging
  5. Optimize the balance between measurement speed and precision

 

1. Make the best possible connection to your device under test

Most impedance analyzers are calibrated to have the best basic accuracy at their front panel. On the MFIA impedance analyzer, for example, the 0.05% accuracy can be easily demonstrated by plugging a high precision 1 kOhm resistor (Digikey part number Y1624-1KCT-ND, calibrated as 1000.010 Ohm at DC) into the fixture on the front panel (figure 1) [1]. The red trace in figure 2 shows that the measurement accuracy from 1 mHz to 500 kHz is (1000.010-999.553)/1000.010= 0.046%, agreeing well with the reactance chart.

Figure 1: A photograph showing a high precision 1 kOhm resistor plugged in the MFITF on the MFIA front panel.

 

Now let’s consider a more realistic scenario where a third-party fixture or cables are added between the instrument front panel and the device under test (DUT). The accuracy will be reduced compared to the measurement at the front panel, unless compensated for by an additional calibration procedure (details explained in tip 2). The blue trace in figure 2 shows what happens when we add one-meter long cables between the front panel and the fixture: even though the measured impedance stays almost the same, the phase deviates by ~1.8 deg at 500 kHz (blue trace in figure 2).

Figure 2: Left: The LabOne sweeper showing the comparison of measurements of a high precision 1 kOhm resistor with the MFITF (red) and with additional 1-meter-long cables (blue). Right: reactance chart of the MFIA, with a red line indicating the basic accuracy (0.05%) of the 1 kOhm impedance within 1 mHz to 500 kHz. (Click to zoom-in to have a better view of the values shown in the graphs)

 

2. Improve measurement accuracy by user compensation

When measuring with fixtures or cables, it is important to calibrate your measurement to the measurement plane, in order to mitigate any parasitic impedance. To do so, you can use the compensation advisor included in the LabOne software. To implement the user compensation, you would need a short and a load in similar size and contact geometry as your DUT. A resistive load in an accurately known value suits well for this purpose.

If you use the short-load compensation, I would recommend to do the short step at last to allow for the short to be measured without reseating. With a good user compensation (figure 3), an equivalent series resistance (ESR) in 10 μOhm level and an equivalent series inductance (ESL) in pH level can be expected. A well-designed fixture is essential to identify such low impedances reliably and repeatably from the DUT [2]. After this step, you can then measure your DUT again by reseating it, which tells the reseating error.

Figure 3: The LabOne sweeper showing the baseline of ESR (upper, in red) and ESL (lower, in green) after a short-load user compensation. The graph is taken from [2].

 

3. Choose the correct terminal configuration

The MFIA offers two terminal configurations: 2-terminal and 4-terminal (also called as 4-probe, or Kelvin setup). Take time to consider which is the best choice for your DUT.

The general guidance is work at 2-terminal when your DUT has a high impedance, such as 100 kOhm, and at 4-terminal for DUTs below this guideline. The advantage of 4-terminal measurement is that it allows the rejection of the contact resistance, which can be significant for low impedance DUTs.

The confidence indicator in the advanced tab can give a rough idea when to switch from 4- to 2-terminal. If the DUT impedance is too low, the measured values will be greyed out and warning signs will pop up. This warning threshold can be set to a different value or disabled if needed. For the aforementioned 1 kOhm resistor, it is seen that switching to 2-terminal leads to extra errors in both impedance and phase measured (figure 4).

Figure 4: The LabOne plotter showing the same 1-meter-long BNC cable connected 1 kOhm resistor measured, switching from 4-terminal configuration into 2-terminal. The switch can be done by simply clicking the corresponding buttons in the red square. Note that the equivalent circuit representation on the right will change as well. The input control here is set in manual mode to reduce switch time.

 

4. Get the most out of auto-ranging

Since the optimal current input range can be unknown beforehand, a good approach is always to start your impedance sweep with auto-ranging. During the sweep, the demodulated current (default: Demod 1 Sample R) can be monitored together with the impedance in the sweeper module. This can give you a nice hint which is the best range to choose from.

Figure 5 shows an example of the current passing through a 100 pF capacitor, measured in different input range settings. Auto-ranging works the best as it prevents underflow and overflow at low or high frequencies. If your later experiment requires only a narrow frequency range, you can then set the manual input range accordingly.

Figure 5: Comparison of different current input range settings when measuring a 100 pF capacitor. Red: 1 μA, green 10 mA, and blue: auto. Underflow and overflow regions are labeled by text. The range control is squared in red.

 

5. Optimize the balance between measurement speed and precision

The LabOne sweeper includes the ‘impedance’ application mode, which automatically sets a narrow but tunable measurement bandwidth during the frequency sweep. This ensures a high precision at a cost of slightly longer measurement time. Readers of interest in the measurement mechanism can refer to the lock-in working principle for a more detailed explanation [3].

In some applications, say, microfluidics [4], or deep level transient spectroscopy (DLTS) [5], you may want to resolve fast-changing impedances, faster than the default bandwidth setting. To do that, you can manually set the measurement bandwidth to a high value (max 200 kHz at 4th order), while slightly losing the signal-to-noise ratio. As shown by figure 6, this allows the instrument to capture the capacitance change in a time scale of 10 μs level, without being cut off.

Figure 6: A screenshot showing the bandwidth setting in the advanced tab (in red square) and the measured transient voltage (blue), current (orange) and capacitance (green) change in the LabOne data acquisition module.

 

How to measure impedance without the IA option?

Impedance is a phase-sensitive property that can be measured by direct current-voltage method [6]. The requirements nicely suit a lock-in amplifier. The MFLI (without MF-MD option) has 1 available oscillator and demodulator in the basic configuration, which can be used in 2-terminal measurement by sensing the current amplitude and phase alone. You would need to assume the output voltage the same as the voltage drop on the DUT. As the current also passes through the instrument’s input and output impedance, such an assumption becomes problematic when the DUT impedance is small [7].

The MF-MD option extends the number of oscillators and demodulators to 4 on the MFLI. This means 4-terminal measurement is possible, by probing the voltage and current simultaneously at the same frequency. In both cases, note that converting measured values to impedance requires post-processing.

If accuracy is important to your application, the MF-IA option can be particularly useful thanks to the measurement accuracy up to 20 times higher. Figure 7 shows the measured impedance with and without instrument internal calibration (equivalent to the calculation using Ohm’s law). Here the difference is ~5 Ohm in 4-terminal, and could be even larger in 2-terminal configuration. With this option enabled, your measurement will also greatly benefit from the auto-ranging capability and built-in real-time circuit models and impedance parameters, especially when your DUT shows a reactive behavior.

Figure 7: LabOne screenshot showing the impedance amplitude change of the same 1 kOhm resistor, by toggling off and on the instrument internal calibration. Be sure to switch it back on for your next impedance measurements.

A quick comparison of these three instrument configurations is summarized in table 1.

Instrument configuration 2-terminal measurement 4-terminal measurement Internal calibration & user compensation Auto-ranging Real-time impedance
MFLI YES YES* NO NO NO
MFLI + MF-MD YES YES NO NO NO
MFLI + MF-IA YES YES YES YES YES

Table 1: A summary showing the impedance measurement capabilities of the MFLI and MFLI with different upgrade options. Note that the MFLI + MF-IA and MFIA are technically the same instruments. * Measuring in 4-terminal requires 2 MFLIs working synchronously together under MDS, or the MF-MD option installed. 

 

Conclusion

This blog post describes 5 tips that can help improve your impedance measurements. Try them out in your next experiments and see how much the result is improved.

If you have further questions or suggestions, please get in touch.

 

Reference

[1] What is the Basic Accuracy of an Impedance Analyzer?

[2] Measuring low ESL and low ESR of a DC-Link Capacitor with the MFIA Impedance Analyzer.

[3] Principles of Lock-in Detection.

[4] Zurich Instruments Microfluidics/Single-Cell Detection & Sorting Application Page.

[5] Zurich Instruments Deep Level Transient Spectroscopy (DLTS) Application Page.

[6] Impedance Analyzer Wikipedia entry.

[7] DC I-V Sweeps on the MFIA.