Project - 741 Op Amp Tester

Discussion in 'The Projects Forum' started by MJaske, May 5, 2013.

  1. MJaske

    Thread Starter New Member

    May 5, 2013
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    I am workinga handheld device that will allow the user to test a 741 Op Amp using error budget analysis to test the DC imperfections of the Op Amp.

    The op amp will plug into the device and the user will click a button then a microcontroller will test the DC values of the DUT (device under testing). If the DC value out the output of the DUT is outside of a ±range then the DUT fails the test. In the range then it passes the test. This pass or fail is given on a display screen.

    The expected output value of an ideal DUT is 10.25 V. A passing value is ±50 mV of the ideal value (Or whatever value I choose for the project).

    Issues:
    (1) If the ±15 V power supply to the Op Amp varies at all, the output value varies.
    (2) The input power of the DUT must be constant at 2 V
    (3) I'm worried about power from the batteries dropping over time, this will change the value of (1) and (2). A constant stable power supply is needed.


    Goal: I need to run the Op Amp, microcontroller, and display using batteries. The inputs of the Op Amp must be stable and the same for every test.


    Can someome point me into the right direction of what I should be researching as a solution. I've looked into using the LM78XX and LM79XX voltage regulators. They seem to work well keep a stable power supply to the DUT circuit.

    I have also been looking into DC-DC converters, I would like to avoid having an on board power supply of 40 V from DC batteries...

    Their has to be a simpler solution than how I am going about things. This is for a senior design project in college. I'm still a student so my education of what is used out in the field is limted.

    I've enclosed a circuit that is working in multisim, I messed with it a lot last night so its a crap design so far.

    Special Thanks,
    Matt
     
  2. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    Do not use a LM7805 as a voltage reference for a precision circuit. You will never get the results you desire. The output voltage tolerance is as much as +/- 250mV. And it is unadjustible.

    Consider a reference voltage IC that is based on a band-gap reference. This will improve accuracy drastically. The more money you pay, the more accurate the reference will be.

    You can get one as a fixed voltage, or adjustable. Adjustable is nice in your application assuming you have a descent DVM to calibrate your project, but remember that there will always be temperature drift which you will need to account for.

    Is this project for fun? Or does it have some practical use? If for fun, it'll be a nice learning experience for you.

    If you *actually* need to sort 741s based on performance for another project (i.e. you have a production line with failures you are trying to eliminate through inspection), this is the wrong approach. Generally, it is better to determine why your circuit is intolerant of the variations from IC to IC, and either improve the overall circuit to allow the variations, or change to another amp with tighter specs.

    Have fun.
     
  3. joeyd999

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    Jun 6, 2011
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    Also, look at your schematic carefully. You are missing a ground node. I'll leave it as an exercise for you to find it.
     
  4. MJaske

    Thread Starter New Member

    May 5, 2013
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    Thank you for your response. The project is for my senior design project, so not really for fun, but it is fun at the same time.

    I'll be making a portable testing device for students to check their 741 before they begin a lab. So the device will be for educational purposes you can say. Our school uses the 741 for many of its labs and students routine connect the 741 incorrectly and ruin many op amps. Having a quick way to test the Op Amp before you begin a lab is very practicle for the school.

    The 741 you see in the circuit is actually the op amp being test.

    The error budget analysis ill be using is from this book, I've attached a picture.

    The project is assigned to me by an advisor. My hands are tied to testing the 741 and if time permitts I will be able to add more IC's if I can to be tested.
     
  5. MJaske

    Thread Starter New Member

    May 5, 2013
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    If you are referring to Vout, Vout will actually go to a microcontroller with a built in A/D converter.

    I had been radomly changing the values of everything and experimenting as much as possible. I must have ran a transient analysis on Vout 200 times last night.
     
  6. ScottWang

    Moderator

    Aug 23, 2012
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    5V to 24V DC Variable Regulator by 7805.

    7805 Datasheet, there is a formular at Page 22 for voltage adjustment.
     
  7. joeyd999

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    Jun 6, 2011
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    If you really wish to be thorough in your testing, take a look at some of the various 741 manufacturer's datasheets. Many times they include schematics of the test circuits that they use to qualify the parts.
     
  8. joeyd999

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  9. MJaske

    Thread Starter New Member

    May 5, 2013
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    If stability is in question, its probably not stable enough. It sounds like a fun read in any case! :)
     
  10. MJaske

    Thread Starter New Member

    May 5, 2013
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    I havent found any on the data sheets I have on hand.

    Their are many test circuits on the net using a simple "light an LED" method. Never found one using error budget analysis.

    The circuit I make will need precision and will be printed onto PCB. Ill be making an enclosure device for the device as well. Ill be making everything from design to finished product.
     
  11. joeyd999

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    Dang, you're right. Just perused a bunch of them myself, and I couldn't find any. Not sure if they once published them for the 741 and don't any more, or if they never did. Sorry.
     
  12. MJaske

    Thread Starter New Member

    May 5, 2013
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    haha, believe me i checked!!! :)

    My teacher wants me to use the circuit in the picture provided from that book. He does not care about testing each individual paramter (CMRR, Ib, ect..). Just pass or fail based off the DC output of the D.U.T.

    So the circuit has been provided. He told me the real part of the project is building of the precision circuit. If I loose lets say 30 mV in total in my circuit design, then good 741's will fail the test even though they should have passed.

    I've been looking at many 741 tesing circuits. Almost all of them use a wall outlet for the power supply, not battieries. Using batteries and keeping a stable and exact DC power supply and the physical building of the circuit is the real project at hand.

    The circuit in the book he said I can use, in fact he even said use the same resistor values if I want.

    Sounded so simple when I received the project, quickly realized it is not lol.

    I have to present this project and have it finished in by the end of the fall semester. I of course want to use all summer to "get ahead" as much as possible. I am really enjoying the concept of this project because it will help the new incoming students with all of their labs and of course if I fail, every EE major in the school will know who made the IC tester that didn't work lol.
     
  13. ScottWang

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    Aug 23, 2012
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    Why don't you just using LM317 and LM337 to replace 7809 and 7909, and the LM317 to replace 7805, they are adjustable, so you can adjust the voltage to reach your goal.

    If you want to measure the op amp more precisely as you using the 1% resistors, the dual power source for op amp must be equal to each other and more precision.

    If the power source is not correct, how can you expect the tester is a good tester.
     
  14. MJaske

    Thread Starter New Member

    May 5, 2013
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    I will look up the two devices you listed right now.

    I used the 7805/7905 because that is what I could find in multisim at the moment. Overall I'm not sure if I'm approaching the power supply issue correctly. I will have to power a microcontroller and a display as well from the same battery source.

    You are correct the power source must be precise every test run, the circuit will depend on it.
     
  15. joeyd999

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    Using a generic voltage regulator as a voltage reference, in this kind of application, is not a good solution, adjustable or not. The LM317 has 1% time and temp stability. This is equivalent 100mV potential drift at 5V (ignoring the tempco and stability of the resistors and adjustment pot.).

    By comparison, an LM4041 adjustable reference has about 100ppm time and temperature stability, or 100 times better! Remember, he is trying to qualify parts based on a few tens of millivolts difference.
     
    ScottWang likes this.
  16. MJaske

    Thread Starter New Member

    May 5, 2013
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    in bold you said it.

    Honestly why I love this project. Starting from almost scratch, building everything myself. So much to learn.

    The projects all the other EE's had assigned are honestly crap lol. They are teamed up with ME's and IE's to build improvements for something they already have or the EE plays a small role in an ME project. I have the only true EE project and I'm riding solo on this.
     
  17. ScottWang

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    Aug 23, 2012
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    You may see here for LM317 and here for LM337, it might help.
     
  18. MJaske

    Thread Starter New Member

    May 5, 2013
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    Willing to bet it is my student version of multisim.

    Installing the corporate pro version now.
     
  19. tubeguy

    Well-Known Member

    Nov 3, 2012
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    joeyd999 makes an excellent point. You should use a precision reference for the 2v reference voltage if you want to achieve the accuracy specified.
     
  20. Ramussons

    Active Member

    May 3, 2013
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    I'm puzzled by the ckt shown in the thumbnail. All the 3 pin regulators have a "ground". Just where's the power supply ground?

    Ramesh
     
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