== This page is obsolete. We should copy Rich's Transmon QPD design. ==
[[https://dcc.ligo.org/cgi-bin/private/DocDB/ShowDocument?docid=13998 | https://dcc.ligo.org/cgi-bin/private/DocDB/ShowDocument?docid=13998]]
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## page was renamed from Electronics/Quadrant Photodetector Design
This page describes the design of a new quadrant photodetector. 

The main applications are for DC beam sensing and optical lever servos. These are non-critical applications.

The DC beam sensing ones are mainly for beams on the PSL and IO optics. We should also add a QPD to each of the ISC readout paths.

The basic idea is to make an updated version of Jay's circuit ([[http://www.ligo.caltech.edu/%7Ejay/drawings/d980325-c1.pdf|D980325-C1]]):
 1. Use voltage regulators. Is there something better than the 7915/7815 ?
 1. Maybe use a different op-amp. The OP497 in Jay's circuit is a good part, but not modern. We want to use something that's not going obsolete soon.
 1. Use metal film resistors.
 1. Allow for whitening in the head via some jumpers. Possibly also give slow DC ouputs.

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=== Choice of Quadrant Photodiode ===
The QD50 and QD100 are commonly used in LIGO for DCPDs. They are Silicon based and have active areas of 50 mm^2^  and 100 mm^2^, respectively. R = 0.5 A/W @ 900 nm.

=== Noise ===
This is the input referred current noise calculated with LISO - OP497 w/ a 100k transimpedance resistor:<<BR>>
{{attachment:qpd-op497.png}}

Next, we look at the noise using the XXNNN quad opamp:

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=== Useful Links ===
 1. Application note [[attachment:AN_940.pdf]] on selection of low noise opamps.