Accurate low-current measurements with a scope

Last week was incredibly busy so I wasn’t able to put the time in to complete the third part of the “Leakage currents & current leaks” post. This will be a short post with a link to a white paper on our website for more details.

Most engineers consider the oscilloscope their first tool of choice for hardware development work. Yet very few engineers ever consider how accurate their scope is. Most of the major oscilloscope manufacturers place great importance on the timing aspects of their products. Multi-gigahertz sample rates are fairly common today in mid-range digital scopes today yet most of those scopes only have 8-bit A/D converters. While timing accuracy is often spec’d in double-digit ppm, voltage measurement error on the same scope can be as much as the signal level you need to measure using a scope’s lowest volts/division setting.

Below are screen shots taken on the same mid-range MSO scope from one of the top scope companies of similar current waveforms using a “standard” scope probe and our CMicrotek µCP100 low current probe. Using a 10mV/division setting on the scope with the standard probe produced a waveform that was way too fuzzy to be useful. The measurements taken with the cursors were almost 2X too high for the peak and plateau portions of the waveform and well over 5X too high for the portions before the peak and after the plateau.

probe_SR_FW_task_shot“Standard” Scope Probe

uCP_FW_task_shotµCP100 Current Probe

For more information on these waveforms and an example of the calculations used to determine the measurement accuracy of a digital scope check out our “Accurate Current Measurements with Oscilloscopes” whitepaper.

My plan is to wrap up the “Leakage currents & current leaks” posts next week. If you find the Low Power Design blog interesting, please help spread the word about it so we can build up a large enough audience to make it worth the time it takes. We are on Facebook, Twitter, Google+, the Element14  Community and have a LinkedIn group.