Non-inveting op-amp

Discussion in 'General Electronics Chat' started by Dritech, Apr 5, 2014.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Hi all,

    I am using a simulation software to test a three input summing amplifier using the OPA337PA IC. As Rf I am using 2.4KΩ and 1.2KΩ as R1 (i.e. gain = 3, to eliminate the average function). The circuit is working fine for output less than 4V, but for output voltages beyond 4V it is clamping at the same voltage (4V). Why is this happening please?

    EDIT: When increasing the op-amp supply to 6V, the circuit is working as expected. So why is it not working correctly at 5V? Isn't this IC a rail-to-rail op-amp?
     
  2. ericgibbs

    Senior Member

    Jan 29, 2010
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    In simulation, is it possible that the sim OPA337 Model is at fault.?
     
  3. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Hi, in the part description it is not labeled ad a rail-to-rail op-amp :confused:
     
  4. ericgibbs

    Senior Member

    Jan 29, 2010
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    Yes in the manufacturers datasheet for the device, BUT you say you are running a simulation, thats why I queried the sim model.

    What load do you have on the output of the OPA.?
     
  5. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
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    What exactly is the sim model please? When selecting the part, I chose OPA377PA.

    The output is connected directly to the LM311 voltage comparator.
     
  6. tindel

    Active Member

    Sep 16, 2012
    568
    193
    That OP amp output voltage is pretty lousy over load current (see the graph in the lower right corner of sheet 6 of the data sheet).

    Sounds like you're driving a high impedance load though... So my guess is that the spice model sucks... Not unheard of... Try duplicating the afore mentioned graph and see if it's anywhere close to similar.
     
    Last edited: Apr 5, 2014
  7. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Hi again,

    When I replaced the OPA337PA IC with the OPA340PA IC, everything worked fine. What is exactly the difference between the two ICs? or is it a simulation problem?
     
  8. kdillinger

    Active Member

    Jul 26, 2009
    141
    3
    I can run a DC sweep of the input voltage for the opa337 and with a gain of 3, +5V supply, and an open load the output pegs at 4.66V.

    Not exactly what one would expect from the datasheet, but this could be an older model.
     
  9. ericgibbs

    Senior Member

    Jan 29, 2010
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    I would say its the OPA337 model thats the problem.

    Just working with another thread using a 2N3055 for Ic/Ib testing, the results were useless, so tried an alternate model 2N3055H, it works fine..

    So all LTS models are not born equal..;)
     
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Your feedback resistors are very low values for that opamp. Try using 100k and 200k instead of 1.2k and 2.4k. You should then get within about 0.1v of your +V.
     
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  11. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5

    That was the problem. Thanks :) How did you get to those values and to the 0.1V accuracy please? Is it mentioned in the datasheet that the resistors should be of a large value?

    Another quick question. As the input resistors of the summing amplifier, I am using 22Kohms resistors (one for each input). Can that cause problems or should I choose lower value resistors?

    Thanks a lot.
     
    Last edited: Apr 6, 2014
  12. ericgibbs

    Senior Member

    Jan 29, 2010
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    You were told a few days ago in your other thread on the 'summer', that the resistor values for your circuit were far too low.

    I did also post that the datasheet recommends a value of 100K for the non inverting input resistors and source impedance of 1K within 1% in order for the summer to give accurate results.
     
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  13. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Hi,

    The problem is that I am varying the input voltages using 200ohms potentiometers (potential divider). Hence, if I select the input resistors at 1K, the potentiometer values will effect the total input resistance. On the other hand, when using values such as 22K, the small resistance of the pots will not have any effect on the total resistance.

    How can I solve this problem please?
     
  14. ericgibbs

    Senior Member

    Jan 29, 2010
    2,499
    380
    hi,
    If you want precision, all the 3 inputs must have the same constant impedance/resistance.

    One way to achieve this is to use an OPA buffer for each Pot input

    EDIT:
    Re-read your post
    The Input summing resistors are recommended to be 100K, its the Source resistances that needs to be 1K 1%
     
    Last edited: Apr 6, 2014
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