From what I've read, in order to separate out the real and imaginary components from what was measured, we have to multiply the V and I by the signal generator, at both 0deg and 90deg. For education purposes, where is that implemented in the attached schematic? If I were to do a brief walk through, we have 6th-order LC filtered DDS sine wave ac coupled and buffered generator driving the DUT. We then measure the differential complex voltage (and current) through TL084 opamps, gained up by 2 and rc filtered through 1kohm and 470pF, and then running it through a programmable gain stage, then another 3x buffered stage to another differential TL082 with PA0 and PA1 ports. Here is where I am not sure what is going on. Are we trying to flip the sign here? If so, it's 180deg and not 90. Maybe the 180deg flip is to get rid of the reactive component? Very confused. Please help. Thanks.