40m QIL Cryo_Lab CTN SUS_Lab TCS_Lab OMC_Lab CRIME_Lab FEA ENG_Labs OptContFac Mariner WBEEShop
  ATF eLog  Not logged in ELOG logo
Message ID: 1391     Entry time: Wed Apr 13 14:58:12 2011
Author: Alastair 
Type: Electronics 
Category: GYRO 
Subject: Trans PD final design 

 So here is the outline of the Trans PD component choice:

1) Shot noise:  at 3mW the shot noise limit is ~3e-11A.

2) The transimpedance is entirely set by the component choices we make for the inductor/capacitor pair in the readout notch, since the diode capacitance and resistance are fixed.

3) We want to make sure we're well away from railing the opamp.  We want approx 0.3V output max, which is a transimpedance of 100.  This includes the gain of 10 from the opamp stage.

4) We are well below any noise level that could be of concern for us in transmission readout.

Since we are well above the noise level we care about in transmission, I've chosen the values to make sure the opamp isn't railing at 3mW input power.  This does mean we will be dominated by voltage noise in the opamp.

Based on this I picked the value of L (which had to be a value we have, 27nH) to give the nearest transimpedance to 10V/A which I checked in Matlab (see attached Matlab graph).  I then checked the model in LISO, and get the correct transimpedance for the full circuit including the opamp.  As you can see the noise is now dominated by voltage noise in the LMH6624, simply because we have such low optical gain.  It seems like the LMH6624 is not the nicest way to do this if we ever want to use more power than 3mW....  the minimum gain of 10 is causing us to reduce the gain at the point where we really want it - the input.

The noise level from this at 100MHz is 1e-10A/rt(Hz), which corresponds to a frequency noise of 3e-9 Hz / rt(Hz) at 0.1Hz.  The equivalent rotation noise is then 1e-15 rads / rt(Hz) at 0.1Hz

Attachment 1: transimpedance.png  16 kB  | Hide | Hide all
Attachment 2: Screen_shot_2011-04-13_at_2.55.30_PM.png  22 kB  | Hide | Hide all
Attachment 3: Screen_shot_2011-04-13_at_2.55.48_PM.png  35 kB  | Hide | Hide all
ELOG V3.1.3-