Forest,

It might be worth investigating using a very small series resistor with=20
a TI INA21X. current shunt monitor.

http://focus.ti.com/docs/prod/folders/print/ina214.html

The chip output reference can be attached to something other than ground=20
and the small value resistor needed allows very low insertion loss and=20
low wattage rating.

Different parts in the family have different gains, so you can choose=20
one that fits your needs.

Data sheet: http://focus.ti.com/lit/ds/symlink/ina214.pdf

I can't spend a lot of time to see if this really fits your app, but I=20
thought I'd mention it here because I bet if you take a look you'll find=20
a nice way to use it and I suspect it will save you some $ over the=20
Allegro part.

Just make sure you have dedicated sense lead connections to the inputs=20
to avoid voltage drop in the main current path which leads to errors. If=20
you do this right you can get good results.

> Mark Skeels
> Engineer
> Competition Electronics, Inc.
> TEL: 815-874-8001
> FAX: 815-874-8181
> www.competitionelectronics.com
> On 6/23/2011 11:38 AM, Michael Watterson wrote:
>> what is resolution of ADC compared to accuracy of the sensor? Does it
>> actually matter if full adc range isn't used? If so save on HW and just
>> scale in SW. Unless you use precision resistors etc adding an op-amp may
>> reduce accuracy?
> I'm going to be using a 10 bit ADC in a PIC.  Vdd/Vss references, 3.3V
> So 3.3/1024 =3D ~3mV/step.
>
> For various reasons (primarily increasing the number of places this part
> can be used), i'd really like to use the +-25A version of the Allegro
> product.   In this case, I'm most interested in 0-+5A.
>
> 0A corresponds to 1.67V.   5A corresponds to (5*0.055+1.67)=3D 1.945V.
> So, a range of 275ma over the range of interest...   or just under 100
> steps.
>
> Or using the other math... 55mV per AMP, or about 1/20th of an amp per
> step.  Not sure that is enough resolution for me.
>
> I was considering moving the ADC references but there just isn't enough
> to gain - you need at least 2V from Vref+ to Vref-, which means I'd be
> lucky to gain another 100 steps.
>
> Although I have to admit, the most usable option I've come up with so
> far is to give up and use the 12.5A version, and move the Vref- to
> around 1.2V, which should put us at 2.1V/1024 =3D 2mV/step, and the about
> 55 steps/amp... or 0.02A per 'step', which seems quite a bit better, and
> then do math in software to get the right values.
>
> -forrest
>
--=20
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