In equation 3, it's required to know the desired gain from the termination resistor to the output. How does anyone know this without guessing? I know the gain I want from my 50-ohm source to the amp's output, -1.5. It's a single-ended source, negative-going signal from 0 to -1V. I desire 1.5Vpp out of the amp. I'm finding, in TINA spice that I need a gain of ~21 in order to get my output. If I plug the resulting resistors into equation 3, my Rg is less than 1 and my termination resistor is 781. In TINA, my feedback resistor is 510, Rs = 50, Av=21. In Eq-n 3, my Rg2 = 48 which seems reasonable. Note that I'm converting this unipolar signal to a bipolar signal by dragging Rg2's input down to approximately -0.35V. This results in my 0 V in being at -1.8Vpp and my -1V in being +1.8Vpp Also, at -5Vin, my Vout is at the common-mode voltage of 0.75. My supplies are Vs+=3.2 and my Vs-=1.8. I have an external load of 1K (my A2D is 1K input).
Anyone see anything wrong with taking a negative-going 0 to -1V signal and converting it to +/-1.5Vpp, offset by 0.75V? In TINA, it all seems to work well but the spice circuit doesn't agree with the equations in the datasheet.
