On 23/06/2011 15:10, Forrest Christian wrote:
> I have a circuit which I now plan to use an allegro ACS711 current
> sensor.  This puts out voltage based on current, where zero current is
> referenced to VCC/2.   As I only care about current in one direction,
> I'd really like to remove this offset and then be able to amplify this
> to a range based on my expected current range.
>
> Vcc is 3.3V.  The  full scale output is from 1.65 to 3.3V.  The current
> range I am working with is probably more like 1.65V to 2.5V. I'd like to
> move this to where this is scaled to a range of around 0 to 3.3V.
>
> The obvious way to do this is to feed this output into a difference
> amplifier made out of an opamp and four resistors.  Wire the positive
> input to the ACS711, and wire the negative input from a VCC/2 voltage
> source made from another opamp in unity gain mode and with a voltage
> divider on the input.  But that seems like a bit of overkill since I'm
> pretty certain I should be able to do this all with one opamp and skip
> the unity gain amp- unfortunately I'm not seeming to have much luck
> doing the correct analysis to make this work.
>
> So, I'd appreciate any opamp wizards which could either just point me
> towards the solution, or even better help me figure out where my 'mental
> model' of opamp operations is falling down in this case, so I don't have
> to ask a similar question again.
>
> Thanks,
>
> -forrest
what is resolution of ADC compared to accuracy of the sensor? Does it=20
actually matter if full adc range isn't used? If so save on HW and just=20
scale in SW. Unless you use precision resistors etc adding an op-amp may=20
reduce accuracy?

--=20
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