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William Murray

Design Margins in Analog & Digital

William Murray
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BillWM
BillWM
11/30/2013 8:59:17 AM
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Re: Re : Design Margins in Analog & Digital
@Yanland -- Antennas often use shorted quarter-wave stubs to shunt ESD to ground while allowing RF to pass --  look them up on GOOGLE -

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yalanand
yalanand
11/30/2013 8:37:38 AM
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Re : Design Margins in Analog & Digital
@DaeJ, I think you can not apply 15 Kv to GPS module because this may destroy the  gate oxide of CMOS transistors inside the GPS module ,if it is damaged once the IC will be useless. So we have to be careful about applying voltages.

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Hughston
Hughston
5/2/2013 11:36:27 AM
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Re: ESD?
I do not do radio front ends, but the radio products I worked on used the inductor of the matching network as front end protection. Two other possibilities are some of these very low capacitance protection devices and an air gap discharge protection. Either of the second two choices might give you capacitance problems. When I was thinking of antennas, I was thinking of the insulated type or internal antennas. Both are isolated, but they can receive an induced transient.

The standard test procedures call for multiple zaps because you often have a lot of entry points for ESD. You want to find them all. And you want to get enough zaps to get that cumulative damage effect. A good design does not let the ESD gun discharge at any voltage. With good isolation you are getting a free pass when you can't discharge.

Your discussion of the current path brings up another test method. Remove the housing if you can and turn off the lights. You should see the ESD going to the same place every time when you hit in the same spot. The PCB might even get burned black. That should get you thinking about burying traces on the board for isolation and giving physical separation between discharge points and your susceptible parts. You might even solder a wire with a loop on the end to that spot, and zap away repeatedly.

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eafpres1
eafpres1
5/2/2013 11:01:37 AM
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Re: ESD?
Hi Hughston--Thanks for the comments regarding my earlier question on cumulative effects.  Your thoughts on cumulative damage make a lot of sense.  Do you think that is the reason test protocols ask for multiple zaps, or is it becuase the exact position and current path varies from zap to zap so it is a crude statistical attempt to "fully test"?

I wanted to go back to the antenna topic.  Antennas are intentional radiators, and for things like FM can just be a piece of wire.  Therefore, there is a direct path of ESD to the front end of the radio.  The system designer should take care of this; it may not be possible to do so in the antenna design.

Once with a large consumer electronics company we designed an antenna that had a hard plastic molded "sheath" covering a rolled-up flexible printed circuit inside.  The housing was ultrasonically welded together.  During ESD testing they found that a zap right on the weld line would impact the system and demanded we "fix" it.  We cried foul--they had not put any isolation (like diodes or, more commonly for antennas, isolation caps) in the design, and they didn't want to do a board spin.  We spent a couple weeks tweaking the weld setup until it just passed.  I think later they fixed the board design.

Your point about creating enough of an external field to couple in is just as valid.  The solution isn't in the antnena, it is in the front end design.

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Hughston
Hughston
5/2/2013 10:29:42 AM
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Re: ESD?
I don't think you will have additive discharge events with ESD because you are charged and then you discharge. But you can only partially discharge at higher voltage, then discharge the rest as you come closer.

You do get cumulative damage to parts with more ESD zaps. One of the hardest aspects of ESD testing, is damage assesment. Your product may just start to reset more often. That reminds me of another important aspect of ESD design, Smart layout of your reset to prevent reset during zaps. However, some companies don't consider product reset to be an ESD failure.

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Hughston
Hughston
5/2/2013 10:18:56 AM
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Newbie
Re: ESD?
Here is something to try with an antenna. Zap a grounded metal plate in the horizontal and vertical planes positioned nearby. The idea is to induce a transient into your product. It also makes a difference if your product is plugged in. If a product is plugged in, then that gives a higher current path for any currents. Even if you have a power source with high insulation, ESD will eventually blow right through it and you will start to notice. Unconnected battery powered products pass ESD relatively easily.

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Brad Albing
Brad Albing
4/29/2013 10:26:21 PM
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Re: ESD?
Probably a business decision regarding testing more severely. May not psss the test, but that points to the need for an improved design. Or you can lower your standards, pass the test, ship the product - and deal with the consequences later.

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Brad Albing
Brad Albing
4/29/2013 2:54:12 PM
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Re: ESD?
Good info. Especially about testing in higher humidity. I guess if you knew your equipment might fail, but you wanted to pass your official agency tests, you could test with higher humidity. If you want to know that your customer won't blow something up and get all cranky on you when they return the equipment, then you should test during low humidity.

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Hughston
Hughston
4/29/2013 11:24:33 AM
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Newbie
Re: ESD?
Normally with ESD you test from the lower voltage ranges to the higher ranges. An antenna should have a high insulation resistance and it should not break down at any voltage, but you can couple into the receiver and cause damage. The antenna protection used to be done with passive componets but protection diodes are very low capacitance now, so maybe that's a better way to go these days.

Lower test voltages have faster rise times and can cause problems not see at higher voltages. That's why you test over the range of ESD voltages. You have a contact and non contact range of voltages. The type of artificial finger affects the test and normally I used the blunt type. Humidity affects the test results and the results are worse with low humidity. Testing in a humid environment is like cheating. Results vary widely with the type of test gun, so use a popular type and pay attention to the return strap of the test gun. You want low inductance on the return ground to get a faster rise time on the pulse.

The best ways to protect against ESD are to 1) have high insulation resistance or large air gaps, 2) design well for EMI, 3) have a good series or shunt protection, 4) use parts with some transient immunity (meaning higher voltage breakdown or internal protection), 5) bury traces near edges where ESD can strike, 6)have a good path to ground for the transient.

What is a good path to ground? The first choice is chassis ground or the input power source capacitor. A second choice might just be away from your sensitive IC. Another possibility is an ESD protection trace around the circuit card, but the ESD current can still jump across the card or induce current elsewhere. ESD likes pointy things and wants to jump there. It's like a lighting rod effect.

Do a lot more testing once you think you have a solution. ESD testing test results can vary from day to day because of the humidity and it is hard to tell when you have a solution.

 

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DaeJ
DaeJ
4/24/2013 8:29:54 PM
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Master
Re: ESD?
 I got GPS antenna module. There are some parameters for each frequency band. There is no ESD information. Should 15KV be applied to GSP module?

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