For the optical levers we are going to use the same QPD that are used in aLIGO optical levers (see T1600085 and D1100290): Hamamatsu S5981

Based on the aLIGO design, I put together a design fof the QPD boards, see the first attached PDF file. Some comments:

• I'm using a quad opamp to implement the transimpendance and a doubel stage of whitening for the readout of each quadrant. I haven't decided yet which is the right whitening
• The bias to the QPD is just coming from the +15V with a capacitor to ground, as in the aLIGO setup. However, I allowed the possibility of supplying a lower noise bias if needed, by removing R0 and connecting an external circuit
• There is no sum or difference computation, each quadrant is sent directly to the ADC with a differential driver (DRV135)
• Each board hosts one QPD, and it is interfaced with a 14 pin header (we're going to use twisted cables for the signals)

The stable power supply will be provided by an additional board, which will also interface 8 QPD boards to the ADC connector, see the second attached PDF. The ADC signal grounds can be connected directly to the power supply ground, left floating, or connected with a RC filter, depending on what we find to be the best solution.

Total cost estimated for PCB manufacturing and components, including the QPDs is less than 3k$, for a total of 12 boards (we need 8, plus some spares)