With help from Awai, Tomura, Kamiizumi, Shimode, Doi and Miayakawa,

We modified a DC QPD circuit (S1504142) such that it can be used for reading out the photo current out of the four independent baffle PDs (3952). The board is ready for installation.

A summary of the hacks:

• Removal of the QPD chip out of the board.
• Reduction of the reverse bias voltage from 15 to 3 volts.
• Addition of a DB9 connector to the board.

More details:

• We took off the QPD, S5981, out of the QPD board.
  • We accidentally damaded the surface of the QPD. Sorry.
  • The QPDs, which are going to be connected to this cirucit board, are FGA21 from Thorlabs. 
• R101 (0 Ohms) was removed. This register was to provide a 15 V reverse bias to the diodes.
• Instead, we palced a 4.3 kOhm register at the place of R101.
  • I choce a lead register for this for no real reason.
• A resister with a resistance of 1 kOhms was added in pararrel to C3.
  • Together with R101, they form a voltage divider to provide a 3 V reverse bias to the diodes.
• A wire was soldered to the annode path of each photodiode. Also, another wire was slodered to the common cathode path.
• The other ends of these wires are soldered to a DB9 female connector. (I should have chosen a male connector!)