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Message ID: 2245     Entry time: Fri Oct 5 12:42:29 2018
Author: awade 
Type: DailyProgress 
Category: PD 
Subject: PMC reflection PD DC path fix 

The DC path of the north PMC resonant reflection photodetector was not showing any signal.

Some work on the North PMC resonant reflection phododetector:

  • Drop in voltage accross R4 is 0.9 V, its 22 Ω so that would be a current of 40 mA: way too much current when dark.
  • I looked inside and there was a lot of blobbed solder joints R4 and C8, C33 was also not fully contacted on the ground side. I wicked all the excess solder off and retouched all the joints that didn't look great. These modifications didn't fix the problem
  • Checked voltage supplies and DC levels in the DC path.  The first OP27 (U5) is showing 1 V on the input but 2 V on the output.  This should be a buffering stage: the op amp is busted. I removed U5 and replaced with new.  Pulled of the pin5 pad but this is not used anyway.  The dark PD doesn't show any voltage drop accross R4 anymore, this is good.
  • Modified the gain of the final DC output stage OP27 (U6).  It is now R16=1 kΩ, R11=10 Ω so gain of that stage is x101 and the total DC transimpedance gain is 2222 Ω.  
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