I have designed a PCB based on a schematic which I am pretty sure that is correct and I have tested the schematic several times on the breadboards and all of them worked properly.

But, after I designed the PCB I found that it only works when I touch the the photo diode pins otherwise it doesn't work normally.

My PCB is only 2 layers and I have not separated (GND,VIRTUAL GND,AND 12V POWER) in to a separated PCB layers.

Also, I have checked all of my OP-AMP voltage and input voltage and compared them with the breadboard, the result was that all of them were same as each other but, I can not understand why PCB doesn't work good and I have to touch the pins to make it works.

I am new employee in a company and it is very important for them that I fix this issue, I tried a lot to fix this issue but until now I have not succeeded.

So, I will be appreciate if any of you guys can give me an advise or guide me to I be able to fix this or issue design a PCB that works properly.

Finally I have to mentioned that this schematic has been designed by very well known company and is 100 present is correct and good but this design is a little bit old maybe for 12 years ago.

I have uploaded the schematic here.

Thank you everybody for your help and time.

enter image description here

  • 3
    \$\begingroup\$ Tip: use ground symbols at the components to eliminate the ground nets running all over the page. It reduces clutter and reveals the node potential at a glance. If you have two grounds then use two different ground symbols. Turning off the grid too before taking a screen grab will also help legibility. \$\endgroup\$
    – Transistor
    Nov 15 '18 at 22:33
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    \$\begingroup\$ the schematic is not very good ...... the output stage does not follow convention so it is unclear .... not all power connections follow convention ..... have a look at every component in the schematic .... you will find one that is totally useless ..... just a plain messy schematic \$\endgroup\$
    – jsotola
    Nov 15 '18 at 23:46
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    \$\begingroup\$ What does do that is wrong? Does it oscillate? Does the output of U4 stick to one of the rails? \$\endgroup\$
    – JRE
    Nov 16 '18 at 3:29
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    \$\begingroup\$ The original circuit may be from GE. That particular schematic, though, was drawn by someone at your company. It is possible to make mistakes when redrawing a circuit, especially if you are working from the board rather than the original schematic diagrams. \$\endgroup\$
    – JRE
    Nov 16 '18 at 6:21
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    \$\begingroup\$ Now you think if I use some capacitor in my board the problem can fix or I have to design it 4 layer same as GE.(4 layers board are a little bit expansive) \$\endgroup\$
    – a2640
    Nov 16 '18 at 6:45

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