Measuring ESD on an osilloscope

[Zapp Brannigan]

Hey guys first time poster here!

I'm working on some old TTL electronics (R65C22) that are potentially being damaged from ESD. These ICs are inside of a tester I'm working with and have a direct connection to an external cable which a user will plug into the tester to interface with it. No surprise that occasionally these ICs will fail when a user connects the cable and will have to be replaced.

Anyway long story short, I'm trying to actually measure the ESD at the pins of the IC that are exposed to the external interface connector. I am consistently seeing a large voltage spike when touching the cable to the tester. This is the ESD event measured at the pin of the IC (I'm intentionally zapping the tester):


The problem is that the scope will measure this voltage regardless of how it's connected to the circuit. Normally I place my scope leads across the pin of the IC and circuit ground, but I can get this voltage spike even with the scope leads shorted together the the pin of the IC. This leads me to believe that the ESD is grounding into the scope which makes my measurement at the IC pin useless.

Does anyone have experience with this sort of thing??? Thanks!

[JohnPi]

If it is true ESD, it will have a vey fast rise time  ~ 5-10 ns and a) your scope may not have enough bandwidth to see it; b) your scope probe may not have enough BW to pass it through; c) you might damage the scope; d) there is a large peak current during ESD -- could be 10 A -- and this generates a magnetically-coupled pulse which the scope may pick up.

You need good shielding on the probe (and a very short ground lead; likely spiraled around the shaft of the probe to minimize the magnetic coupling) -- but  I wouldn't recommend this as if this is ESD, you are at risk of damaging the scope.

If you are trying to prove it's ESD, you can get a high voltage e-field tester to confirm what V is on the cable before connecting it. If you are trying to make the tester more robust, put some diodes to GND/VDD on the leads and then a small R (~ 100 Ω) between that point and the actual IC's input pins. It is well known that the friction from pulling a cable can cause a build-up of charge on it.

Modern testers and ICs are all ESD protected, but not necessarily against the large-ish ESD that could come from a cable (general spec is 1 kV, 100 pF)

[Andreas]

Hello,

when you look at what is needed to calibrate a ESD-Gun (to measure the current of up to 30A half way right) then you get a impression how far you are away from a proper setup when you want to measure something with a probe.
A probe is also always changeing the waveform due to capacities.

https://www.esdguns.com/calibration-setup-accessories/46-faraday-cage-for-esd-test-target-esd-gun-calibration-setup.html

Btw: the rise time of "fast" ESD pulses is below 1 ns so you need a bandwidth in excess of 350 MHz.

with best regards

Andreas