Isolating USB is cheaper and easier than it used to be:

http://www.ubasics.com/usb_isolator

Since you have power on both sides you don't need the ADuM5000, just
the ADuM4160 - and that should be about $5 total for the isolator.

And is necessary if your power supply is on a different earth ground
than the computer's power supply.  Since USB cables are short,
generally the assumption is that everyone shares the same ground, so
USB is not often isolated.

But it's tricky - I've seen people blow up USB ports (and more!)
through ground loops with even properly grounded equipment, so adding
isolation as an extra measure of protection is something worth
considering.  It only takes a few mA of constant ground loop current
over a few months to cause lots of very interesting effects.

On Mon, Mar 22, 2010 at 2:21 PM,  <thomas@tgohome.com> wrote:
> Ok, I basically have a circuit like this:
> __________
> =A0 =A0 =A0 =A0 =A0|
> =A0 =A0 =A0Vdd |------+---- 5V
> =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0/
> =A0 =A0 =A0 =A0 =A0| 4.7k \
> =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0/
> =A0 =A0 =A0 D+ |------+---- to USB D+
> =A0 =A0 =A0 =A0 =A0|
> =A0 =A0 =A0 D- |----------- to USB D-
> =A0 =A0 =A0 =A0 =A0|
> =A0 PIC =A0 =A0|
> =A018F4550 =A0| =A0 =A0 =A0 =A0 -- to USB V+
> =A0 =A0 =A0 =A0 =A0|
> =A0 =A0 Vusb |-------+
> =A0 =A0 =A0 =A0 =A0| 470n ---
> =A0 =A0 =A0 =A0 =A0| =A0cap ---
> =A0 =A0 =A0GND |-------+--- to USB GND
> __________| =A0 =A0 __|__
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 ___
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0_
>
> Now, USB V+ is not connected; it is power from the computer (or host) at 5
> volts. USB GND is connected to my chip's ground as well as the ground of
> the power supply for the chip. I wonder however, should I opto-isolate my
> D+ and D- connections (to avoid ground loops, voltage spikes and other
> nasties), and use the USB V+ and USB GND wires to power the opto-isolator?
>
> I am asking this because I've always seen RS-232 interfaces isolated, and
> my configuration is odd (normally a stand-alone unit with a stand-alone
> power supply, and an optional USB data download/control interface.) Is
> there any harm from isolating an interface which doesn't requite it?
>
> Any help appreciated. Thanks!
>
>
>
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