A new current measurement methodology: It’s all about counting the electrons going by!

One thing near and dear to us here at the Power and Energy Division is making accurate current measurements. What exactly is current? It’s basically the flow of electric charge per unit of time. In a conductor it’s the flow of electrons through it per unit of time. 

The ampere is the fundamental unit of current in coulombs per second, which equates to 6.241x10¹⁸ electrons per second. Accurate current measurement is one of the core values of virtually all of our products. Some of the precision SMU products can measure down to femtoamp (fA) levels (10^-15 amps). This is where we tend to muse that we’re getting down to the levels where we’re virtually counting the individual electrons going by.

While there are a few different ways of measuring current, by far the most common is to measure the voltage drop across a resistive shunt. With careful design this provides the most accurate means of current measurement. There are a lot of non-obvious factors that can introduce unexpected errors that many are not aware of, leading them to believe they have better accuracy than what it really is. A good discussion of what it takes to truly make accurate current measurements was covered in a previous posting “How to make more accurate current measurements”(click here to review). We go through great pains in addressing these things in our products in order to provide accurate and repeatable measurements.

Unlike the volt and the ohm, which have quantum standards for their electrical units, the ampere instead relies on the standards for the volt and ohm for measurement, as a quantum standard for the ampere that directly relates it back to charge is still lacking. However, that may change in the not too distant future. A group of scientists were awarded the Helmholtz Prize in metrology for realization of the measurement of the ampere based on fundamental constants. Basically they’ve created an electron charge pump that moves a small, fixed quantity of electrons under control by a clock. You can say they’re literally “counting the electrons as they go by”. This could become the new SI standard reference for current measurement. To me this is very fascinating to find out about. More can be learned on this from the following link to the press release “Helmholtz Prize for the “new” ampere”(click here to review).  I am curious to see how this all plays out in the long run. Maybe it will lead to yet another, and better, way to make more accurate current measurements in products we all use today in our work in electronics!

Upcoming software release unleashes the N7900 APS’s potential without any programming

Our N7900A Advanced Power System (APS) is well named, being the most advanced power product we’ve introduced to date. In many ways it is based on our N6700 series modular DC power system and N6705B DC power analyzer, incorporating their capabilities, including:

• High precision programming and measurement
• Seamless measurement ranging
• High speed measurement digitization of voltage, current, and power
• Long term data logging of voltage, current, and power
• Output ARB and List capabilities
• And quite a bit more

In addition the N7900A APS brings quite a few new and unique capabilities as well, including:

• Much greater output power
• Logic-configurable expression signal routing for advanced custom triggering and control
• Optional external dissipater unit for full two quadrant operation
• Optional black box recorder for post-test diagnostics when needed
• And quite a bit more

To take advantage of these advance capabilities does require a bit of programming, which is to generally be expected for an automated test environment, but in low volume design validation and R&D this can slow down the desired quick time-to-result. The N6705B DC Power Analyzer, in Figure 1, has a full-featured front panel menu and graphical display that lets design validation and R&D users quickly configure and run complex power-related tests on their devices. In comparison, the N6700 series, pictured in Figure 2, does not have all the front panel capabilities of the N6705B and can be looked on as the ATE version of this product platform, requiring programming to take advantage of its advanced capabilities. The N6705B shares all the same DC power modules that the N6700 series uses.

Figure 1: The N6705B DC Power Analyzer, primarily for bench use

Figure 2: The N6700 series Modular DC Power System, primarily for ATE

The N7900A APS is very similar in form and function to the N6700 series, not having all the advanced front panel capabilities that the N6705B has for bench-friendly use of its advanced features. I am really pleased to be able to share with you that this is now changing! While we are not creating a bench version of the N7900 APS, we are upgrading our 14585A Control and Analysis software, which emulates the front panel of an N6705B and more, to work with the N7900 APS as well. The 14585A will soon let you quickly and easily create and configure complex power-related tests based on using the N7900 APS.  I am fortunate enough to be working with a beta version of the software. Some examples of things I was able to do in just a few minutes were to capture the inrush current of an automotive headlight, shown in Figure 3, and superimpose an AC sine wave disturbance on top of the DC output, shown in Figure 4.

Figure 3: Auto headlamp inrush current captured with 14585A software and N7951A APS

Figure 4: Sine wave voltage disturbance on top of DC generated by 14585A software and N7951A APS

The updated release of the 14585A Control and Analysis software is just a few weeks away. More about the 14585A software can be found by clicking on the following link <14585A>. With the 14585A being a great way to implement ideas and tests quickly, using the N7900 APS, look for me and others coming up with some interesting applications in future posts here on “Watt’s Up?”!

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!