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Mon Feb 8 10:45:50 2010, Steve O'Connor, Electronics, Pre-amplifier, replace Pot with fixed Resistor        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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