[time-nuts] Slightly OT: inexpensive USB analog-digital converter?

John Ackermann N8UR jra at febo.com
Fri Jan 18 08:39:50 EST 2008

Didier Juges wrote:
> Be careful that if you do not isolate the A/D converter analog circuits from
> the USB ground, you may create all sorts of nasty ground loops with the
> computer. When dealing with power supplies, it's always best to be isolated.
> I do not know enough about USB to know if it can be simply isolated with
> opto couplers, but I know that RS-232 is very easy to isolate. So if your
> data rate is not great, a serial solution would be easier to implement (even
> if you go through a serial-USB adapter for convenience on the PC side)
> Linear Technology, Analog Devices and Maxim have hot side current sense
> chips that are accurate and cheap. They are specialized diff amps, not
> regular IA. LT for instance has one that has less than 50uV offset (LT1787,
> see Design Note 227)

That's a really good point.  Thanks for reminding me of the potential 
ground loop problems.


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