# Shift and enlarge voltage out of DAC

I have a DAC that can provide voltages between 0-2.5 V and I need a -12.5 - +12.5 sine wave. I am not too steady on the circuit calculations. Would it be possible to amplify by 10 and set the positive input on the op amp to a 1.25 V ref? Like I have drawn in the picture. Or do i perhaps have to do it in two steps?

• You can also generate the offset by injecting a current into the virtual earth terminal. Here, that would require a resistor from a -ve reference voltage. – Brian Drummond Jun 30 at 9:48

Would it be possible to amplify by 10 and set the positive input on the op amp to a 1.25 V ref? Or do i perhaps have to do it in two steps?

You need to offset by +1.1363636 volts and no, you don't need two steps.

Think about the op-amp output at 0 volts and the signal input at a steady 1.25 volts. The potential divider formed by R and 10R means that the inverting-input needs to be at: -

$$+1.25\cdot\dfrac{10}{10+1} = \text{+1.1363636 volts}$$

To achieve that you need to apply +1.1363636 volts to your op-amp's non-inverting input: -

• @jusaca - sorry about the mix up - I was already editing the answer. – Andy aka Jun 30 at 8:17
• I figured, now it's all fine ;) – jusaca Jun 30 at 9:12

Your solution will not go from -12.5V to +12.5V, but actually from -11.25V to 13.75V (because you will have a constant offset of 1.25V).

An easy solution, if your are using the DAC only for AC, might be to ground the noninverting input of the OpAmp and decoupling the output with a capacitor. That way you don't have to create a 1.25V reference and your output will swing symmetrically around ground. Of course this won't work if you need to supply some DC offset or very low frequencies.