As more background on this last blog entry on measuring current with the ADALM1000 I thought it might be good to answer the question, What is a Source Measurement Unit anyway?


A Source Measure Unit (SMU) is an instrument that combines a sourcing function and a measurement function on the same pin or connector. It can source voltage or current and simultaneously measure voltage and/or current. It integrates the capabilities of a power supply or function generator, a digital multimeter (DMM) or oscilloscope, a current source, and an electronic load into a single, tightly synchronized instrument. Here is a link to the Wikipedia definition.

Figure 1 Block diagram of one ADALM1000 SMU channel


Early on there were curve tracers made by companies like Tektronix. They displayed the voltage vs. current characteristic curves of electron tubes and transistors. Later Hewlett Packard expanded the concept to incorporate multiple source-measure units (SMUs) in one instrument in their semiconductor parameter analyzers. Two (or more) SMUs can be combined to make a two-port analyzer (TPA) such as a transistor parameter analyzer (also TPA) or curve tracer. SMUs are integrated as part of automatic test equipment (ATE) pin drivers, commonly used to test digital ICs in production. ATE pin parametric measurement units for digital ICs such as the AD5520 do not require very high voltage or current ranges and tended to emphasized speed and high pin counts.


Bench-top curve-tracer technology of the past was expensive. As an alternative, a bench DMM  might cost less than $100 or even $10 for a hand-held DMM, with a 3 1/2 digit display. It is important to separate the function of an SMU from a regular multi-meter, both are useful but have very different purposes. A multi-meter can measure voltage and it can also measure current, but not at the same time. It also doesn’t output a variable voltage or current. Three DMMs and two programmable power supplies could be combined as a curve tracer (VCE, IC, IB) if they are controlled together to sweep values as a I-V plot. For example the sweeping of one variable (VCE) while holding the other variable constant (IB) to produce a family of curves (of IC) can be programed by a computer via the GPIO bus. The display of the resulting data would then be displayed on the computer screen.


DMMs can be low-cost but fully integrated bench SMUs can be several thousand dollars. Much of the reason for the high price comes from the ability to deliver significant amounts of power to the device under test (DUT). In the early analog curve tracers from Tektronix, the control of maximum VCE was done by a variable power transformer (variac) and the collector drive waveform was a rectified power-line sine-wave.


The picture emerges that if the required DUT voltage and current are modest and can be supplied by off-the-shelf op-amps, then a much lower-cost SMU can be designed, and all of us can own an SMU and check diode, BJT and MOSFET device characteristics. To actually see the I vs V curves of a transistor tells a lot in one display.


The $40 ADALM1000 is such a Source Measure Unit (SMU) at its heart. But can also be viewed as a separate oscilloscope and function generator. However, because the output function ( generator ) and input function ( oscilloscope ) share a common pin when considered as separate only one function can be used at a time. This can be confusing to some users who are maybe not so familiar with a SMU.


Why is it important to have a programmable source measure unit?

 

For some kinds of testing it might not be important to have a programmable instrument. You may just want to read a value once or a small number of times. However, in many cases it  might be required to collect lots of data so that a plot or graph of the performance over time is generated. However doing this manually is time consuming and error prone.

There are also lots of different experiments that require automated data collection to get faster or more accurate measurements, or to take measurements over a long time-scale (months or even years). Here, you will certainly need a computer to collect the data and export it to a database for analysis.

Why is it important to have negative voltages?

Not all experiments will need negative voltages and in some cases you can avoid this, however many different types of devices work differently if a positive or a negative voltage is applied. To fully understand how such devices work, we need to be able to change the sign of the voltage applied. Each SMU channel in the ADALM1000 can only produce voltage from 0 to +5 V with respect to ground. Fixed +2.5 and +5V outputs that can both source and sink current are provided. The DUT could be connected between the +2.5 V output and the SMU output rather than to ground to sweep the voltage across the DUT from -2.5 V to +2.5 V. In addition, because the ADALM1000 has two SMUs the DUT could be connected between the two SMU outputs. By sweeping one channel from 0 to 5 while sweeping the other from 5 to 0 the voltage across the DUT from -5 V to +5 V.

As an example, consider a diode - a device that only allows electricity to pass through it in only one direction. In order to evaluate if a diode is working we need to see if it will pass current in both directions. We can do this in one of two ways. We can measure the diode in one direction, and then manually turn it around and then measure it the other direction and then combine the data sets together. However, we could just measure current flow when we apply both positive and negative voltages. In fact, this technique is so useful it is used to characterize many types of devices that have diode-like behavior, solar cells and light emitting diodes are both good example cases. Figure 2 shows how to connect a diode to the ADALM1000 to sweep the voltage from -5 V to 5 V.

Figure 2, Sweep diode from -5 V to +5 V

With CH A programed to sweep from 0 to 5 V while CH B is programed to sweep from +5 to 0 V the difference between the channels appears across the resistor, used to limit the current, and the diode. The time domain waveforms are shown in figure 3. Green trace is CH A voltage, orange trace is CH B voltage and yellow trace is CH B current ( the CH A current is not shown but would simply be the inverse of the CH B current ).

Figure 3 Voltage and current waveforms vs time

We can plot these measurements vs each other and perform some simple math at the same time. What we want to plot is the current through the diode vs the voltage across the diode. To calculate the voltage across the diode we can subtract the voltage drop across the resistor ( V = I R ) from the difference between the CH A and CH B voltages. The following Python equation ( used in ALICE ) does that:

VBuffB[t] - VBuffA[t] - IBuffB[t] * 100

Where the 100 is the value of the resistor. The plot of the diode current vs that equation is shown in figure 4.

Figure 4 Plot of diode current vs voltage from -5 V to +5 V

What are the uses of a source measure unit?

Many everyday objects will have been tested with an SMU as part of the factory test and quality control process. If you use LEDs to light your home or have solar panels on your roof, these will have been tested with an SMU as part of the manufacturing process.

The ADALM1000 is designed for use by engineering students who are studying the next generation of electronic devices. Understanding how a vast number of materials and devices conduct electricity, ranging from carbon nanotubes and quantum well heterostructures to biomembranes and biosensors requires an SMU. In short, you can use the ADALM1000 to understand the electrical characteristics of any component at DC or low frequencies over a voltage range from - 5 to + 5 V, measuring current from +/- 0.1 to 180 mA.

Can you give me a specific example of a measurement that needs a source-measure unit?

Take the example of a solar cell. In research labs engineers are looking at ways to make more efficient, lower cost solar cells. In order to understand how well a solar cell is working a small scale test device is produced, perhaps a few square mm to a few square cm in size and then its performance is characterized. These test cells are too small to generate any usable power beyond lighting say an single LED, but they are large enough to characterize the basic operating range and efficiency. This example Lab uses the ADALM1000 to measure a small solar-cell.

The key characteristic of a solar cell is how efficiently it converts sunlight energy into electrical power. This can be done by illuminating the test cell with a known intensity of light and measuring the electrical power produced per unit area. Since power is simply voltage multiplied by current, the starting point is to measure the terminal voltage (V) and current produced (I).

The voltage generated can be measured by connecting a voltmeter across the cell terminals while it is illuminated. Similarly, a current can be measured using ammeter across the cell terminals. If we divide the measured current by the area of the solar-cell we get the current density. However, there is a problem; If you multiply the voltage by the current (or current density) then this only tells us how much power (or power per unit area) we can generate if we had a ideal device. The reason is that a voltmeter has a nearly infinite internal resistance, and when we measure the voltage by itself no current will be flowing. In this case there is zero power being generated (measured voltage x zero current = zero). This measurement is called the open circuit voltage. Similarly, when we place the ammeter across the terminals to measure the current, we are testing the cell when it has been short-circuited, because an ammeter should have near zero internal resistance. In this case there is current flowing but no applied voltage. Again there is no power generated (measured current x zero voltage = zero). This measurement is called the short circuit current.

For any practical (real) solar cell, the voltage that it outputs will depend upon how much current is being produced and this is why an SMU is used so that the voltage can be varied while measuring the change in current.

The graph in figure 5, shows a typical IV curve for a particular small solar cell (in this case a 3 cm X 3 cm solar cell from a solar garden light). The current is negative because the current is going into ( sunk by ) the SMU channel. The current at 0 V is the short circuit current and the voltage at 0 current is the open circuit voltage.

Figure 5, Solar cell I vs V plot

The IV curve tells us how the voltage and current change and allows us to calculate the actual amount of power that a solar cell generates. Figure 6 plots the power in mW vs the voltage across the cell. The power is simply V times I. The following Python equation calculates the power in mW:

VBuffA[t]*IBuffA[t]*1000


Figure 6, Solar cell power vs voltage


The peak of the graph is the point at which maximum power is generated (the so-called maximum power point). The power is negative because the SMU is absorbing the power produced by the cell.

If we used the technique from figure 2 we could also measure the solar cell when a negative voltage is applied ( reverse bias ). This gives us some useful information, firstly it tells us that the device doesn’t break down under reverse bias. This is a sign that the device is of good quality. Secondly, it tells us whether there is any extra current available. By applying a negative voltage we can effectively "suck" charges out of the device that wouldn’t otherwise be extracted. While these "sucked" charges can’t be used to generate power ( we’re actually putting power into the device at this point rather than extracting it ), it allows us to understand some of the photo current loss mechanisms. Thus measuring IV curves is one of the most important tools used in solar-cell development and optimization. Similarly taking IV curves is extremely  important to understanding a wide range of other device types including LEDs and OLEDs, transistors, sensors and many more.

As always I welcome comments and suggestions from the user community out there.
Doug

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