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Entry  Mon Feb 8 10:45:50 2010, Steve O'Connor, Electronics, Pre-amplifier, replace Pot with fixed Resistor 
    Reply  Mon Feb 8 17:44:38 2010, Aidan, Electronics, Pre-amplifier, replace Pot with fixed Resistor 
Message ID: 11     Entry time: Mon Feb 8 10:45:50 2010     Reply to this: 12
Author: Steve O'Connor 
Type: Electronics 
Category: Pre-amplifier 
Subject: replace Pot with fixed Resistor 

 

            Preamp for Bulls eye detector                
                             
  It was felt that the Pot used at the input stage to remove offset added Noise            
  To test this the Pot was replaced with a fixed resistor and the offset removed at the second stage        
  Noise was measured after the first stage and at the monitor point first with the pot and then with the pot replaced with a Resistor  
                             
              First stage gain =1+500/10 test point 1 gain = 51      
              second stage gain=10K/1K test point 2 gain = 510    
  1K Pot (R19) is present                      
                             
  Chan #1                          
    dbVrms/Hz       nV/Hz       Referred Input Noise  nV/Hz      
                    gain = 51        
    200Hz 100Hz 50Hz   200Hz 100Hz 50Hz   200Hz 100Hz 50Hz    
  Test Point #1 -141.1 -140.0 -136.8   88.1 100.0 144.5   1.7 2.0 2.8    
                    gain = 510        
  Test Point #2 -119.4 -120.4 -118.4   1071.5 955.0 1202.3   2.1 1.9 2.4    
                             
                             
                             
  Pot replaced with Resistor (R4)                    
                             
  Chan #1                          
    dbVrms/Hz       nV/Hz       RIN        
                    gain = 50        
    200Hz 100Hz 50Hz   200Hz 100Hz 50Hz   200Hz 100Hz 50Hz    
  Test Point #1 -142.7 -142.7 -141.9   73.7 73.3 80.8   1.4 1.4 1.6    
                    gain = 500        
  Test Point #2 -122.0 -121.1 -120.7   794.3 881.0 922.6   1.6 1.7 1.8    
                             
                             
  When the Pot was replaced with R4, the offset was removed with the Pot at the second gain stage          
  R4 was not a thin film metal resistor                    
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