I am trying to design a very basic oscilloscope and my first PCB schematic and I am having trouble wrapping my head around how to level shift the front end analog signal so it can be accepted by my ADC. I am using a AD9238 ADC with a 2Vpp input and I want the scope to accept a signal of +/- 3.5 V.
I have a level shifter that feeds into a differential op amp to give me a differential input in my ADC. What I am having trouble understanding is how much gain to give at the differential op amp, and what to level shift to.
I have set the Vocm pin on my differential op amp to Vdd/2, and my voltage source is 3.3V so that gives me a common mode of 1.65V I believe. So since my ADC accepts 2Vpp and common mode is 1.65V does that mean I am looking to level shift from +/-3.5V to .65 to 2.65V? The ADC also has an internal Vref that can be set to 1V, and if I instead use that for Vocm pin on the differential op amp would I then be looking to level shift to 0 to 2V?
The op amp for level shifting I am using is the lf351, the differential op amp is the THS4121CD and the 40msps version of the AD9238 ADC.