Attenuators and ESD - Page 1
Forum section: electronics / projects

author: ezalys
author_rank: Frequent Contributor
timestamp: November 10, 2021, 04:54:35 pm
content: I have a device that's extremely sensitive to ESD and I want some way to protect it. The device itself is a capacitor with some exotic semiconducting material in it and maybe 20 monolayers of boron nitride. It can be charged to maybe 2 volts without blowing up. The device itself is 70 fF and a square micron in size. Crucially, it's permanently connected to a 10 foot piece of coax, with the side opposite to the device connectorized. I only interact with the connectorized side. This means that the total capacitance I see from the connector is about 2*pi*3*epsilon0*10 feet, where the 3 is the e_rel of teflon, and this works out to be about 500 pF.What I want to know is if placing an attenuator at the connectorized side is going to save the device from ESD. I've never done an ESD calculation before, but from what I understand, I look like a high voltage, very low impedance source. I was hoping that the 50 ohm impedance of the attenuator might save the device, but I've seen countless instruments with ESD discharge warnings on the front. If something as trivial as a 3 dB attenuator could save the detector in these devices, I would assume they would have added such a thing.

author: ezalys
author_rank: Frequent Contributor
timestamp: November 10, 2021, 05:15:28 pm
content: Ok I did a simple calculation with the HBM that seemed to suggest that with a 10 kV 100 pF source thru 1.5 kOhm into 500 pF shunted by 50 ohms you get maybe 10 volts across the device max. I guess the 500 pF of the cable saves you to a degree.So that raises two additional questions... in an environment where I wear a grounding strap, should I stay with 10 kV, not worry, or pick an intermediate voltage?If I want to be very safe, what kind of ESD protection diodes won't corrupt a high speed signal? 1 GHz bw DC coupled.

author: ejeffrey
author_rank: Super Contributor
timestamp: November 10, 2021, 05:44:10 pm
content: A 3 dB attenuator won't do much more improve robustness by 3 dB.What protection is practical depends on the signals needed. In general it is very hard to protect an extremely wideband device, which I expect an exotic semiconductor with a 70 fF capacitance needs to be.Good ESD protection usually involves something to act like a clamp: TVS diodes, zener diodes, gas discharge tubes, filtering to block fast transients before the clamp kicks in, and resistors to limit the current flow when the clamp is tripped. That likely interferes with your use of the device unless you are just doing DC curve tracing.The best protection for ESD then is just to handle in an ESD safe environment: esd compatible materials and tools, redundant ESD straps, the whole bit.I also think you are going to be screwed by EMI. 3 meters of coax makes a pretty good antenna and it isn't hard to induce enough voltage to blow out your device. Something as simple as turning on or off the lights is likely to blow out your device. Theoretically the induced noise is common mode but imbalance in the connection at either end of the coax can cause differential to common mode conversion with enough energy to blow out such a fragile device. I've run into these issues with Josephson junctions which have very similar properties: sub-micron devices with negligible capacitance and break down voltages around 1 volt.

author: ejeffrey
author_rank: Super Contributor
timestamp: November 10, 2021, 05:46:32 pm
content: Quote from: ezalys on November 10, 2021, 05:15:28 pmThe cable capacitance doesn't help you as much as you hope. It's a transmission line so a discharge will be conducted as a pulse. What you actually get is a voltage divider between the 1500 ohm source and the 50 ohm cable impedance. That gives a division ratio of only 30:1.

author: ezalys
author_rank: Frequent Contributor
timestamp: November 10, 2021, 06:39:21 pm
content: So the current strategy is to use a microwave switch to short the sample to ground while switching cables around. When mounting the sample one just grounds everything and hopes for the best while using the precautions you had mentioned.I don't think EMI has been that much of an issue. We've had good luck with the setup as described. I just need shorter cables and would like to be able to remove and reconnect cables to the cryostat.You wouldn't happen to be with google would you? We may have met when I was an undergrad with John.

author: David Hess
author_rank: Super Contributor
timestamp: November 10, 2021, 08:58:13 pm
content: Attenuators tend to break down at high voltage so the attenuation is less of a factor for protection.Ejeffrey is exactly right. At high frequencies, the transmission line looks like a resistive impedance and not a lumped capacitance.