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

Robert Atkinson robert8rpi at yahoo.co.uk
Fri Jan 18 09:58:30 EST 2008

Hi John,
  One other possible problem is that the existing shunts may be in the hot side of the supply. This could exceed the common mode range of the ADC or even the amplifier (if used).
  Robert G8RPI.

John Ackermann N8UR <jra at febo.com> wrote:
  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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