I am getting -8V at the ground pin

Discussion in 'General Electronics Chat' started by nado, Jan 15, 2013.

  1. nado

    Thread Starter Member

    Dec 17, 2012
    40
    1
    After I fixed one problem with the help of the group members, I have another puzzle.
    At the pin 12 of U305, I am getting -8~+8. I removed the chip. Now I am getting -10V. Data sheet of the LM339N says it is ground pin and Fluke's schematic doesn't show the pin number 12. So I think they omit the pin 12 from the schematic for the obvious reason.

    I enclosed the schematic in pdf. Please take a look at. I read the hand writing of U305 in the schematic as "LM339"
    Thank you in advance for whatever the suggestion is.
     
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    Last edited: Jan 15, 2013
  2. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    Pin 12 is a ground for the output circuit. It should be connected to circuit common.
     
  3. nado

    Thread Starter Member

    Dec 17, 2012
    40
    1
    Sorry I have to clarify what I am doing. I am fixing my Fluke 8840A DMM.
    My question is why they use pin 12 as -Vcc?

    I read the datasheet in detail. It says, "Operation from dual supplies also is possible, as long as the difference between the two supplies is 2V to 36V, and VCC is at least 1.5V more positive than the input common-mode voltage."
     
    Last edited: Jan 15, 2013
  4. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    Pin 12 determines the low level output voltage. It it's connected to -Vcc then the low level output will be at -Vcc. That's unrelated to the datasheet quote you posted.
     
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