Explaination for Photo diode circuitry

[nikhillife11]

Hello All,

I need a help with the attached circuit below. Here I know the reason for using a OP-AMP and IC AD694.
OP-AMP is used for current to voltage conversion whereas AD654 is used for voltage to frequency conversion. My problem is with the area in that RED box. Why are so my components used and how does it work. I see a diode which i assume is for reverse blocking. But I can see a zener too. Whether it is used to maintain a voltage of -2.5V is what I want to know. Can anyone explain me the logic behind that circuit in RED BOX.



Thanks and Regards

Nikhil :-D

BMP image replaced with GIF format which can be viewed in the browser and is smaller in size [alexan_e]

 

[FvM]

D7 and C10 don't belong to the red box. The remaining parts are generating a small negative offset voltage for the V/f converter.

You didn't ask about it, but the input stage as shown isn't a current-to-voltage converter and won't work without a DC feedback.

 

[nikhillife11]

Hello FVM,

Can You explain this in detail and what about D7 and C10. Whats there use here. Also how is this negative offset voltage created and whats that offset voltage. Whether as per my assumption is it -2.5V.

Thanks and Regards
Nikhil

---------- Post added at 06:33 ---------- Previous post was at 06:02 ----------

Also,

As i know Photo diode is a current sourcing device,So the input stage. But if its not a current to voltage amp then what is this op amp used for. Can you guide me on the same and share any good link or ebook for opamps .

thanks and regards

Nikhil