The primary side consists of: power supply, input signal divider (1/10, 1/100, 1/1000), low input capacitance buffer and operational amplifier that drives IR emitters. Feedback of the amplifier includes additional IR emitter and photodiode (the same as on the secondary side) - it's needed for linearization of characteristics of the emitter and detector.
The secondary side consists of: power supply and amplifier that converts photodiode current to the output voltage.
If emitters and detectors have similar characteristics (preferably they are paired), then voltage on R21 and (R42+R46) should be identical (good linearity).
The main problems with the first revision was susceptibility to electromagnetic disturbances: switching noise from internal converters and external SMPSs, also picking up low frequency noise by input voltage divider. Also there were some issues with power supplies or other design issues.
Initially, the infrared emitter was LED, then I started testing VCSEL. VCSEL provided more power at lower current and it was much faster (about 40MHz achieved with VCSEL, with LED it was only about 10MHz). The used VCSEL could be easily overloaded so some active protection was needed in next revision.