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2018

Why Perform Current-Voltage (IV) Measurements?

IV measurements obtain the current vs. voltage or resistance characteristics by providing a voltage/current stimulus and measuring current/voltage reaction. This is a basic electrical measurement and a fundamental way to understand the characteristics of various materials and devices under test (DUT).

 

Figure 1 shows the IV curves of some common electrical components. In the first graph, we see a linear relationship between voltage and current, so we immediately know that we are looking at a resistor. The graph in the center shows an exponential relationship between voltage and current. This probably means we are looking at a diode (or something that exhibits diode-like behavior). Finally, the graph on the far-right indicates that we are looking at some transistor curve. The characteristic of IV curves are able to provide us with immediate insight into a component, making them crucial in science and engineering. 

 

Resistor, diode and transistor IV curves

Figure 1. IV curves of common electrical components

 

So, how do you make quick and accurate IV measurements? The most common tool for measuring IV curves is a source measure unit (SMU).

 

What is a Source Measure Unit (SMU)?

An SMU combines the capabilities of a current source, voltage source, current meter and voltage meter into a single unit. This gives it the ability to evaluate the IV characteristics of devices across all four measurement quadrants without the need for any additional equipment. You get substantial cost and space savings compared to having multiple instruments.

 

Besides being able to output and measure voltage or current very accurately, SMUs also possess a compliance feature that allows a limit to be placed on the voltage or current output to prevent device damage.

 

As Figure 2 shows, a source measure unit packs an amazing amount of measurement capability into a very small package. SMUs can act as ideal 4-quadrant sources, meaning they will always try to maintain their programmed current or voltage until they reach the limit of their output power or a user-defined limit.

 

Circuit diagram of a simplified equivalent circuit (2-wire measurements) in an SMU

 

Figure 2. Simplified equivalent circuit (2-wire measurements) in an SMU


In addition, while sourcing current or voltage, SMUs can simultaneously measure both current and voltage. Figure 2 represents a single SMU channel, and this channel can exist as a standalone product or as one SMU channel within a mainframe that supports multiple SMU channels.

 

Notice that the “Low Force” side of the SMU can be connected to chassis ground via a relay. In fact, the default boot-up state of most SMUs has them configured with their low side grounded. However, it is sometimes convenient to float the “Low Force” side connection (for example when making differential measurements). In those cases, keep in mind that disconnecting the low side from ground is possible.

 

Why Use an SMU for IV Measurements?

Without an SMU, here are some common test challenges that you may face when performing IV measurements:

  • Difficulty in controlling and synchronizing multiple instruments
  • Complex cabling and setup
  • Difficulty in obtaining accurate, reliable measurements


When using multiple instruments simultaneously for an IV measurement, it is not easy to obtain good performance and accuracy due to the cabling and grounding with various instruments. In contrast, an SMU typically provides superior measurement performance that goes down to sub pA and sub µV resolutions.

 

SMUs integrate many capabilities into a single channel. It is possible to source voltage and measure both the sourced voltage and current (or source current and measure both the sourced current and voltage). But usually, either only voltage is sourced while current is measured, or only current is sourced while voltage is measured. Hence, the most common SMU use models are shown here (also in Figure 3):

  • VSIM – Source voltage and measure current
  • ISVM – Source current and measure voltage

 

Two of the most common modes of SMU operation

Figure 3. Two of the most common modes of operation for an SMU

 

For why would you need to measure a source if the SMU will always hold its sourced value? When you specify a sourced value, you also must specify some limit on the measure unit value. And if for some reason the measured unit hits its limit, the value you expect to source may not actually be what you set. Below is a simple example to explain this (Figure 4).

 

IV curve of a sweep voltageFigure 4. IV curve of a sweep voltage

 

Suppose we are sweeping the voltage applied to a DUT (sourced value) and measuring the current flowing through the DUT. If the IV curve of the DUT hits the set current limit before the applied voltage reaches its stop value, then the applied voltage will remain at the voltage level for the rest of the sweep. In fact, all voltage and current data points beyond the limit value will be the same. However, unless you tell the SMU to measure its applied voltage, you will not be able to see it happening. If you are only measuring current, then all you will see is that the current does not change for the last part of the sweep.

 

All SMUs have some sort of indicator to tell you that a measurement has hit the limit. However, if you do not see this indicator, then reviewing the measured data is the only other way to catch this.

 

Source Measure Units Make IV Measurements Easier

So, what are reasons that make an SMU the preferred choice for IV measurements – as opposed to using multiple discrete instruments?

 

It ultimately boils down to two things:

  • Convenience, and
  • Form factor.

 

A good example of this is shown in Figure 5, which shows measurement of current and voltage at the inputs and outputs of a four-terminal DUT. Performing these operations with discrete sources and meters is complicated and time-consuming, and requires many manual connection changes to modify a measurement setup.

 

In contrast, the same measurements can be performed using a 2-channel SMU with a very 'clean' setup. One channel can be connected to the device input, and the other channel can be connected to the device output. All the necessary connections to measure different current and voltage parameters can be performed without the need to modify any physical connections.

 

IV measurement with multiple instruments versus IV measurement with a 2 channel SMU

Figure 5. IV measurement setup comparison: using multiple instruments (left) vs. using a 2-channel SMU (right)

 

Summary

IV measurements are an important part of testing as they provide unique insights into your device under tests. Source measure units (SMUs) are recommended instruments for IV measurements as they are able to provide better measurements, easy to set-up and operate, and take up minimal bench space – which makes them a cost effective investment.

 

If you'd like to know more about how to make accurate resistance measurements – which is one of the more challenging areas of measurement science – download this Application Note: Resistance Measurements Using the B2900A Series of SMUs. You will learn how you can overcome measurement issues such as residual test lead resistance, thermal electromotive force and leakage currents in the measurement path.

Do you have issues where the voltage at your load is lower than what you’ve set at your power supply? Do you always need to guess the amount of voltage to increase (to compensate for lead losses) just to get the right amount of voltage to appear at your load? If you have these issues, remote sensing can help! Remote sensing is a life saver, especially when you are setting test stations on your manufacturing floor or performing part qualifications.

 

Download the free "4 Ways to Build Your Power Supply Skill Set" eBook.

 

Remote sensing allows you to have your desired voltage appear at your load. It works by sensing the voltage that appears at your load instead of the voltage that appears at the output terminals of your power supply. This is accomplished by connecting the load directly to your power supply’s sense terminals using two separate wires. By measuring voltage across the load, the power supply will adjust the output voltage until the voltage across the load reaches the desired voltage. No need to manually compensate for voltage drop across your load leads.

 

How Does Remote Sensing Work?

You’ve probably seen that sometimes the voltage at your load is different than the voltage at your supply. What’s causing this accuracy issue, and why does remote sensing help? To answer this, let’s use an example. In Figure 1, we have a power supply set for 5 V output. If your load is located at the output of the power supply, you’ll get almost 5 V at your load. Now, imagine that the load is 6 feet away from your power supply. You’re now transferring power to your load using a pair of 6-foot wires. If you’re using 14WAG wire for your connection, each wire will have a resistance of about 0.015 Ω.

The resistance of your copper wire doubles for every 3-gauge increase in wire size

Now, when you have 10 A flowing to your load, each wire will cause a voltage drop of 0.15 V (10 A x 0.015 Ω). You now have a total drop of 0.3 V on the wires. Instead of 5 V, you now have only 4.7 V (5 V – 0.3 V) across your load.

 

Figure 1 shows sense lead tied to output terminals

Figure 1. Sense lead tied to output terminals

 

The thinner the wire, the less voltage you have across your load. In the table below, you can see that wire resistance increases as wire size decreases. As a general rule, the resistance of your copper wire doubles for every 3-gauge increase in wire size.

 

AWG wire sizeResistance in mΩ/ft (at 20°C)
2216.1
2010.2
186.39
164.02
142.53
121.59
100.999

Table 1. Wire size vs. wire resistance

 

Let’s use the same setup, but now with remote sensing. To set up remote sensing, connect the sense terminals directly to the load. Wire size doesn’t matter for remote sensing ─ more on that shortly. When using remote sensing, the power supply will regulate the voltage across the load so that 5 V appears across the load. In this case, the power supply will increase the voltage at the output of the power supply to 5.3 V to offset the 0.3 V drop across the load wires. This will give you 5 V across your load. This is all done automatically by the power supply. No need for manual adjustments and calculations.

 

Figure 2 shows sense lead being connected directly to load.

Figure 2. Sense lead connected directly to load

 

The sense terminals on the power supply function like a voltmeter and have high input impedance. This means current flowing into the sense terminals is negligible and wire size does not significantly affect accuracy. You can use thinner wires for sense, but make sure these wires are properly shielded to reduce noise.


As you can see, remote sensing works pretty much like 4-wire resistance measurements. Instead of a small source current used in resistance measurement, we now have large current following through the leads and load. Remote sensing is especially useful if you have to connect to your load through long wires, complex relay topologies, or connectors.

 

Best Practices for Connecting Sense Leads

We just learned that remote sensing can significantly improve the accuracy of your output voltage at load. However, connecting your sense leads incorrectly can do more harm than good. To avoid this, let’s talk about best practices for connecting your sense leads to get the best results.

 

1. Use Two-Wire Twisted, Shielded Cables

Whenever possible, use two-wire twisted and shielded cables for your sense leads. A twisted pair, shielded cable protects your sense leads from noisy environments. You want to make sure the sense terminals are getting the cleanest possible measurements from your load. Noisy sense measurements will lead to fluctuations of your output voltage.

 

2. Make the Right Ground Connections to Avoid Ground Loops

If you are using a shielded cable for your sense leads, make sure to connect the shield to ground at only one point. Connecting your shield to ground at multiple points may look like a good idea because you are making more solid connections to ground, but it creates ground loops.

Ground loop current can cause noise to appear on your sense leads

How is that possible? Well, not all grounds are at the same potential to each other, especially grounds located far apart. When you connect these grounds together through your cable’s shield, current will flow between these points. This is called ground loop current. Ground loop current can cause noise to appear on your sense leads.

 

Figure 3 shows ground loop current flowing between to ground points.

Figure 3. Ground loop current flowing between to ground points

 

Figure 4 shows how in a correct ground connection, the shield is only connected to ground at a single point.

Figure 4. In a correct ground connection, the shield is only connected to ground at a single point

 

Figure 5 shows the physical connection on a typical DC power supply using a 2-core twisted and shielded cable

Figure 5. Physical connection on a typical DC power supply using a 2-core twisted and shielded cable

 

3. Keep the Sense Leads and Load Leads Separate

Do not twist or bundle your sense leads together with the load leads. Crosstalk will occur between the sense leads and load leads, causing inaccurate measurements on the sense leads.

 

4. Connect Your Sense Leads Properly

It may seem obvious, but you should have a solid connection between your sense terminals and load. An open connection at the sense terminal may cause the power supply to quickly increase output voltage because the sense terminal detects no voltage. This can be disastrous for your load!

 

Fortunately, Keysight power supplies use internal sense protect resistors. These resistors prevent the output voltage from rising too high if there’s an open connection at the sense leads.


Conclusion

Using remote sensing significantly improves your power supply’s accuracy with little investment. I encourage you to take advantage of this feature. Most modern power supplies come equipped with remote sensing. Use the best practices we discussed above to get better accuracy from your power supply.

 

I’d love to hear your questions and feedback in the comments section below!

 

Download the 10 Practical Tips You Need to Know About Your Power Products application note for more ways to improve your power supply operation and measurement capabilities.