Re: gEDA-user: Re: Looking for a project

[Peter Clifton <pcjc2@xxxxxxxxx>]

On Sat, 2007-03-31 at 16:36 -0400, al davis wrote:
> On Saturday 31 March 2007 15:31, Kai-Martin Knaak wrote:
> > I'd vote for a floating, differntial input preamplifier for
> > the oscilloscopes. This avoids the notorious ground loops
> > that pick up all kinds of noise. An optional, decent
> > amplification will be handy too. Sometimes analog signals are
> > just too tiny to emerge from the input noise of the scope.
> > The main benefit is the ability to measure floating voltages
> > not referenced to common ground potential.
> 
> That is perfect for a starter.  The design is so simple that it 
> could be used as a homework assignment in a beginning 
> electronics course.  That is, low bandwidth, with op-amps.  A 
> wideband one would be more challenging.
> 
> The significant contribution here is to do the whole job, with 
> PCB layout, assembly procedure, test procedure, and make the 
> whole package GPL.

One word: _Saftey_.

We use differential isolating probes in the lab for all sorts of
measurements, and for power systems, they must be isolated. I'm not sure
what the standards require, but I'd probably design for many kV. (to
ground).

Such devices are on my "would be nice" list too, but reaching a spec is
hard. Some options:

Direct to digital - you can get sigma-delta ADC chips with output clock
of 10Mhz, with integral isolating magnetics. You "just" need a PSU and
the signal processing.

You could use a matched, dual port opto-isolator (might drift though) -
depending on how accurate you want to be. I've seen this used in
motor-controller current sensing.

You could choose to reference to ground with a high MegOhm divider,
keeping currents drawn from the test-points (say 600V off ground)
minimal. CMRR is a problem though.

I made a little 50x gain, non-isolating differential amplifier for some
instrumentation around a little self zeroing instrumentation amplifier,
the AD8230. My simple circuit used a separate floating PSU for the
amplifier, and a ground-reference chip to centre that as a split-rail
about the output circuit's ground.

http://www.analog.com/en/prod/0,,759_782_AD8230,00.html


Regards,

-- 
Peter Clifton

Electrical Engineering Division,
Engineering Department,
University of Cambridge,
9, JJ Thomson Avenue,
Cambridge
CB3 0FA

Tel: +44 (0)7729 980173 - (No signal in the lab!)



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