On Thu, Jun 23, 2011 at 7:10 AM, Forrest Christian wrot= e: > 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 > -- Well, how about an out-of-the-box idea? Are you aware of the other ACS7xx parts? They have a couple that are unidirectional and do not have a vcc/2 "zero" point. Look at the ACS713 or ACS715. The other thought depends on the rest of your system. An interesting approach is to play with the negative and positive Vref inputs. Most of the time Vref is set to Vdd and Vss, but there's nothing stopping you from setting the lower Vref to Vdd/2 which scales the A/D converter to the effective range of the sensor. I had one application where I changed the Vref on the fly (internal Vdd/Vss to read one sensor, external references t= o read another sensor) -Denny --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .