Proper oscilloscope setup yields correct ESD measurements
Automotive electronic components are designed to have, and are tested to determine, a certain level of immunity to ESD (electrostatic discharge). The tests address a variety of conditions the components will encounter including packaging, handling, vehicle assembly/service, and intended operation. To properly test any system, subsystem, or IC, you need to measure the voltage waveform used to test a DUT. Unfortunately, typical oscilloscope setups can yield incorrect results. You need to know how to properly trigger an oscilloscope. Plus, you need to get the most from an oscilloscope's vertical resolution to maximize test results.
Automotive EMC (electromagnetic compatibility) specifications typically reference ISO 10605:2008 in describing test setups, procedures, and equipment. Equipment includes an ESD simulator whose components such as the RC networks produce waveforms that represent human ESD models.ISO 10605 also provides a method for verifying ESD simulators. The method assures test repeatability over time and among multiple laboratories and/or ESD simulators from various manufacturers. Proper oscilloscope setup is essential to correctly assess ESD simulator outputs.
ISO 10605 specifies ESD test levels from 2 kV to 25 kV in both polarities. Typically, you apply test voltages in steps by increasing the voltage to an established limit. In addition, you must subject component surfaces, interfaces, and electrical terminals to direct air and contact discharges while unpowered, and while configured and operating in a predetermined mode. The component, while powered, may also be exposed to indirect discharges that produce a radiated disturbance. The component needs monitoring for deviations in operation as well as inspected for damage or degradation of performance upon test completion. Similar procedures are also performed to the full vehicle.
The verification of the ESD simulator includes characterizing the discharge pulse waveform. The second edition of ISO 10605 identifies rise time, first peak current, current at t1 and current at t2 as the parameters of interest (Figure 1). The values of t1 and t2 vary with the value of R and C in a given RC network for the purpose of verifying its time constant.
Figure 1. Measurement parameters of interest include rise time, first peak current, current at t1, and current at t2 on the ESD pulse.
Figure 2 shows an ESD simulator gun applying a contact discharge into a current shunt target that is connected to the oscilloscope’s 50 Ω DC coupled input through a double shielded cable and inline attenuators. (A detailed description of ESD simulator operation can be found here).
Figure 2: An ESD gun discharges into a current shunt target. The resulting pulse waveform is captured and measured with an oscilloscope.
Although this measurement is a standard requirement, it's often inaccurately performed because of the oscilloscope's threshold setting and vertical sensitivity. Traditional pulse measurements require an oscilloscope to find the steady-state high and low values of the pulse and then compute pulse parameters such as rise time based on these steady state levels. A problem occurs when using industry default measurement thresholds.
The red histogram in Figure 3 identifies the Top and Base of a waveform. For clock signals, the default thresholds automatically identify the 0% and 100% levels of a waveform, and timing measurements such as rise time are correctly calculated for a clock waveform.
Figure 3. Default oscilloscope measurement parameters incorporate the IEEE clock pulse definitions, which use the top and base of a waveform. These values correctly identify thresholds for clock pulses, but will incorrectly define the 0% and 100% levels of an ESD pulse.
This method for threshold placement is, however, incorrect when applied to an ESD pulse. In this case, the standard IEEE Top and Base thresholds will misidentify the 100% threshold at the semi-stable portion of decay labeled as "top" in Figure 4 and will also misidentify the 0% threshold as the prolonged decay area labeled "base" rather than using the Zero Volt and Maximum ESD pulse values required in standards IEC 61000-4-2 and ISO 10605. Because these standards require that the 100% threshold to be placed at the Maximum level of the waveform, and the 0% threshold to be placed at the Zero Volts level, default threshold placement will result in an erroneous rise time calculation of an ESD pulse.
Figure 4. Default oscilloscope measurement thresholds incorrectly identify the Top and Base of an ESD pulse using IEEE clock pulse definitions, resulting in ESD measurement errors.
Oscilloscopes will, by default, misinterpret the prolonged decay area of the pulse, highlighted with a dashed red line in Figure 5, as being the 100% steady-state Top level of the waveform (and the peak of the waveform misidentified as being overshoot). Using default thresholds, the oscilloscope will incorrectly calculate the pulse's rise time. Considering the vertical distance between the dashed red line and the solid green line of Figure 5, is it easy to envision how the rise time could be miscalculated with an error margin between 100% and 800% error, relative to EMC standard specification requirements. A critical step to prevent this measurement error from happening is to configure the automatic thresholds to instead be the Zero Volt level and the waveform Maximum level, as shown circled in green.
Figure 5. The desired ESD pulse threshold is identified with a green line, and the default threshold is illustrated with a dashed red line on this acquired ESD pulse.