5-22 Ratio and Delta
Superconductor Application #2 - fixed magnetic field
Another typical test on a superconductor sample (DUT) is to source an increasing-amplitude
current (I) through the DUT, while maintaining the magnetic field (H) at a fixed level. The I-V
curve in Figure 5-9 shows that the measured voltage across the DUT remains at ~0V for low
currents (I). This is the flat portion of the curve where the DUT remains at 0Ω. At some point,
the voltage drop across the DUT will start increasing as current through the DUT increases. The
actual resistance of the DUT can be calculated at any current source point using Ohm’s law:
R = V/I
where: R is actual resistance of the DUT.
V is the measured voltage across the DUT.
I is the known current that flows through the DUT.
Figure 5-9
I-V Curve (Fixed H)
Such a test system is shown in Figure 5-10. A Keithley SourceMeter (Model 2400, 2410, or
2420) is used to source current through the DUT and two Model 2182s are used to provide
simultaneous voltage measurements.
Model 2182 #1 measures the voltage across a precision reference resistor (R
REF
) and stores
the readings in its buffer. These stored readings allow you to reference current amplitudes to the
voltage measurements of the DUT. Model 2182 #2 measures voltage across the DUT and stores
the readings in its buffer.
For example, assume you want to measure DUT voltage at current sweep values of 10µA,
20µA, and 50µA. When the sweep is started (10µA output), Model 2182 #1 measures 1mV
(10µA × 100Ω = 1mV) and stores the reading in its buffer at location 1. At the same time,
Model 2182 #2 measures the DUT and stores that reading in its buffer at location 1. At the next
sweep point (20µA), Model 2182 #1 measures 2mV and stores the reading in its buffer at
location 2, and Model 2182 #2 measures the DUT and stores the reading in its buffer at location
2. At the last sweep point (50µA), Model 2182 #1 measures 5mV and stores the reading in its
buffer at location 3, and Model 2182 #2 measures the DUT and stores the reading in its buffer at
location 3.
Measure
V
Fixed H
Source I
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