Performing DC I-V Sweeps on the MFIA

January 10, 2020 by Tim Ashworth

Characterizing the impedance of components and materials with the MFIA is done by necessity with an AC signal. However, the MFIA and MFLI can also take DC measurements. This blog post shows how easy it is to take DC measurements, specifically I-V measurements, and highlights a few points to be taken into consideration in order to get the best possible results.

For the I-V measurements in the blog post, we used a high precision 1 kΩ component (Y1624-1KCT-ND) mounted on an MFITF carrier in 4-terminal configuration. The resistor (on the carrier) was first measured by a Keysight 34420A Microohm meter to give us the DC value to a high accuracy.

To measure at DC on the MFIA, the frequency should be set to zero, as highlighted by lozenge number 1 in Figure 1. As the current may change over several current input ranges, it is important to enable the auto-ranging as shown in lozenge 2. Auto-ranging is a feature unique to the MFIA and the MFLI with MF-IA option. The actual DC offset voltage can be viewed in the advanced area of the IA tab, highlighted by lozenge 3 in Figure 1. The output range of Hcur can be selected in the signal output range menu highlighted by lozenge 4 in Figure 1. Please keep in mind that the limit for 4-terminal measurements is +/- 3 V, so use 2-terminal for offsets up to +/- 10 V. The swept parameter can be chosen in the sweeper settings highlighted by lozenge 5 in Figure 1. Finally, the measured current can be added to the vertical axis group of the sweeper by selecting "Demod 1 Sample R" from the menu highlighted by lozenge 6 in Figure 1.

Figure 1: Overview of the settings required for DC I-V sweeps on the MFIA (or the MFLI with the MF-IA option). The five key settings are numbered as follows: 1. Set oscillator frequency to zero. 2. The auto-ranging should be enabled 3. The actual static DC offset voltage can be read here. 4. The output range of Hcur can be set here. 5. The swept parameter should be set to "Output 1 Offset" from the parameter tree. 6. The current can be added to the sweeper by selecting "Demod 1 Sample R" to the vertical axis group.

Now the impedance analyzer and sweeper tabs are ready to go. We connected the component to the MFIA and ran a sweep. Figure 2 shows the resulting DC I-V sweep from -10 V to +10 V in a 2-terminal configuration (blue curve), while the orange curve represents the 4-terminal measurement (which is limited to +/-3 V due to the compliance input voltage limit of the Hpot and Lpot). Using the cursors on the sweeper, we read off the current at +10 V to be 12.85 mA.

Figure 2: Sweeper tab of LabOne showing a DC I-V sweep taken on the MFIA (or the MFLI with the MF-IA option). The blue trace shows the 2-terminal measurement from -10 V to + 10 V, and the superimposed orange trace shows the 4-terminal trace from -3 V to + 3 V.

At first glance, this is not the current we expect when we put 10 V over a 1 kΩ component. We would rather expect 10 mA. This discrepancy can be explained due to two factors; the output/input impedance and the fact that the displayed current corresponds to twice the RMS current (see this blog post for more details). The latter can be resolved by dividing by root two, and the former can be resolved by including the impedance of the output (fixed at 50 Ω) and the impedance of the input (this varies depending on the current input range, see Figure 3).

Figure 3: Schematic of the current input at LCUR, showing the transimpedance amplifier. The table below lists the gain, bandwidth and input impedance of LCUR for a given input range.

Referring to the table in Figure 3, we see that the input impedance for our I-V presented here is 50 Ω at +10 V offset. Taking into account this, and the output impedance of 50 Ω, we expect a current of 9.09 mA. To confirm the value read from the sweeper with the cursor, we can use the plotter to measure the current over a longer period and take an average. Figure 4 shows the results taken with the plotter for an offset voltage of + 10 V, which show the value of current to be 12.847 mA. If we divide by root two to take into account that this value is twice the RMS value, we get; 9.085 mA. This agrees with the expected value of 9.09 mA within a margin of error better than 0.1%. Please note that the typical accuracy of the measured current at LCUR is just 1% of the range, as the measured current does not take into account the internal calibration which allows the MFIA to measure to a basic accuracy of 0.05%.

Figure 4: LabOne Plotter tool showing the current measured for an offset DC voltage of + 10 V. This tool allows averaging and includes tools to calculate the standard deviation and min/max with real-time updates. The histogram averages the values over the full 20 s, whereas the cursor area tool averages over 8.6 s in this case. Both measurements of current agree to three decimal places: 12.847 mA.

Conclusions

In addition to impedance analysis with AC signals to high accuracy, the MFIA and the MFLI with the MF-IA option can also measure I-V sweeps at DC to a lower accuracy. The results presented in this blog post show an example where the measured current is within 0.1% of the expected value. However, the typical accuracy for DC measurements is 1% of the measured range. This may be more than enough for many applications, especially considering that these measurements can be achieved without changing the cabling and with a couple of parameter changes.