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Date: Tue, 21 May 2013 12:08:01 +1000
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Subject: Re: [geda-user] ITEAD?
From: Stephen Ecob <stephen DOT ecob AT sioi DOT com DOT au>
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On Tue, May 21, 2013 at 11:30 AM, Peter Stuge <peter AT stuge DOT se> wrote:
> Stephen Ecob wrote:
>> one of the engineers of our "partner" company was very happy to
>> explain how he'd modified it to give better linearity
>
> What were his modifications?
>
> Did he change some of your assumptions in order to accomplish the
> better performance?

The PCA was an isolator for reading current shunts.  The 50mV peak
shunt signal was boosted to about 5V and galvanically isolated.
It was based on a linear optocoupler.  This optocoupler had a small
but significant nonlinearity.  To obtain high accuracy we
characterised this nonlinearity and cancelled it out in SW after the
signal was digitised.

I wish I could say that this linearity correction was hidden away in
our SW and that the IP thieves ended up with inaccurate readings after
"improving" our HW, but unfortunately we put an option in the UI to
turn the nonlinearity cancellation off.
I guess they would have realised what was happening after a bit of
initial confusion, and turned the nonlinearity cancellation off,
yielding an accuracy close to that of our genuine product.
I do hope, though, that they pulled out a bit of hair when they first
realised that their "improved" HW was yielding worse results at the
system level :-)