40m QIL Cryo_Lab CTN SUS_Lab TCS_Lab OMC_Lab CRIME_Lab FEA ENG_Labs OptContFac Mariner WBEEShop
  40m Log  Not logged in ELOG logo
Entry  Sun Jul 24 20:36:03 2011, haixing, Summary, Electronics, AA filter tolerance analysis 6x
    Reply  Sun Jul 24 20:47:21 2011, rana, Summary, Electronics, AA filter tolerance analysis 
       Reply  Sun Jul 24 22:19:19 2011, haixing, Summary, Electronics, AA filter tolerance analysis low_pass_new.png
          Reply  Tue Jul 26 21:11:40 2011, haixing, Summary, Electronics, AA filter tolerance analysis 
Message ID: 5022     Entry time: Sun Jul 24 20:36:03 2011     Reply to this: 5023
Author: haixing 
Type: Summary 
Category: Electronics 
Subject: AA filter tolerance analysis 

Koji and Haixing,

We did a tolerance analysis to specify the conner frequency for passive low-pass filtering in the AA filter of Cymac. The
link to the wiki page for the AA filter goes as follows (one can have a look at the simple schematics):
http://blue.ligo-wa.caltech.edu:8000/40m/Electronics/BNC_Whitening_AA

Basically, we want to add the following passive low-pass filter (boxed) before connecting to the instrumentation amplifier:

low_pass.png

Suppose (i) we have 10% error in the capacitor value and (ii) we want to have common-mode rejection
error to be smaller than 0.1% at low frequencies (up to the sampling frequency 64kHz), what would be
conner frequency, or equivalently the values for the capacitor and resistor, for the low-pass filter?

Given the transfer function for this low-pass filter:
transfer_function.png     f0.png
and the error propagation equation for its magnitude:
error.png
we found that the conner frequency needs to be around 640kHz in order to have
DT.pngwith Dc.png
 



ELOG V3.1.3-