Offset Null for Single Supply Op Amp.

Discussion in 'General Electronics Chat' started by FrancescoC, Apr 8, 2016.

  1. FrancescoC

    Thread Starter Member

    Nov 22, 2014
    30
    5
    Hi,
    I was reading the datasheet for an LN1006 Op Amp from Linear Technology.
    The device is equipped with offset null connection and the datasheet says to connect the wiper of pot to -V.
    Because I want to use a single supply, I was wandering if you can connect the wiper to +V instead.

    I am sure I have seen this done before. Or am I mistaken?
    Regards

    FrancescoC
     
  2. OBW0549

    Well-Known Member

    Mar 2, 2015
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    Why in the world do you want to connect the offset pot wiper to V+???? As you said, the datasheet says to connect it to V-. So connect it to V-.
     
    #12 likes this.
  3. #12

    Expert

    Nov 30, 2010
    16,298
    6,808
    Why would you want to pull the emitters of a matched pair of NPNs to a higher voltage?
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,014
    3,234
    I could not find an LN1006.
    Do you mean LT1006?
     
  5. hp1729

    Well-Known Member

    Nov 23, 2015
    1,954
    219
    That "-V" is -V of the op amp whether it is connected to a negative voltage or ground.
     
  6. FrancescoC

    Thread Starter Member

    Nov 22, 2014
    30
    5
    Hi,
    Sorry is not LN1006 is LT1006.
    The label V- (not -V) is on Pin4. It is the negative supply.
    My point is, if I use both Negative and Positive supply, the wiper of the pot is connected to the negative supply.
    Therefore I can use offset null. Section 4 of the datasheet explains this.

    So what can I do if I wish to Offset-null the op-amp when using a single supply?
    Can it be done at all?

    Regards

    FrancescoC
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,123
    3,048
    There's no such thing as a dual supply, there is only a supply of two voltages, one higher than the other. If you must, call your lowest voltage -5V and your highest voltage V+ minus 5. There, now you have a negative supply!
     
  8. #12

    Expert

    Nov 30, 2010
    16,298
    6,808
    "The LT1006 is the first precision single supply operational
    amplifier. Its design has been optimized for single supply
    operation with a full set of specifications at 5V."

    "Note 4:
    Optional offset nulling is accomplished with a potentiometer
    connected between the trim terminals and the wiper to V–."

    I don't understand what is so difficult about connecting the wiper of the pot to the most negative supply voltage which is connected to the chip.
    Why is this difficult for you?
     
    OBW0549 likes this.
  9. OBW0549

    Well-Known Member

    Mar 2, 2015
    1,310
    885
    You're making this a lot more complicated than it really is: for single-supply operation, the V- pin of the opamp is connected to circuit ground rather than to a negative supply voltage; therefore, simply connect the wiper of the offset nulling pot to ground since that is where the V- pin is connected.
     
  10. FrancescoC

    Thread Starter Member

    Nov 22, 2014
    30
    5
    Hi,
    I think I may have misunderstood something.
    I thought that to offset-null an op-amp a negative voltage supply is required.

    Are you telling me that this is not the case?
    I have not been able to see an example of offset-null with a single supply op-amp.

    Regards

    Regards

    FrancescoC
     
  11. OBW0549

    Well-Known Member

    Mar 2, 2015
    1,310
    885
    Yes, you have.

    Wrong.

    That is correct.

    When we operate an opamp from a single supply (i.e., without a negative supply voltage), all we are doing is connecting the opamp's negative supply pin to circuit ground-- that is, circuit ground IS the negative supply for the opamp. And therefore, in this case, that is where you connect the wiper of the offset trimpot.

    The reason you haven't seen any examples of this is most likely because it was assumed to be blatantly obvious, and therefore in no need of explanation.
     
  12. FrancescoC

    Thread Starter Member

    Nov 22, 2014
    30
    5
    Hi,
    Thank you for the straight answer.
    It is also blatantly obvious that I know very little about opamps!
    Thanks to everybody for the help.

    Regards

    FrancescoC
     
  13. dannyf

    Well-Known Member

    Sep 13, 2015
    1,811
    362
    That's generally true: as you typically don't know the direction of the offset.

    To put it another way, in a single supply circuit, you may not be able to fully null the offset.
     
  14. OBW0549

    Well-Known Member

    Mar 2, 2015
    1,310
    885
    That is simply not true. In all my 45+ years of doing analog circuit design, I've NEVER encountered any opamp, or opamp circuit, for which that is so. In all cases, such as this one, in which an opamp provides connections for an offset trim pot, the adjustment is adequate for nulling out even the worst-case offset, positive or negative, regardless of whether the opamp's V- terminal is connected to ground or to some negative supply voltage.

    For opamps that DO NOT provide offset trim pot connections, nulling offset externally can be a bit more tricky in single-supply circuits than in dual-supply circuits; but that's not the case here.
     
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