Have a look at datasheet of OPA27/37 for instance and you will see, that also antiparallel diodes can be found between inverting and noninverting input of this OPamp. These diodes are often used to protect inputs and are often already implemented directly on die. As potential difference between both inputs is nearly 0V under normal condition, resulting leakage current through these diodes is often, like here, negligible. But keep glass bodies of diodes isolated from heavy ambient light.
These diodes are very helpful, when using TLC277, because they provide a very safe latch-up protection scheme!!

33pF capacitors are also often used in the feedback loop of OPamp. It's a bit difficult to texplain, what they do:
Between output of OPamp and inverting input there's a certain internal phase shift. Capacitive load at inverting input or at output does introduce an additional phase shift (phase lag), which tends to move internal phase shift to dangerous levels, where oscillation might occur. 33pF capacitors introduce a 'healing' phase shift of opposite sign (phase lead), which moves internal phase to sane value again.
You will find this topic very often discussed, when having a look at datasheet of many OPamps.



Don't forget this 1kOhm resistor! As it is mounted in series to both 20kOhm resistors, it acts like a 2kOhm resistor, to be added to each 20kOhm, giving 22kOhm! Calculate by yourself, which currents are flowing at a certain input voltage and analyze what the voltage drops are at every resistor. You will see, that full wave rectification takes place. For ease of analysis assume, that 1nF capacitor is not mounted, as it will only show influence at much higher frequencies than mains frequency.

Kai