Setting Up Your Oscilloscope for ESD Pulse Testing

Figure 1: An ESD calibration test setup. 
The ESD gun discharges its waveform
into a properly attenuated current target. 

Electrostatic discharge (ESD) pulse tests are a type of conducted immunity testing done to confirm that a device can withstand a sudden transient electrostatic discharge. It is done by using an ESD gun to shoot a pulse of the required voltage at a device while testing that the DUT continues to operate properly. The ESD pulse shape simulates a person, carrying a static charge, touching a device. When their fingertip first touches the device, there is a leading edge with a high peak and fast decay, often visible in the real world as a spark flying from the fingertip. This is followed by a second edge due to the charge in the rest of the human body that propagates toward the fingertip with a time delay.

Oscilloscopes are most often used to “test the tester” in ESD pulse test setups, confirming that the pulse from the ESD gun is the right shape and meets the requirements of the standard to which the device is being tested. A typical calibration test setup is shown in Figure 1. The pulse from the ESD gun is fired directly into a current shunt target connected to the oscilloscope through an attenuator required to keep the signal within the limits of the oscilloscope’s 50 Ω input, which is used for this testing. Then, key parameters of the ESD pulse are measured per one of several standards, such as IEC 61000-4-2.  

ESD standards require a range of measurements. The most common are the initial edge 10% to 90% rise time, peak amplitude, pulse width, amplitude and current levels at specified times from the initial edge (e.g., T1 and T2), and time to half value. 

Following are four, important things to do to make sure you get the best ESD pulse measurements from your oscilloscope.

1. Set Measurement Thresholds Properly

Figure 2: The rise time measurement for the ESD
waveform must be based on the threshold levels of
10-90% of the 0 to maximum pulse amplitude.
Click any image to expand it.

EMC standards define the rise time to be measured between the 10% to 90% levels relative to the full pulse amplitude of 0 to the maximum value.  However, the default rise time measurement in most oscilloscopes is based on IEEE Standard 181, which measures 10% to 90% rise time levels relative to the difference between the waveform top and base amplitudes. Top and base are the mean amplitude values of the upper and lower levels of a rectangular pulse. Since the ESD pulse is not rectangular, basing measurements on top and base will result in errors.  For accurate ESD measurements, the reference level for all amplitude measurements should be the % 0-Max level setting, as shown in Figure 2.

The measurement markers on the screen show the reference values at 0 and the maximum, with measurement thresholds at 90% and 10% of the reference level. The % 0-Max setting must also be used in setting up the Width @ Level measurement.

2. Use Specialized, “Shortcut” EMC Measurements

A number of “shortcut” parameters to measure EMC pulses are available with the EMC Pulse Parameter option.  These parameters automate measurements that would normally involve manual cursor placements, such as measuring the waveform amplitude at a specified time after the leading edge of the pulse. The option also adds settings to standard measurements that enable them to perform better when used for EMC tests.

Figure 3: The setup of EMC Level After Pulse
parameter. Two instances are shown,
one for the level after a 30 ns delay
and another for level after a 60 ns delay.

For example, IEC61000-4 requires the pulse amplitudes at intervals of 30 ns and 60 ns after the leading edge.  This can easily be setup using the EMC Level After Pulse (EMClap) parameter shown in Figure 3. The vertical lines at 30 ns and 60 ns are measurement markers that show where on the waveform the parameter is reading the waveform amplitude. 

Another useful EMC Pulse Parameter is the EMC Time to Half Value (EMCt2hv). It measures the time between the first rising edge and a user-defined Mid Percent value on the falling edge.  Figure 4 shows the setup for this parameter using a Mid Percent value of 50%.

Figure 4: Measuring the time where the leading
edge falls to half its maximum value using the
EMC Time to Half Value parameter. 

The upper and lower blue horizontal parameter markers show the range of the search between 10 and 90 percent of the % 0-Max value. The middle marker shows the 50% level. Vertical parameter markers show the leading edge and the crossing at the 50% amplitude value, which is read in the parameter field P6. 

3. Use Parameter Limiters to Measure the Correct Edge

Sometimes perturbations on the waveform may confuse the parameter measurements.  For instance, the ESD pulse we have been measuring has two peaks. If the second peak had a slightly higher amplitude than the crossing threshold—causing it to be considered a new pulse by the measurement system—then the Width @ Level measurement would include both pulse widths, whereas the standards call for measuring only the leading edge. 

Figure 5: Using P Limiter to only measure
the width of the first peak.

In such a situation there is a Math on Parameters operator, P Limiter, which limits the number of pulses measured by a specified measurement parameter. 

Figure 5 shows parameter P2 set up with P Limiter to limit the P1 measurement to 1 pulse, so that only the first measurement in each acquisition is accepted.  The corrected Width @ Level result of 4.85 ns appears in P2, since the second 26 ns width has been eliminated from the measurement.

4. Use Trends to Export Consecutive Parameter Measurements

EMC standards like ISO10605, 2008 introduced consecutive parameter measurement requirements, such as 10 measurements in a row to be made and documented.  This task is easily accomplished by using a math function called Trend. Trend operates like a data logger, capturing the sequential values of a user-defined number of parameter measurements. This technique allows you to capture up to 1 million measurement values in the order they occurred and plot them as a waveform. 

Figure 6: The trend plot of ten
consecutive measurements of pulse rise time.

Figure 6 shows how to set up the Trend function for the acquisition of 10 measurements of the pulse rise time.

The ESD pulses are acquired individually using Single trigger mode.  The parameter statistics for the  P1 rise time measurement show that 10 measurements have been made and displays the calculated statistics for those 10 values. The lower trace is the Trend of the rise time, with the vertical axis showing the rise time in ps.  The horizontal axis corresponds to each measurement sequentially from 0 to 9, a total of 10 acquisitions/pulses. 

Figure 7: Use the Save Waveform feature to save
the consecutive measurement data that make up
the Trend to an Excel .csv or other file format.

Once the data for consecutive measurements is acquired by the Trend plot, it can be easily transferred to a computer simply by saving the Trend waveform to an Excel .csv file, as shown in Figure 7.

Any waveform can be saved in a variety of formats, with options to simultaneously output data from all waveforms displayed.

For more EMC testing tips, watch our on-demand webinar, "How to Get the Most Out of Your EMC/EMI Lab Oscilloscope Pt. 1".

See also:

Making EMC/ESD Pulse Measurements

Why IEEE's Pulse Definitions and ESD Pulses Don't Mix

How Does Sampling Rate Affect ESD Pulse Measurements?

Dynamic Range, Signal Integrity, and ESD Pulses