I saw this application example in the LM324 datasheet.
In a case like this:
where R1/R2 is equal to R4/R3 and R4/R3 is large (330 like above), the first op-amp seems to act like a unity gain buffer; its output isn't much different from its non-inverting input.
Also, the inputs (and thus, the output) to the op-amp are already offset by Vcc/2 and buffered. Is there still any need for op-amp U1A above? Can both inputs just be fed into U2A directly without compromising the "High Input Z" quality (or anything else) of the original circuit? I'd also like to understand what is meant by "CMRR depends on this resistor ratio match" in the first image.