I typically use microchip rail-to-rail in and out opamps, such as the=20 MCP6024, so that isn't a big issue. On 6/23/2011 11:53 AM, smith steve wrote: > Deepening on the accuracy you meet do it in the micro you can > establish an offset value for no current this will effectively give > you 11 bits of resolution for a 12bit conversion and you don't need > any odd voltage supplies for the opamp to ensure true rail to rail > operation > > Steve > > On Thursday, 23 June 2011, Michael Watterson wrote: >> 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 t= o >>> 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 'menta= l >>> model' of opamp operations is falling down in this case, so I don't hav= e >>> to ask a similar question again. >>> >>> Thanks, >>> >>> -forrest >> 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? >> >> -- >> http://www.piclist.com PIC/SX FAQ& list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist >> --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .