J. Kissel, F. Clara

At my wits' end with measuring the analog response of the OMC DCPD transimpedance amplifiers via the OMC DCPD whitening chassis's preamp monitor interface (LHO:78090), the only thing left to question was the SR785 we'd been using that had been out by the SQZ and HAM6 racks -- a trusty race horse that had been in service out on the floor for a long time. 

Fil and I have discovered that indeed, it was time to put this race horse out to pasture.

We pulled the unit from the floor, grabbed a random spare electronics chassis we had lying around (Fil chose a D0902783-style anti-aliasing chassis), and measured it's transfer function using a suspected good, and the suspected bad SR785 from the floor.

Our suspicions were proved correct: the floor SR785 that we'd been using for months (if not years) is broken in a nasty, subtle way.

Check out the attached comparison of transfer functions of the same electronics channel, but between 
    - The "bad" floor SR785, where the CH1 and CH2 inputs are in differential, (A-B) mode :: the configuration that's needed for the measurements I was doing for months,
    - The "bad" floor SR785, where the CH1 and CH2 inputs are in singe-ended, (A only) mode :: totally fine
    - The "good" EE shop SR785, where the CH1 and CH2 inputs are in differential :: totally fine
    - The "good" EE shop SR785, where the CH1 and CH2 inputs are in singe-ended, (A only) mode :: totally fine

If you compare the ratio of the "bad" floor SR785 in differential mode to the "good" SR785 in differential mode against the "measurement setup" with pages 3,4, and 5 of the plots from LHO:78090, then you see exactly the same "gain is 2 [V/V], and has a frequency dependent hump above 1 [kHz]" that drove me insane this past Tuesday.

GUH. It's been a rough couple of months when you find out you've been using a calculator that computes 2 + 2 = 5...

The unit is now boxed up and off for "re-calibration," which (a) it hadn't had in a really long time, and (b) usually means that we'll get it back fully-functional.