Choosing the Best Current Input Range for Impedance Measurements

October 20, 2021 by Meng Li

Auto-ranging is one of the key features of all impedance analyzers. It makes it possible to run a sweep (of frequency, AC amplitude, DC bias, or other parameters of interest) continuously and without manual adjustments of the input ranges. This helps particularly when working with reactive components, whose impedance changes by several decades during a frequency sweep. It is important to choose the correct ranging conditions to decide when to trigger the range switching events. In most cases, auto-ranging delivers the best user experience. In certain scenarios, you may want to have a bit more control over when the ranges change during a sweep. The current zone ranging feature in the 21.08 LabOne release enables precisely this functionality.

Case Study 1: Increase Reverse Sweep Precision at Low Frequencies

Sometimes, to get a quick overview of the first few data points, it may be preferable to sweep the frequency reversely. This is particularly useful when one wants to know if a device or its contact is good or not, as many electrochemical devices reveal their equivalent series resistance at kHz to MHz range.

An example can be see in Figure 1, where we show the reverse sweep traces in auto-ranging (red trace) and in current zone ranging (blue trace). At relatively high frequencies, the two almost overlap with each other. But due to a high noise profile at low frequencies, auto-ranging might not make the correct range choice. The measured data are noisy below 0.1 Hz. With current zone ranging, we can guide the LabOne software to choose the suitable ranges. We can even use the 1 nA and 10 nA ranges that are not included in the auto-ranging scheme. In this way we manage to improve the measurement precision, achieving the same smoothness as in the forward sweep. We demonstrate a smooth result on a 10 pF air gap capacitor until 1 mHz, amounting to 14.79 TOhm in impedance.

To make the best use of the feature, a conditioning forward sweep including 'Demod 1 Sample R' is necessary to measure the frequency response of the current and to predict the switching frequencies. This takes the same time as the reverse sweep, but it only needs to be done once for a set of similar devices. We can then not only avoid underflow and overflow conditions, but also reduce the amount of range switching needed. Alternatively, if external noise is weak and the physics of the device under test is well-known, a simple calculation using Ohm's law will also suffice.

10pF-current-zone-ranging

Figure 1: LabOne screenshot showing the reverse sweeps of a nominal 10 pF air gap capacitor using auto-ranging (red) and current zone ranging (blue). From top to bottom, the 4 traces are impedance amplitude, impedance phase, capacitance, and current amplitude, respectively. The settings of the current zone ranging feature are located in the Impedance Analyzer tab.

Case Study 2: Reduce Phase Steps at High Frequencies

Transimpedance amplifiers often use different compensation capacitors at different current input ranges. This difference is more likely to appear as a step in the measured phase, particularly at high frequencies.

Figure 2 summarizes the results on the MFIA. The Sweeper data includes auto-ranging (orange trace), manual ranging at 1 uA (red trace), manual ranging at 100 uA (blue trace) and current zone ranging (green trace), where we switch from 1 uA to 100 uA at 5 kHz (arbitrarily chosen). With 300 mV of excitation voltage, it is no surprise that the returning current (~300 nA) would call for the 1 uA range. Indeed, the sweep starts here but then it switches to 10 uA at 1 kHz, and finally approaches 100 uA at 50 kHz. Switching is needed as the 1 uA range has a limited bandwidth that rolls off the phase by too much at high frequencies (red trace). The 100 uA range, on the other hand, suffers from a higher intrinsic 1/f noise. Therefore the measurement around 100 Hz is very noisy (blue trace).

The steps due to range switching are in fact very small, and normally only visible after zooming in. In terms of amplitude, this is just 350 Ohm/1 MOhm = 0.035%, well within the accuracy defined in the reactance chart. The phase is ~0.3 deg at 50 kHz. The phase step at 1 kHz is smaller as the compensation capacitor in parallel is less conducting there. This suggests that we may use the current zone ranging to switch to the 100 uA range earlier, thus reducing the phase step.

By combining the two ranges (1 uA and 100 uA) thanks to current zone ranging (green trace), we show that the measurement not only guarantees a high precision but is also almost 'stepless', i.e., only 30 mdeg at the 5 kHz switching frequency.

1MOhm-current-zone-ranging

Figure 2: LabOne screenshot showing sequential sweeps on a 1 MOhm resistor. The upper plot shows the impedance amplitude and the lower plot shows the phase. The label of each colored trace can be found in the History tab. The settings of the current zone ranging feature are located in the Impedance Analyzer tab.

Conclusion

In this blog post, we presented two cases where current zone ranging helps achieve higher precision in different frequency ranges. With the proper setting, current zone ranging allows you to define the most suitable current input ranges; the entire measurement process can be automated, and the measurement accuracy will not be affected.

If you are interested in this feature, please get in touch to set up an instrument demo.