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Unity Gain opamp configuration

lyle ogle
Prodigy 40 points
Other Parts Discussed in Thread: TLC279, TLC271

I am trying to use a TLC279 as a unity gain buffer for a current transformer since it needs a fairly high impedence as not to load  it down.  However with the input grounded on the amp I am getting a large offset voltage of up to 1 volt.  I did a previous breadboard with the TLC271 single opamp and it work great with much lower offset.  I picked a quad package to make it simpler.

I have some but I ask the you to see what would be the best since I am not an analog person.

Any ideas? 

 

Regards,

 

Lyle

  • 0
    TI__Guru* 93105 points

    Hello Lyle,

    Would you be able to share your TLC279 application schematic with us? That would provide helpful information.

    You mention the output drives a current transformer. What potential does the transformer primary terminate to? What sort of resistance does the winding present? Although I haven't seen your circuit yet you may want to check the output with an oscilloscope to make sure the circuit is stable. If it isn't odd DC voltages may be present.

    Regards, Thomas

    PA - Linear Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 40 points

    Thomas thanks for the response.  We are feeding the opamp + input with a split core current transformer to measure current.  It is a 0 to 5vdc signal.  The circuit is a simple unity gain configuration input is on the plus input minus connects to the output.  in on 3 ,2 goes to 1 and 1 is the oputput.

     

    Output is stable

     

    How do you paste from a schematic editor into here?

     

  • 0 in reply to lyle ogle
    TI__Guru* 93105 points

    Hi Lyle,

    You can provide the circuit as an attachment by selecting the "Options" tab which resides along with the "Compose" and "Preview" tabs. The file can be a PowerPoint slide, or something as simple as a bitmap file from Paint. Many different file extensions are accepted.

    Or, an image can be pasted into the area where you type your text. You will note that when you are writing a message there is a "Description:" title, and then the block of icons directly below it. Select the small icon in the second row, near the end, that looks like a white box with a green image inside. I have no idea what the icon is supposed to be, but if you place your pointer over it, it reads "Insert image." Click on it and it leads to place where you insert the file where your image file resides. After you go through the process your image will appear along with your text.

    Thomas

  • 0 in reply to Thomas Kuehl
    Prodigy 40 points

    Here you go, very simple

     

     

    thanks,

    lyle

  • 0 in reply to lyle ogle
    TI__Guru* 93105 points

    Hi Lyle,

    The buffer circuit connection looks straight forward, so that generates more questions. Does the circuit function at all as the input moves from 0 to 5V?Is the offset present at all levels of input common-mode voltage (CMV), or just a portion of the input CMV range? What is the output load that the TLC279 drives? What sort of power supply decoupling is being used on the supply pins? Are the other sections of the TLC291 properly connected when you test the buffer circuit? Have you checked the other output of the other 3 amplifier sections to make sure they are stable?

    Thomas

  • 0 in reply to Thomas Kuehl
    Prodigy 40 points

    Thomas,

    I am using a bench supply and it is bypassed pretty good.  The inputs of all used stages have a 15meg input pulling them down,  the last stage has a pin pulled to supply to keep it stable.  As I vary the input and it passes the Delta the output follows the input great (no offset).  It is just the offset with no or little input that is causing some issues.  I can traverse 0 to 8 volts and the amp performs as expected once we get past the offset.  Presently the output is open circuit

    I can hook up a TLC271 which is a single opamp not quite the same thing a little more to it.  It works great, I want one package :(

     

    You can't get any more simple for sure.

     

    Thanks,

     

    Lyle

     

    P.S. Just put a 90k load on the output and it zero'd out for now.  Thanks a bunch. I was ignoring it was not loaded.  Good way to end a Friday will get back and make sure i was seeing what I on Monday...... have a good one!