How to Make More Accurate Current Measurements

There are a number of ways to make current measurements, including magnetically coupled probes, Hall-effect devices, and even some more exotic field sensing probes, but a good quality resistive shunt really cannot be beat in terms of accuracy, bandwidth, and overall general performance.

We likewise make considerable use of high performance shunts in our DC power products to provide extremely accurate current read-back of load currents, spanning the full range of output loading. Not only is the quality and design of the shunt itself critical, but how you treat it and make use of it are all equally important to get great current measurement performance. At the surface it may seem simple; it’s just measuring the voltage drop across a resistor. In reality it is no simple task. It requires appropriate metrological resources to validate the performance.  There are a lot of potential sources of error to recognize, quantify, and contend with.

When working with folks I sometimes encounter those who prefer to develop in their own current measurement into their test systems, instead of relying on the current read-back system already build into their system DC source. There are times when this is the right thing to do and is fine when done correctly. However some of the time there is the preconception that the DC source cannot provide an accurate measurement. The reality is there is a wide selection of DC sources available spanning a wide range of performance, Most likely something will be available that adequately addresses one’s needs. A second issue is, when developing current measurement capabilities for a test system, is truly recognizing all the potential sources of error. It goes well beyond having a good DVM and a good shunt resistor in the test system.  

A colleague here in our R&D group, Mark Peffley, wrote a comprehensive article that was just published. It covers a myriad of things in depth to be taken into consideration in order to make accurate current measurements, including:

• Temperature dependencies
• Self-heating and thermal equilibrium
• Temperature gradients
• Thermo-electric effects
• Additional sources of offset errors
• Voltage drop considerations
• Shunt selection practical considerations
• And more!

So using a shunt is a great foundation for making highly accurate current measurements. That’s why we use them in our power products. But, as Mark points out, there is a lot more to it than just Ohm’s law. When using one of our power products we factor all these things in so that they become a non-issue for the user. However, if you do plan to add current measurement into your test systems then I highly recommend reading Mark’s article “Obtain Accurate Current Measurement” (click here to access) as it is a great reference on the subject!

Paralleling power supplies for more power without compromising performance!

A year ago my colleague here, Gary, provided a posting “How can I get more power from my power supplies?” (Click here to review). He describes connecting power supplies in series for higher voltage or in parallel for higher current. Along with suggested set ups a list of requirements and precautions are also provided.

Connecting multiple power supplies in parallel operating as voltage sources is always problematic as there will be some imbalance of voltage between them. That’s why, in this previous posting, one unit operates as a voltage source and the remaining paralleled units operate in constant current. The compliance voltage limit of all the units operating in constant current need to be set higher than the master in operating in constant voltage in order to maintain this operation. This is illustrated in Figure 1.

Figure 1: Operating power supplies in parallel for higher power

As long as a high level of loading is maintained the paralleled units remain in their respective operating modes (in this case at least 2/3 loading). However, what happens if you cannot maintain that high level of loading? It is possible in practice to operate at lighter loads with this approach. In this case it is important to set the voltage levels of all the units the same. Now what happens is when the units are fully loaded they operate as already described, with the lowest voltage unit remaining in constant voltage. But when they are unloaded the lower voltage units transition to unregulated operation and the highest voltage unit then maintains the overall output in constant voltage. This is shown in Figure 2, for 0 to 1/3 loading.

Figure 2: Conditions of power supplies connected in parallel at light loading

There is a bit of performance compromises as a result. The transition between the lowest and highest voltage limits adds to the voltage regulation. Also, due to different units experiencing mode crossover transitions between constant voltage, constant current and unregulated operating modes transient voltage performance suffers considerably.

An improvement on this direct paralleling approach is having a master-slave arrangement with control signals to maintain current sharing across units. Our N5700A and N8700A series power supplies use such a control arrangement as depicted in Figure 3, taken from the N5700A user’s guide.

Figure 3: N5700A Connection for parallel operation (local sensing used)

With this arrangement the master unit, operating in constant voltage, provides an analog current programming output signal to the slave unit, operating in constant current. In this way the two units equally share the load current across a wide range of load current.

Still, having multiple units with only one in constant voltage does not provide as good of dynamic performance as a single voltage source of higher power.  A unique and innovative approach was taken with our N6900A / N7900A series Advance Power System (APS) to support seamless parallel operation without compromising performance. The paralleling arrangement for our N6900A / N7900A series APS is depicted in Figure 4.

Figure 4: N6900A / N7900A series APS Connection for parallel operation

The N6900A / N7900A series APS paralleling arrangement also uses an analog control signal for driving current sharing. However with this arrangement there is no master or slaves. All units remain in constant voltage while equally sharing current. This provides the user with an easy way to scale a power system as required without having to worry about compromising performance.

Power Supply Programming Part 2: What Type of Programming Language to Use

Happy Halloween Watt’s up fans!  Today I want to look at what programming language you are going to use to write your program.  Instead of recommending a particular language, I am going to break it down by graphical versus text based programming.

Let me start by saying that there really is no correct answer to this question.   This is a matter of personal taste.   

I am going to start with a bit about my background.  Unlike most of my colleagues, I did not specialize in analog electronics in college.  I focused more on computer engineering.  Due to this specialization, I have taken quite a few programming based courses.  I prefer sitting down and programming using a text based programming language because of my background. 

 Graphical programming languages are a very popular option (Agilent VEE is the one that I am most familiar with).     I find these programs are great for doing short programs.  If something can take up less than one page on your computer screen, then it works pretty well here.  These languages also make building user interfaces really easy since there are a lot of easy to access functions for controlling and displaying instrument data.  I personally find that they get very unwieldy if you want to send and read a lot of data with an instrument.  I also find the looping constructs to be strange.   People have told me that these graphical languages look very similar to circuit diagrams and I can see how people would prefer that kind of view to just plain text programming.

I am going to make a confession.  If I have to write a program quickly and I do not have to show it to anybody, I still will write it in HPBASIC.  I find it to be very easy to do simple instrument programming.  There is no need for drivers, once it is set up properly; sending and receiving information with an instrument is a breeze.    Large programs do not fare very well in HPBASIC though.

My preferred way to program these days is Visual Basic (using VISA-COM IO).  If you look at the power supply example programs that we provide, there is a lot of VB in there.  I feel that a text based program allows you to write much more compact code. It takes up a lot less screen space than an equivalent graphically based language.  Something like Visual Basic is also more versatile since it is not only for test and measurement but for more general applications.  The looping constructs work very nicely here and to me the flow makes more sense.  I also find typing quicker than connecting boxes.  Text based programming does have some cons though.  For one, the graphical languages are written from the bottom up to do instrument control.  They have built in functions and data manipulation that make thing easier.  The graphical languages also have some really good libraries for building User Interfaces. 

 The real correct answer to the questions is the best language to use is the one that you are most comfortable programming in.  If you think that I missed any pros or cons please feel free to share in the comments.

Protect your DUT from over-current in more ways than one

Last month, I posted about one of our new families of products: the N6900/N7900 Series 1- and 2-kW Advanced Power System (APS) DC Power Supplies (click here). I typically like to post about more general power topics rather than focus on specific Agilent products, but this product has some really interesting features from which you can benefit. After 33 years of working on power here, there aren’t too many new products that get me excited, but this is one of them! So here is a story about an application for it.

Earlier this month, I visited one of our customers that had a device under test (DUT) whose input was sensitive to too much current. That is typically not a difficult issue to protect against using Agilent power supplies with over-current protection (OCP). Set the current limit to a value that you don’t want to exceed, turn on OCP, and the power supply output will go into protect (turn off) when the current limit value is reached. Simple enough! But this customer had an additional requirement. In addition to an OCP value as just described, he also wanted to shut down the output if the current exceeded a lower current for more than a specified amount of time. So he wanted the power supply output to go into protect (turn off) if either of the following conditions occurred on his DUT (I changed this example to protect his information):

       1.  DUT input current exceeds 6 A for any amount of time, or
       2.  DUT input current exceeds 4.5 A for 80 ms

To be honest, at the time of the visit, I wasn’t sure if our new product could do this. The product is so new and so feature-rich that I am not yet familiar with all of its capabilities. But when I returned to my office, I set it up and found it was very easy to do! Here is the solution:

I used the advanced signal routing and logical trigger expressions built into our N7952A APS to setup both requirements. I could have sent SCPI commands to setup the same trigger configuration, but our free Power Assistant Software (N7906A) made this even easier. Figure 1 shows the software with the configuration.

If, after creating the configuration, I want all of the SCPI commands that correspond to it for a program, I could use the software feature “SCPI to clipboard” that creates them from the configuration. See Figure 2.

Take a look at this feature in action. Figure 3 shows a scope trace of the current waveform. As you can see, currents that are less than 4.5 A do not trip the protection. And currents above 4.5 A for less than 80 ms (and below 6 A) also do not trip the protection. But as soon as the current exceeds 4.5 A for 80 ms (and remains below 6 A), the protection tripped – the output shut off causing the current to go to zero amps.

This is just one example of how versatile the N6900/N7900 APS power supplies are. For more information about how these advanced power systems can help you in your power application, please use this link: www.agilent.com/find/aps. To explore this advanced signal routing and logical trigger expressions feature even more, take a look at a post from one of my collegues: http://gpete-neil.blogspot.com/2013/10/protecting-your-dut-during-test-with.html

Quickly Measure a High Brightness LED’s (HBLED) Forward Electrical Characteristics

It’s not hard to notice (or extremely hard not to notice!) how high brightness LEDs, or HBLEDs, are quickly becoming commonplace all around us in our daily lives. LEDs are no longer relegated to being an indicator light on a display panel. HBLEDs have drastically ratcheted up their output to become sources for illumination.  More and more autos use them for their tail and brake lights. It’s easy to see the “instant on” they have when the auto in front of you hits its brakes, not to mention the deep purity of color they have in comparison to the incandescent predecessors.  They are also turning up in the headlights, the traffic lights, even high power street and parking lot illumination lights, and in countless other places. A lot of testing, characterization, and development work has, and continues to take place, to achieve this level of performance from HBLEDs. This includes making careful measurements of electrical power being provided and the corresponding luminous efficacy outputted, in order to assess its performance.

In my title above I am using the term “quickly” for two reasons in my posting today. First, it is important when trying to capture the forward characteristics of an HBLED that it is performed in a minimum amount of time in order to minimize temperature change due to self-heating.  The temperature an HBLED is running at has in impact on its performance. Minimizing the amount of temperature change improves accuracy of test results in determining the performance of the HBLED, for a given operating temperature. My second reason for using quickly is providing a means to make these HBLED measurements with just a little time and effort.

It turned out using the N6784A four-quadrant SMU module in an N6705B DC power analyzer mainframe worked out really well on both counts of quickly. This set up is depicted in Figure 1.

Figure 1: HBLED test characterization set up

While the N6784A is an extremely fast voltage source it is even a faster current source. With current rise and fall times of just a few microseconds was a simple matter to generate sub-millisecond-long high amplitude pulses of current with fast settling edges to provide the necessary stimulus for performing the forward electrical characterization of the HBLED. This allowed testing to take place in minimum time and avoid significant heating of the HBLED die.

One of the outcomes of the testing is shown in Figure 2, displayed graphically by the 14585A software.  Here a ramped current pulse was used instead of a flat top pulse. The HBLED’s voltage and current were simultaneously digitized as the current was ramped up. This gave a way of characterizing the HBLED’s forward voltage drop for all levels of drive current, from zero to maximum.

Figure 2: HBLED forward characterization results

The N6705B DC Power Analyzer mainframe and 14585A companion software made quick work of the setup, testing, and display of results.  A ramp waveform from the library of pre-defined ARBs was selected and used to generate the current ramp. In this instance it was set to ramp up to 1.2 amps in 1 millisecond. The oscilloscope mode was used to set up the simultaneous capture of voltage and current, synchronized to the current ramp stimulus. As voltage and current were captured it is also a simple matter to display the power, being the point-by-point product of the voltage and current. The electrical power in can then be correlated with a light output measurement on the HBLED for evaluating its performance.

Not only is this setup able to measure the HBLED’s forward characteristics, as the N6784A can source negative voltage and measure down to nanoamp levels it can quickly test the HBLED’s reverse leakage characteristics as well.

Using the power supply status subsystem to improve your test throughput

Continuing on my throughput theme here, one recommendation is to take advantage of the power supply’s status subsystem. Some power supply operations take notably longer than most to complete than others. Two notable examples:

• Initializing a triggered measurement
• Initializing a triggered output transient or output list event

When developing programs you can include long, fixed wait statements to make certain these operations have completed before proceeding. However, this can easily add many tens of milliseconds or more of unnecessary waiting, increasing overall test time.  A better way is to take advantage of the DC power supply’s status subsystem features that eliminate unnecessary waiting for these operations.

Triggered measurement and output sourcing events can substantially speed up testing by providing actions tightly synchronized with other test activities. But they do have some up-front set up overhead time needed for initializing them. Instead of using a fixed programming delay following an initialization operation it is better to take advantage of the Operation Status Group register in the status subsystem, which is illustrated in Figure 1.

Figure 1: Agilent N6700 series DC power system operation status group

The “WTG meas” bit (#3) or “WTG trans bit (#4) in the condition register can be monitored with a loop in the test program to see when they turn true. At the moment the measurement or output sourcing event is initiated and ready for a trigger the test program will then proceed with its execution without incurring any unnecessary additional waiting. This saves a considerable amount of time as illustrated in Figure 2.

Figure 2: Operation-complete wait time distribution

Instead of waiting for the full worst-case each and every time, the wait is now just the actual time. When repeated over and over for all DUTs being tested, the net result is the average of the actual wait time, which in most cases is just a small fraction of the worst case time! The net result can be many tens of milliseconds test time savings, making an improvement in test throughput.

The first five hints of my compendium “10 Hints for Improving Throughput with your Power Supply” can be viewed here: (click here to access).  For those reading our “Watt’s Up?” blog here are getting the opportunity to preview one of the remaining 5 hints yet to be released!

New Agilent Advanced Power System: More on High-Power!

Last week, I announced two new families of high-power system DC power supplies from Agilent Technologies:

• N6900/N7900 Series 1- and 2-kW Advanced Power System (APS) DC Power Supplies
• N8900 Series 5-, 10-, and 15-kW Autoranging DC Power Supplies

Here is the press release on these two new families:

http://www.agilent.com/about/newsroom/presrel/2013/04sep-em13103.html

In my post last week, I concentrated on the N8900 Series of autoranging power supplies. Those are basic DC power supplies with outputs up to 15 kW (can be paralleled to 100 kW and more). Today, I am focusing on the N6900/N7900 Series of Advanced Power System DC Power Supplies. These power supplies really do live up to their “Advanced Power System” label. I’ve been working here on power products since 1980 and have supported several feature-rich product families in that time: most notable were our AC source products (6811B, 6812B, and 6813B) and more recently, our battery drain analysis source/measure units (N6705B with N6781A). The new N6900/N7900 Advanced Power System rivals those products for rich features and quite honestly, just like our marketing slogan says, they really should help you “overcome your toughest power test challenges”. Why? Read on…

First the basics:

• There are ten 1 kW models each in a 1U package
• There are fourteen 2 kW models each in a 2U package
• Rated output voltages range from 9 V to 160 V
• Rated output currents range from 12.5 A to 200 A
• Outputs can be paralleled up to 10 kW

Here is what these products look like:

Now for a few details. There are two performance levels:

• N6900 Series is designed for ATE applications where high performance is critical
• N7900 Series is designed for ATE applications where high-speed dynamic sourcing and measurement is needed

Both performance levels have advanced power features including:

   Sourcing

• Precision voltage and current programming (N6900 is 14-bit; N7900 is 16-bit)
• Programmable output resistance
• Current sinking up to 10% of rated current (up to 100% with added N7909A power dissipator)

   Measurement

• 18-bit voltage and current measurements
• Power measurements
• Amp-Hour and Watt-Hour measurements

The higher performance N7900 products add more features to the above:

    Sourcing

• Precision 16-bit voltage and current programming (N6900 is 14-bit)
• Output lists to quickly step through voltage or current levels
• Arbitrary waveform generation

    Measurement

• Low current measurement range
• Seamless ranging for dynamic current measurements
• Adjustable sample rate
• Measurement array readback
• External data logging

And even more capabilities:

• Extended current measurement range that measures 2.25 x higher than the rated current
• Sampling up to 200 kS/s
• Extensive triggering capability
• Extensive protection features such as open sense lead detect, over- and under-voltage and current, and over-temperature
• And my personal favorite: you can track power events by adding a black box recorder (N7908A). The N7908A Black Box Recorder is a user-installable option that performs continuous background logging of output voltage, current, power, and system status to its own dedicated mass storage device. Features of this option include:
• Automatic logging starts when the power supply is turned on
• Logs in a circular buffer of about 380 MB
• Select one record every 10 ms (24 hours of logging) or one record every 100 ms (10 days of logging).
• Each record saves the average, maximum, and minimum  values for voltage, current, and power in addition to power supply status bits and events
• Logged data is preserved after a power cycle. A time-stamped event is logged each time power is turned on.

The black box recorder is pretty cool, no? If you have a mission critical power application, this option is a must to keep track of any power related events that might affect your device under test.

One of my colleagues, Neil Forcier, posted about these products on his GPETE blog earlier this month. Here is a link to that post: http://gpete-neil.blogspot.com/2013/09/the-new-advanced-power-system-designed.html

For more detailed information, take a look at the datasheet: http://cp.literature.agilent.com/litweb/pdf/5991-2698EN.pdf

The datasheet contains some fantastic details about the products including 9 tests challenges that are directly addressed by these powerful products. In fact, you can read about each of the 9 test challenges here: www.agilent.com/find/TestChallenges

With all of these advanced features built into this family of products, I think you can now appreciate why we called it the Advanced Power System!