Teraohm Impedance measurements with the MFIA Impedance Analyzer

Introduction

This blog post describes impedance measurements of a 1 TΩ commercially-available resistor using the MFIA impedance Analyzer. This further demonstrates the performance of the MFIA to accurately measure challenging components with very high impedance.  Thanks to a clear reactance chart, MFIA users know what accuracy they can expect starting from the basic accuracy of 0.05%. As the accuracy depends on parameters such as frequency and impedance, the reactance chart of the MFIA is a key reference to ensure best possible measurements. The latest chart now covers impedances up to 1 TΩ and frequencies down to 1 mHz (see excerpt in figure 1), meaning MFIA users can measure high impedance and at low-frequency with the confidence of a well-defined accuracy.

Figure 1: Excerpt of latest MFIA reactance chart showing the high-impedance/low-frequency region. The full reactance chart can be seen here.

 

Teraohm measurements are useful in many applications such as dielectric development, high-Q capacitor characterization and bio-impedance. Teraohm resistors such as the component measured in this blog post are commonly used in devices such as charge-amplifiers, electrometers and piezo-electric drivers. It’s important to be able to accurately characterize them at the frequency relevant to the device, which is where the MFIA can help thanks to its high dynamic range.   This blog post presents measurements of the absolute impedance, phase, and current through the resistor over the full frequency range of the MFIA; 1 mHz to 5 MHz in a single seamless sweep.

 

Experimental Setup

Figure 2: Teraohm through-hole component (Ohmite MOX112523100AK) solder-mounted onto BNC connectors, and then plugged directly onto the MFIA without the MFITF fixture. This configuration is a two-terminal setup.

 

The component under test is a commercially available 1 TΩ resistor (Ohmite MOX112523100AK). The component was solder-mounted onto BNC connectors, and then connected directly to the MFIA without the use of the MF-ITF fixture (see figure 2). The set-up is a two-terminal configuration (which can be selected from the impedance analyzer tab). These settings can be accessed in the impedance analyzer tab as shown in figure 3. The test signal was set to 8 V by selecting the full output voltage range (10 V) in the advanced settings of the impedance analyzer tab. To further improve the signal-to-noise ratio, we increased oversampling to 800 in the advanced setting tab of the sweeper (Sweeper->Settings->Advanced Mode->Min Samples).

 

Figure 3: Impedance analyzer tab showing the 2 Terminal configuration, a test signal of 8 V and a manually selected input range of 1 nA.

 

Absolute Impedance and Phase Results

Using the sweeper tab, the frequency was swept from 1 mHz to 5 MHz and the absolute impedance was recorded as shown in figure 4. Figure 4(a) shows the absolute impedance (red trace) initially at 1 TΩ starting to decrease at 2 Hz to a final value of 1.27 MΩ at 5 MHz. The oversampling was set at 800 for the trace in figure 4(a). A higher definition second sweep was made between 10 mHz and 100 mHz as highlighted in figure 4(a) by a dashed blue oval. For this higher-definition sweep, the current input range was set manually to 1 nA, and oversampling was set to 2000 in the advanced sweeper settings. The resulting linear-scale trace seen in figure 4(b) shows an absolute impedance value of 997 GΩ. This is consistent with the nominal value of 1TΩ, considering the tolerance of the component is specifed as 10%. Furthermore, the horizontal dashed cursors in figure 4(b) represent the 1% deviation limit, showing that the accuracy of the MFIA is well within the specified value of 10%.

Figure 4: (a) LabOne Sweeper tab showing the absolute impedance as a red trace on a log-log scale. The abs(Z) starts at 1 TΩ at 1 mHz and drops to 1.27 MΩ at 5 MHz. The dashed blue oval highlights the area of the second sweep shown in figure 4(b) . Figure 4(b) is scaled linearly, and shows the absolute impedance between 10 mHz and 100 mHz. The horizontal dashed lines in figure 4(b) represent a 1% deviation from the center value of abs(Z).

 

In addition to the absolute impedance, the corresponding phase signal between voltage and current was also recorded simultaneously. The gold trace in figure 5 shows the phase. It shows purely resisitive behavior from 1 mHz to the onset of capacitive behavior starting at 1 Hz. The phase reaches a minimum of -90 degrees at 1 kHz, which indicates capacitive behavior.

 

Figure 5: Screengrab of the LabOne Sweeper tab showing the phase as a gold trace. Starting from an initial value of 0 degrees (resistive behavior), the phase starts to decrease at 1 HZ to approximately -90 degrees at 1 kHz (capacitive behavior). 

 

Low-current Input Range

A key strength of the MFIA is the array of eight switchable current-input ranges, ranging from 10 mA to 1 nA, as shown in table 1. The current inputs can be selected manually, or set to auto-ranging. At the low-current range of 1 nA, currents of picoamps can be measured reliably and repeatably; such high sensitivity means teraohm impedances can be measured with confidence.

Table 1: Table showing the eight current input ranges of the MFIA. The full table can be found in Table 5.10 of the MFIA user manual here.

 

The current passing the through the device under test is measured simultaneously with all other impedance parameters, and can be seen in figure 6. The blue trace shows the current starts at 5.3 pA at 1 mHz, and starts to rise at 2 Hz, reaching 3.7 uA at 5 MHz. The current change from picoamps to microamps is a six order-of-magnitude change, but the MFIA can measure currents from 0.1 picoamps to 10 milliamps; an eleven order-of-magnitude range!  The possibility to measure the current through the device allows the user to keep the current within the tolerance of the device.

Figure 6: Screengrab of the LabOne Sweeper tab showing the phase as a gold trace. Starting from an initial value of 0 degrees (pure resisitive behavior), the phase starts to decrease at 1 HZ to approximately -90 degrees at 1 kHz (capacitive behavior). 

 

Conclusion

The MFIA impedance analyser has been used to characterise a commercially available 1 TΩ resistor. The parameters measured were; absolute impedance, phase, and current through the resistor. We measured the absolute impedance to be 997 GΩ with an accuracy of better than 1 %. Measuring teraohm impedances is challenging, and the MFIA achieves this due to the eight current input ranges, including a 1 nA input range. The MFIA also leverages its internal state-of-the-art lock-in technology to get best possible measurements.