I am using DAC1 on an Arduino Duo board to create a waveform that runs 20 hz and varies from 0 to 5 volts. I want to shift the output from 0 - +5 volts to -5 - 0 volts. Have tried several different op amp circuits and nothing is working for me. seems like it should be a fairly simple thing to do but the solution is eluding me.