I want to get a +-15V differential signal (60Vp-p) into an ADC that has a single 3.3V supply. When I asked a friend to proofread it, he was concerned about the noise performance of R5-R6 vs. using R1-R4 to make U2A output the desired level to start with:
(input protection not shown for clarity)
My understanding is that an opamp's noise level does not depend on signal level, therefore a hotter signal is better. So I only attenuated U2A enough to match the full input range and left the rest to R5-R6.
(Another benefit is that if something goes wrong and U2A becomes a comparator, then the ADC input is still within the limits. Probably not likely, but possible.)
My friend thinks I would actually have less noise by putting all the attenuation in R1-R4, deleting R5-R6, and driving C1 directly. Is this true?