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. =A0This puts out voltage based on current, where zero current is >> referenced to VCC/2. =A0 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. =A0The =A0full scale output is from 1.65 to 3.3V. =A0The cu= rrent >> 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. =A0Wire 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. =A0But 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 > 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 .