Hi, I'm currently working on making a level shifting circuit that is supposed to take an input voltage (somewhere from the 0-5V range or 2.5-4.5V range) and shift it between 0-2.5V as the input for an xmega chip. We are currently trying to use a differential op amp circuit with the op amp AD8036 and are using this as a reference for building the circuit:

we were having trouble building the correct gain for this circuit though, since the calculations we did by hand got us negative resistances, so if anyone has any tips as to how to select the correct resistances, or if another circuit would be better, the help would be awesome.

Thanks

 

The problem is that you cannot make a non-inverting opamp with a gain of less than 1. Check the pages on this site for non inverting op amp circuits to see the derivation of the gain formula.

Hint: it looks like 1 + (R1/R2). So for any positive values of R1 and R2 the gain is going to be greater than 1. Looks like you need either two inverting opamps or a passive divider and a non-inverting stage.

With a single ended 3.3V supply you may have trouble reaching the supply rails. Better plan for a bit more headroom.

 

ahhhh, i see
that would explain much of the difficulty there xD
cuz yea, when i was doing the calculations, i also got something like the 1+R1/R2
we were thinking of using a 5V supply for the op amp though, would that be better for reaching the supply rails?

thanks