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high and low levels. To properly calculate the average current the waveform needs to be exported and all the measured points need to be integrated to get the average value. Oscilloscopes do a good job of capturing a single burst. However, the measurements are more complex if we want to verify how many times the sensor activates in a timeframe and how often it sends out a TX burst. Oscilloscopes can easily do a good job with measurements taken over the short term, but sensors may have operational cycles of minutes or hours, which can be complex to capture and measure. Measurement innovations The Keysight N6781A source/measure unit (SMU) for battery drain analysis overcomes the limitations of traditional measurements with two innovations: seamless current ranging and long-term gap-free data logging. The SMU is a module that can be used with the Keysight N6700 low-profile modular power system or N6705 DC power analyzer. The seamless current ranging is a patented technology that enables the SMU to change the measurement range while keeping the output voltage stable without any dropout due to ranging. This feature enables you to measure the peaks with high current ranges and measure the sleep current with the 1-mA FS range, which has 100 nA of offset error. This low offset error (100-nA offset error is 10% at 1 μA or 1% at 10 μA), orders of magnitude better than a traditional DMM. The seamless current ranging is combined with two digitizers to measure voltage and current with simultaneous sampling at 200 kSa/s (5-μs time resolution). Digitized measurements can be captured over 2 seconds and displayed with full time resolution and proportionally longer time with lower resolution. However, for long-term measurements, the internal data logger in the Keysight N6705B modular DC power analyzer integrates the 200-kSa/s measurements over a user-specified integration period (20 μs to 60 seconds) without losing any samples between the integration periods. As the data logger is gap-free, all the samples fall in one integration period or in the next one -- no samples are lost. With the data logger, engineers can now measure the current and energy drain performance of a wireless sensor for up to 1000 hours of operation. Measuring the sleep current is just a matter of placing the markers and directly reading out the values provided. The measurement in Figure 4 is made with a single acquisition over a long period of time; we get the complete picture of the current drain as well as an accurate measurement of the sleep current at 599 nA. With pan and zoom capability, it’s possible to look at the current level and time spent at every power level. Details that traditional measurement tools do not see can now be identified and measured. A clear example is the trailing pulses identified by “???” in Figure 4. The software revealed this surprise: the device drain pulsed energy at ~90 μA peaks for 500 ms for an average current of 3.3 μA. When we add this current drain to the 599 nA sleep current, it moves to 730 nA, 22% higher current than we expected. This type of surprise can be one of the reasons for underestimating energy requirements and delivering a shorter battery life than anticipated. In wireless sensor power optimization, engineers get great value by understanding the details. Knowing how much energy it takes to send out a single packet of information is very important when balancing user experience against battery drain and answering questions such as “should I send information once every second, every 5 seconds or every 10 seconds?” Engineers can accurately estimate the battery drain impact of any firmware change and validate it in a reasonable time with real measurements. Joule measurements made easy Joules are useful in battery life estimation, as every activity has a defined amount of energy. We can also compare device performance using Joules/transmitted bits. But engineers rarely use Joules because they need to be calculated from voltage, current and time. With the Keysight 14585A control and analysis software, energy in Joules can be measured directly. For example, you might measure the energy consumed by transmitting a packet captured with a triggered measurement. This is one benefit of having two digitizers for voltage and current with simultaneous sampling that enable pointby point power measurements. Joules can be easily read out as a value between the markers, and designers can go a step further by defining Joules/transmitted bit. Harwin Shield Clips EETimes Europe Sept 15.qxd:Lay EZ-Shield Clips - Surface Mount EMI/RFI shield clips - Eliminates the need for post reflow operations - Allows easy shield placement and removal - Now available in 10 different sizes P I C K & P L A C E R E A D Y B Y D E S I G N For technical specifications go to: www.harwin.com/ shield-clips www.electronics-eetimes.com Electronic Engineering Times Europe October 2015 35


EETE OCT 2015
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