problem in op-amp subtracting circuit

Discussion in 'General Electronics Chat' started by yehdev_cc, Feb 20, 2011.

  1. yehdev_cc

    Thread Starter New Member

    Jul 25, 2010
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    hi,
    I tried to design a simple circuit to interface analog signals (either 0-10 V or 4-20 mA) to a microcontroller A/D converter, the circuit uses op-amp to subtract and amplify the two-wired signal terminals as shown in the figure in the attachments.

    Simulations using 'NI Multisim' were successful, however, practical real samples of op-amp response values were not as successful :

    Vo V+ V-
    ----------------------------
    3.98 - 2.783 - 2.765
    2.99 - 2.2 - 2.081
    2.49 - 1.93 - 1.74
    2.01 - 1.682 - 1.4
    1.5 - 1.243 - 1.066

    1.44 - 0.9 - 1.0
    2.13 - 0.8 - 1.48
    9.08 - 0.778 - 6.27


    as you can see, it looks like that op-amp feedback mechanism can not 'follow' the changes applied to V+ successfully after certain level, and it started to diverge instead :confused: ... any clue ?
    thanks in advance ...
     
    Last edited: Mar 2, 2011
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    If the input voltages of the op-amp are driven beyond acceptable range for the given IC, the circuit will not work properly.

    Your figures seem to show input voltages below 0V, which is almost certainly not OK for an op-amp with only a single positive supply. It is possible that dual (positive and negative) supplies would resolve this issue. To determine the acceptable range, consult the IC data-sheet.

    I cannot see how this would happen with the schematic shown - is there some difference between the schematic and the practical application?
     
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  3. Audioguru

    New Member

    Dec 20, 2007
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    The TL071 and most other opamps have inputs that do not work when they are within a few volts (within 4V on the datasheet) from the positive or negative supply voltage. It is called the allowed input common-mode voltage range and your input voltages are too low.
    Therefore their inputs are usually biased at half the supply voltage or a dual-polarity supply is used.

    Additionally, the TL071 and other older opamps with FET inputs have a condition called "Opamp Phase Inversion" where the output suddenly goes high if an input voltage becomes within a few volts from the negative supply voltage like in your circuit.
     
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  4. yehdev_cc

    Thread Starter New Member

    Jul 25, 2010
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    thank you for the replies.

    I didn't get any voltages below 0V, when would this show up ?
    and the answer is Not really, there is no serious differences between the circuit and the schematic, except for resistors' values.

    it's the first time I've ever read about this effect ... thank you for pointing it out ... but isn't TL071 a rail-to-rail op-amp ?

    and so, the best suggested solution is to use negative voltage supply for the op-amp instead of ground(0V), isn't it ?
    are there other solutions that can be considered in case negative power supplies are not available ?
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    No.
    The datasheet shows that the inputs might not work properly if their voltage is within 4V from the negative supply voltage. Your input voltages are very close to 0V which is the negative supply voltage.
    The datasheet also shows that the outputs do not get less than 1.2V from the positive or negative supply voltage.

    Simply bias the inputs at half the supply voltage with 2 resistors and a filter capacitor, and use coupling capacitors. But then the output will swing a few volts above and a few volts less than half the supply voltage that is good for audio.
     
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  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    No, the TL07x and TL08x family opamps are not rail-to-rail, or even close to it.

    The common-mode input range I use for that family is +V-1.5v to -V+3v. So, if you have a single 12v supply, the opamp can only "see" inputs that are in a range of ~3v to ~10.5v.

    Try replacing the TL071 opamp with an LM358 or LM324 opamp. They are old and slow, but have a common-mode input range that can "see" within a few millivolts of ground. Either that, or use a negative supply that is at least 3v more negative than your input signal.
     
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  7. nigelwright7557

    Senior Member

    May 10, 2008
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    You need a rail to rail op amp.
     
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  8. yehdev_cc

    Thread Starter New Member

    Jul 25, 2010
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    Thank you very much, I'll try either LM358 or LM324, they're more applicable -in my project - than using negative supply or biasing the voltages.
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    The LM358 and LM324 have crossover distortion and operate poorly above 2kHz. They have crossover distortion.

    An MC34071 single opamp, MC34072 dual or MC34074 quad opamp has the same 0V input and output features and operation with a single supply as low as 3V as the LM358 and LM324 but has no crossover distortion and operates well at frequencies as high as 100kHz.
     
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  10. yehdev_cc

    Thread Starter New Member

    Jul 25, 2010
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    Thank you very much 'Audioguru', I tried many op-amp's, but the one you suggested gave the best results, although i wouldn't find it easily around here.

    just another question, where exactly in the datasheet can I check if the op-amp does give the required voltage levels ? when I checked the 33078 datasheet, it looked like that it might works fine for its voltage ranges, but it didn't when it was put down to practical test .. !
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    The datasheet for the MC33078 lists that its minimum power supply is plus and minus 5V or a total of 10V.
    Its minimum output voltage is listed as plus and minus 13.2V into 2k ohms when it has a plus and minus 15V supply.
    It has a graph (figure 8) that shows its typical output saturation voltage loss at various load resistances and at various temperatures when its supply is plus and minus 15V.
    It has a graph (figure 15)that shows its typical max output voltage with loads of 2k and 10k at various supply voltages.
    It shows a graph (figure 16) that shows its max output swing at various frequencies.

    What problem did you have with it?
     
  12. yehdev_cc

    Thread Starter New Member

    Jul 25, 2010
    23
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    Thank you very much for the reply ...

    Quoting the following from my first post :
    I need to get an output voltage below 1.4v to make the voltage represent the current in a leaner manner, but it doesn't, rather it starts to rise again.
    MC33078 does go to about 1.2v, and then starts to rise again when the non-inverting input continues to decrease.
    I checked the datasheet now, and I found that it should be predictable, please find the figures in the attachments.

    but for the MC34074, according to the saturation graphs, if I draw a very little current (very high input impedance of an ADC), I should get a an output voltage above 0v with few millivolts, do I get it right ?
    (I attached the datasheet I used for the MC33078, because I couldn't find saturation graphs in it, does it have any ?)
     
  13. Audioguru

    New Member

    Dec 20, 2007
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    The output of the MC33078 goes down to about +1.2V then begins to rise when the input voltage goes lower because its input common mode voltage range does not go lower than +1V or +2V above its negative supply because then the inputs stop working.

    The inputs of the MC3407x work perfectly down to 0V and its output typically goes down to +0.1V with no load and down to +60mV with a 100 ohm load to its negative supply.
     
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