Op Amp Zero off set

Discussion in 'The Projects Forum' started by BobW55, Mar 14, 2015.

  1. BobW55

    Thread Starter Active Member

    Jul 22, 2008
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    I am working with an unknown 8 pin op amp that is rated for a min of 5 volts.
    I am only using 3 volts. Would this cause me to NOT be able to adjust it to zero?
    It has a UV sensor hooked to it, and in total darkness I still get over 700 mv output, even with the zero adjust pot maxed out.

    Bob
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Using opamps outside the specifications can lead to unwanted effects.

    Take a suitable opamp.
    Have a look at page B-2 of the following PDF for more info:
    slod006b Op Amps For Everyone.pdf

    Bertus
     
  3. BobW55

    Thread Starter Active Member

    Jul 22, 2008
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  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    That is a nasty practice.
    They probably want to protect their schematic, so it can not be reproduced.

    Bertus
     
  5. Papabravo

    Expert

    Feb 24, 2006
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    With Vcc below the minimum operational level you don't really have an opamp. You have something, but parts of the internal workings are probably non-functional. Trying to get zero volts out of a single supply opamp can be challenging under the best of circumstances unless you have a part the operates to the negative power supply rail. The best way to guarantee this is to use a bipolar supply or create a vitual ground at Vcc/2. Then you can define "0" output as mid scale and play away to your hearts content.
     
  6. BobW55

    Thread Starter Active Member

    Jul 22, 2008
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  7. Dodgydave

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    Jun 22, 2012
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    Last edited: Mar 14, 2015
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Dodgydave, are you sure it is CA3240?
    ca3240-a.pdf

    Bob says it is likely an 3140:
    CA3140.pdf

    The 3140 has some schematics in the datasheet for offset compensation:

    CA3140_offset_compensation.png

    Bertus
     
  9. Papabravo

    Expert

    Feb 24, 2006
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    Can be used in single supply mode with diode to limit output swing. No indication it can go below 5 V though.
     
    Last edited: Mar 14, 2015
  10. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    ca3140 has dc off set
     
  11. BobW55

    Thread Starter Active Member

    Jul 22, 2008
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    The board you see in the picture I posted is what I have. I have no schematic of the unit. Only data sheet I can find is in Chinese.
    http://us.100y.com.tw/pdf_file/57-A24B-UVM-30A.pdf Took me forever to translate which trim pot was which.
    Tried changing the zero offset pot from 1K to 10K dropped the output a few MV.
    Being that I do not have a schematic, nor can I see the traces on the board to reverse engineer it, I am kind of stuck.
    Will have to try and add some resistors to either side of the Zero adjust pot and see what that does for me.
    While I understand the basics of an Op Amp, my expertise is VERY limited. I also intend to hook this up to a 5 volt supply and see if that is what is making it
    not read a zero output.
    May have to abandon what I have and build my own from scratch.
    Thanks everyone for all the input.

    Bob
     
  12. Dodgydave

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    Jun 22, 2012
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    dc off set is pins 1-5, with th wiper to pin 4, what happens when you short pins 2 and 3 together, does the output on pin 6 go to zero..
     
  13. BobW55

    Thread Starter Active Member

    Jul 22, 2008
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    Dave,
    Will give that a try tomorrow.
    May have to rebuild the amplifier from scratch and keep the sensor part.
     
  14. BobW55

    Thread Starter Active Member

    Jul 22, 2008
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    Here are some test results.
    Running from a 6 volt supply
    Shorting pins 2-3 drops to 1.5mv, may not have had the best contact on the pins, hard to do with surface mounted stuff,
    Lowest I can drop the in the dark voltage to is 38.9mv on the out put of the board.
    Got the same results using a 3 volt power supply.
    Right now there is a 10K multi turn pot on the zero adjust. Have 2 thoughts,
    1) Try a 1 meg trim pot.
    2) build my own amplifier for this thing and just use the sensor from the existing one.
    Only thing I can add is that the amplifier is amplifying voltage, not current to voltage. The sensor part has a small current to voltage amp of some sort on it already.
    Course I could spend the $300 that Davis wants for their uv sensor but what fun is that.

    Bob
     
  15. crutschow

    Expert

    Mar 14, 2008
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    Do you know what the sensor dark current is?
     
  16. Dodgydave

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    Jun 22, 2012
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    Try a 1nF cap across pins 2,3
     
  17. BobW55

    Thread Starter Active Member

    Jul 22, 2008
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