OP-AMP filtering/amplification mystery

Discussion in 'The Projects Forum' started by iceman529, Feb 19, 2014.

  1. iceman529

    Thread Starter New Member

    Jun 17, 2013
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    Hi, I am working in a project where i am measuring the current with an micro-controller. I am using a Rogowski coil for A->V afterwords i have an amplification by 41.16(AD623), afterwords an filtering/amplification phase that i cannot understand, i attached a screen of the schematic and also here is a simulation of the filtering amplification part of the design. I understand how the last amplifier works, the non inverting amplifier.

     
  2. crutschow

    Expert

    Mar 14, 2008
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    Those are two Sallen-Key active low-pass filters.
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Working from right to left, U2B and U2A are each 2-pole lowpass filters. Depending on the component values, they either combine to form a single 4-pole filter response (3 dB down at the corner frequency), or they are each 2-pole sections so the combo would be 6 dB down at the corner freq. C16 and R3 form a single pole highpass section. Also, C20 has a highpass effect at a much lower frequency.

    ak
     
  4. iceman529

    Thread Starter New Member

    Jun 17, 2013
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    Thank you for the fast reply, after reading the Wikipedia page and a couple of other pages related to the Sallen-Key filter topology and some application notes form TI I still do not understand from where i have the gain factors after each U2A and U2B looking at an application note of TI snippet I got really confused, because i should not have any gain. I calculated the attenuation, hoped that i would find my answer here using the formula from Wikipedia got some numbers that didn't make any sense to me. [​IMG]
    Can someone explain from where i get the gain or at-least give me a gain factor that i can use to calculate to make the calculation i the MCU.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    The filters U2-A and U2-B have a gain of 1 at frequencies well below their corner frequency.
     
  6. iceman529

    Thread Starter New Member

    Jun 17, 2013
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    yes that is what i was expecting but if you look at the simulation that is not true, i put 70mv AC in, after the first amplifier i get a 41mv, after the second i get 139mv (all are peak to peak values), if someone can explain me from where i get this amplifications i will be really grateful.
     
  7. iceman529

    Thread Starter New Member

    Jun 17, 2013
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    Please help me in solving this mystery :confused::confused::confused::confused:
     
  8. crutschow

    Expert

    Mar 14, 2008
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    What simulation?
    What frequency?
    Post a picture of it.
     
  9. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    The simulation is linked in the original post. However I don´t think that the falstad simulator is up to the task for a circuit like this, better get something more proven like the free LTSpice.
     
  10. iceman529

    Thread Starter New Member

    Jun 17, 2013
    16
    0
    HERE is the link for the simulation. The simulation works almost as the real life circuit it just ads a couple of millivolts to the peak to peak values, but on the real circuit I still see the same behavior a negative gain after the fist op amp afterwords a positive one after the second. The frequency is 50hz(I sample the power line so a 49.5-50.5Hz is expected)
     
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