L. Dartez, J. Kissel, F. Mera, M. Pirello, D. Schaetzl, D. Sigg
IIET Ticket 24198
WP 10933

In order to take advantage of the new low-noise, fast, 18-bit ADC (that we also switched over to today; LHO:66974), and improve the headroom for violin modes (among several other excellent new user-interface and noise advantages), we've swapped the out the first attempt at the A+ era whitening chassis (D2100630-v3) for the newly arrived second attempt at the A+ era whitening chassis (D2200215) for the primary, OMC DCPD signals coming out of the vacuum system.

We've installed the new chassis -- serial number S2300002 -- under the previous version in ISC-R5 U-height U24. Once installed, we moved all of the relevant cable connections from the previous chassis (S2101608) and plugged them into the front and back of new chassis.

There were some hiccups after connecting the DCPDs to front-end of the whitening chassis -- namely the current limiting resistor in the power regulator circuit (that which reduces the chassis input +18V to the +15V that the preamp) became current limited, and was dragging down the voltage supplied to the DCPDs to a measly 2V. Dean first ID's the problem because the status LED on the front panel became "barely lit" after the chassis was powered on *with* the DCPDs connected. We then spent a bit of time understand what was happening, including measuring the current draw of the DCPDs (using the old chassis, in-situ, chassis power and DCPD power on, measuring the current draw at the output of the regulator by measuring voltage across the 10 mOhm resistors R55 R68 R81, using the available surrounding test points).

   Supply          Current draw
  Regulator          at Output
    +18 to +15V:      120 mA
    -18 to -15V:       80 mA
    +18 tp  +5V:       50 mA

Finding this, and seeing with our own eyes (after pulling S2101608 from the rack and popping the lid) that our S2101608 instantiation of D2100630 chassis had an undocumented (no e-traveler update, no drawing update, no aLOG) change of the current limiting resistor on the +15 V regulator (R61 on page 7 of D2100630-v3) from 1.5 kOhm down to 499 Ohm, increasing the current limit from 100 mA to 250 mA, we made an on-the-fly design change/choice to replace the equivalent resistor in the new chassis assembly -- R8 in D2200045 -- with 750 Ohm, and thus we now have a current limit of 200 mA on this +15V supply leg. Dean has already graciously already updated D2200045 to -v2 with this change.

To be covered in separate aLOGs / Comments:
    - Pictures of the chassis installed in the rack
    - Measurements of the frequency response in calibration-group level detail
    - Fits and subsequent compensation of that response
    - Comments about the output voltage noise
    - Comments about a new output voltage offset
    - Comments about the full chain's OMC DCPD dark noise w.r.t. the new ADC noise
    - Comments about using the remote full-chain driven excitation test system to confirm that the compensation is still relatively flat
    - Comments about changes needed to be done to the Beckhoff system to regain remote control of the chassis