USB DAC and ESD problems (?)

[00940]

Here is the situation:

I've built an USB DAC with a PCM2707 as interface. There is a first enclosure for the xformers, then a second enclosure for the circuit. Obviously the first is directly linked to main earth for safety reasons and the second enclosure is connected to the first by a dedicated earth link.

In the second enclosure, everything is isolated from the box except at the headphones jack, where the signal ground is (deliberately) connected to earth. The whole thing is noise-free. The laptop is plugged in the same power strip as the DAC. The laptop PS is grounded.

Now my problem is that sometimes, when I touch any of the enclosures of the DAC, I get a small electrical shock and the DAC stops playing. Simply disconnecting and reconnecting the USB plug is enough to get it back working. Static electricity being a fact of life during winter (mixing wool pullovers and synthetic clothes ), I've two questions:

- Do I risk killing the ICs inside the DAC ?
- Is there anything I could do to prevent what's happening ?

Big thanks in advance for any inputs.

[VictoriaGuy]

I think you are describing the reason why some people working on sensitive electronics repair wear a ground strap?
Anti-Static Wrist Straps Blue AS3 Black AS2 ESD Protection
A bit cumbersome for ordinary audio listening, though.

John

[00940]

A tad cumbersome indeed

Wrong forum to post ?

[VictoriaGuy]

Originally Posted by 00940

Wrong forum to post ?

If you are referring to my post, I didn't think so...
Isn't the OP's problem that he is 'zapping' his equipment with static charge built up on his body (synthetic and wool clothing rubbing together) ?
However, a post "Winter static problems!" in some other part of the forum might get more attention, I agree.
John

[wwenze]

How is the signal ground connected to mains earth? What about the transformer secondary?

[marce]

Whats the gnd scheme on the PCB, it sounds like the ESD spike is causing some sort of reset, this use to be common on some designs many years ago (1980's), before ground planes became common on boards.

[00940]

Ok, here's the current grounding scheme. I wonder if I should add a ground link from the ground star to the digital supply ground or the DAC power ground. For now, the only ground link between the DAC and the rest is close to the analog outputs of the DAC.

The DAC PCB has a continuous groundplane on top.

[marce]

Having a look at at H.Otts book, Electromagnetic Compatability Engineering, unfortenatly there is no instant simple solution. If you can get hold of the book Chap. 15 covers ESD.
I'll read through it and see if anything pops out.
An EMC engineer here at work has made one sugestion of tying each PCB's GND plane to the chassis with a 1nF 2.0KV - 2.5KV cap. The problem is going to be to try and steer the ESD spike away from the board.

[00940]

Thanks a lot for the help marce, it's much appreciated

Checking everything also made me realize that the usb connector shell was making contact with the box while I thought the shield was floating. I'm going to fix that and try the cap trick on the DAC PCB.