Unexpected 741 Op Amp in simulator and unsure of cause

Discussion in 'The Projects Forum' started by tjayh913, Jun 20, 2012.

  1. tjayh913

    Thread Starter New Member

    Jun 20, 2012
    3
    0
    I have randomly assigned myself the project of designing an analog to digital converter out of fairly basic parts. My envisioned design will involve a digital to analog converter as a subcomponent first, so I am current working on that. I am current in the early stages and am attempting to use a unity gain 741 as a buffer to my voltage dividers. However, when I attempt to simulate it in multisim, the 741 is outputting around 800mV when it should not (Figure 1). When I did further analysis (figure 2), I found the very behavior of the op amp to be very odd. Can anyone find a flaw in my design or some non ideal 741 behavior that I was not aware of that would explain this?

    Figure 1


    [​IMG]
    Figure 2
    [​IMG]
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,446
    3,361
    A 741 opamp needs more than +5V to operate.
    Typically, it requires a dual power supply, +15V and -15V.
     
  3. tjayh913

    Thread Starter New Member

    Jun 20, 2012
    3
    0
    Thank you. I gave its own -15V and 15V power supply and it now only outputs a more reasonable 500uV.
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    A 741 is an ancient op amp design with poor AC and DC characteristics. You should use something more modern.

    If you want to use just a 5V single supply you can use a rail-to-rail type op amp that is designed to operate from 5V.
     
  5. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
    is LM358 OK as standard OpAmp these days? I never tried it at 5 volts...and yes it can work from single supply.
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,127
    3,048
    No such thing as standard, but yes, the LM358 is pretty good (can sense the negative rail with single supply) as long as you don't need high frequency, over 2kHz or so. I don't recall the exact limit, but if it matters, you have to look it up anyway. I use the LM358 as my general purpose op-amp because I bought a small pile of them a while back. Cheap and easy to find.
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    The LM358 is ok as a general purpose op amp as long as you don't need frequency response above a few kHz (GBW <1MHz), or low DC offset, or have the output go completely to the positive rail (the output will go to ground).
     
  8. tjayh913

    Thread Starter New Member

    Jun 20, 2012
    3
    0
    Couldn't you actually run it closer to a megahertz at unity gain? My design is based on using the digital to analog converter to match the input every sample, so to be accurate this must all happen in a small fraction of the time beftween samples. (So it might run at 10MHz, but only do pulses of say 25 cycles every 20kHz).

    In a rough form of psuedocode it would work something like this:
     
    Last edited: Jun 22, 2012
  9. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    Depending upon the supply voltage (which somewhat affects the GBW) the response would be about 3dB down near 1 MHz. In your case the settling time is likely of interest which can be found on page 7 of the data sheet. The follower large signal response is 5-7μs to settle and the follower small signal respose is about 2.5μs to settle.
     
Loading...