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  =20
So 3.3/1024 =3D ~3mV/step.

For various reasons (primarily increasing the number of places this part=20
can be used), i'd really like to use the +-25A version of the Allegro=20
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. =20
So, a range of 275ma over the range of interest...   or just under 100=20
steps.

Or using the other math... 55mV per AMP, or about 1/20th of an amp per=20
step.  Not sure that is enough resolution for me.

I was considering moving the ADC references but there just isn't enough=20
to gain - you need at least 2V from Vref+ to Vref-, which means I'd be=20
lucky to gain another 100 steps.

Although I have to admit, the most usable option I've come up with so=20
far is to give up and use the 12.5A version, and move the Vref- to=20
around 1.2V, which should put us at 2.1V/1024 =3D 2mV/step, and the about=20
55 steps/amp... or 0.02A per 'step', which seems quite a bit better, and=20
then do math in software to get the right values.

-forrest

--=20
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