LM3915 question - Is it normal that I still measure 9mV on the input of my LM3915 n°2 after adding a

Discussion in 'General Electronics Chat' started by george.dyson, Jun 24, 2015.

  1. george.dyson

    Thread Starter New Member

    Jun 24, 2015
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    Hi Guys,

    I'm jumping in with my first post on this forum, because I have an issue with the LM3915 wich is very similar to lowprofile's. The difference is that I am using a very extended range bargraph except that the gain of my opamp is 39 dB instead of 30dB.

    I had the same problem, of input of my LM3915 n°2 floating high. I used a 100k pull down resistor between LM3915 n°2's input and groud wich solved the problem even though I still measure more or less 9mV on that input when it's not loaded. But, after the 39 dB opamp, those 9mV become enough to turn on the first led driven by the LM3915 n°1.

    My LM3915's are driven by the Precision Half Wave Rectifier circuit suggested in the datasheet. (attached)

    My questions is : Is it normal that I still measure 9mV on the input of my LM3915 n°2 after adding a 100kOhm pull down resistor ?

    Should I be bringing that pull down resistor value down to 30 or 10kOhms to prevent the first led from my "low levels bargraph" from turning on when OV or no signal is connected to the input, or should I also use a 100k pull down resistor on the input of LM3915 n°1.

    Thank you very much for your help to a newbie.
     
  2. Brownout

    Well-Known Member

    Jan 10, 2012
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    My gut says that should mitigate your issue to a large degree. The voltage you're seeing is most likely due to bias current for the input pin multiplied by the pulldown resistance. That is, assuming there are no mistakes in the circit construction.
     
  3. george.dyson

    Thread Starter New Member

    Jun 24, 2015
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    Thank you, I will probably go in that direction.
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    3915 max. input bias current is 100 nA; times 100 K = 10 mV. Also, the max input offset voltage is 10 mV. Either of these or some combination probably is the source of your error voltage.

    Datasheet Datasheet Datasheet page 3.

    "If you can read, you can cook." - REA

    ak
     
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  5. george.dyson

    Thread Starter New Member

    Jun 24, 2015
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    Thanks for that fast reply AnalogKid

    But I must admit, I am a newbie in electronics, and I don't know what Input Bias Current means...
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    The input to *any* electronic component only functions if there is current into or out of it. Opamps, logic gates, power transistors, whatever - no current, no function. For opamps this is called the input bias current. That current comes from or goes through external components, and creates a voltage drop across those components. So if the input bias current is 100 nA, and it is going through a 100K resistor to ground, that develops a voltage of 10 mV across the resistor.

    ak
     
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  7. Brownout

    Well-Known Member

    Jan 10, 2012
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    What is the frequency of your input signal?
     
  8. george.dyson

    Thread Starter New Member

    Jun 24, 2015
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    It's very low frequency. between 0 and 50Hz
     
  9. Brownout

    Well-Known Member

    Jan 10, 2012
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    Here's what the TI datasheet says about this circuit:


    So this issue was predictable. There are a couple things you can do about it, if you've tried lowering the resistor and still have the issue. You may try using an op-amp that has an offset null adjustment input. Typically, there would be two pins between which you would connect a variable resistor, the wiper connected to a power rail, and null out any input offset. This would be my recommendation. Go to any online supplier and do a parametric search of opamps specifying offset null adjustment. If you don't have offset null pins, you can add an external resistor, such as these shown on this page. Scroll about half way down and you will find several ways to connect the resistor, depending on what your'e doing. In some of these configurations, adjusting the offset will change the gain, so you'll need to readjust the gain, which will in turn change the offset, etc. Two or three iterations of adjusting the gain and offset should get it right. Here is the page:

    http://electronics.stackexchange.co...-of-op-amps-which-have-no-explicit-offset-nul
     
    Last edited: Jun 26, 2015
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  10. george.dyson

    Thread Starter New Member

    Jun 24, 2015
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    Thanks fort that excellent piece of information. It's very clear now.
     
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