Op-amp comparator switching before threshold

Discussion in 'General Electronics Chat' started by msinger, Dec 23, 2009.

  1. msinger

    Thread Starter New Member

    Dec 23, 2009
    5
    0
    Problem: Op-amp comparator is switching before reference (threshold) voltage is reached.

    As far as I can tell the comparator (non-inverting, +5v single supply, tlc2274) takes the output of low pass filter and "digitizes" it. The reference voltage on the inverting input is a voltage divider formed by a potentiometer. The reference can be adjusted from +5 to 0v. The wierd thing in this circuit is a 2200pF cap connected between the output and inverting (reference) input. :confused: I only know enough to be dangerous in regards to op-amp circuits. Does anybody have an idea what it wrong with this circuit? What is the capacitor for?

    Thanks it advance,
    MikeS

    PS, U2B is the device reference on the schematic attached in reply #8.
     
    Last edited: Dec 23, 2009
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Please post a schematic, showing the application.
     
  3. MasterSnow

    Member

    Jan 18, 2009
    22
    0
    If I'm reading this correctly, seems to me that the 2200pF cap would turn the thing into a linear amplifier at higher frequencies. At what frequency are you switching the comparator?
     
  4. msinger

    Thread Starter New Member

    Dec 23, 2009
    5
    0
    Regarding frequency: the comparator receives a 250uS-1ms pulse (pulse width varies) every 65mS from the preceeding stage.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I don't mean to be overly fussy, but:
    1) It appears that you posted in .jpg format. .jpg is a "lossy" format; it makes schematics fuzzy. Please use .png format.
    2) The schematic is not complete; both sides are lopped off.
    3) The schematic is too small, making it hard to read. This might be alleviated if it were in .png format, but hard to tell at the moment.

    If your schematic capture program can't export in .png format, then
    A) Display your schematic on your screen as large as possible, yet showing the entire circuit.
    B) Press Ctrl+PrintScreen to capture the image in a buffer.
    C) Start MS Paint.
    D) Paste the image from the buffer (select Edit/Paste or hit Ctrl+V)
    E) File -> Save As...
    F) Give it a filename, and select .png format from the options below the filename.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Gosh, I still can't read it. :(
    I guess you used a compressed option? That made it very fuzzy. None of the values are readable.

    Try saving it as uncompressed in a higher resolution (say, 75bpi to 150bpi) as a .bmp, then load it using MS Paint, and save it as .png format.
     
  7. msinger

    Thread Starter New Member

    Dec 23, 2009
    5
    0
    I tried uncompressed bmp first, but it was too big. I PDF'd, but can't orient it and it is still a little fuzzy. Still working on it. Is the whole schematic needed? I am just trying to understand the comparator (or what is called a comparator in the schematic). It is driven by an op-amp and feeds into an op-amp. I will try to pdf directly from schematic capture progam and see if that is better.
     
  8. msinger

    Thread Starter New Member

    Dec 23, 2009
    5
    0
    This should do it.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    MUCH better!
    Unfortunately, I have company so no time to look at it at the moment.

    However, there are plenty of experienced folks on here.

    Seems like C31 is a bit on the small side; 0.1uF caps are pretty much the norm.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I started putting together a sim of just that portion of the circuit using Tina-TI 7.0.

    I don't know what your input signal looks like offhand, and really don't want to build the entire circuit.

    What's the amplitude, frequency and shape of the waveform?
     
Loading...