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LM358 voltage converter

Discussion in 'Electronic Basics' started by Ricardo Fernandes, Apr 20, 2004.

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  1. Hi there I need an advice please! I'm doing a very simple voltage
    converter ie, I have an output signal 0-20v from a external detector,
    and I want to analize that signal in an ADC, but the input of the ADC
    is 0-2.5v. I did an voltage divider and then I puted an LM358 like
    this

    0-20V------
    |
    __ +---------+
    || | +15v |
    56k || | |-\| |
    -- +-|- \ |
    | | )---+--- Output (0 - 2.5V)
    +----|+ /
    -- |_/| LM358
    || |
    8k || |
    -- gnd
    |
    gnd

    This simple circuit is powered by a tranformer wich has 1 output 0V
    and +15V.
    I've tested and I cannot get the desired output. I've tested with a
    breadboard, then the 0-20V input I simulated with a function generator
    (sine wave), I powered the AMPOP with vcc=+15v and vee=0v. When I
    watch the output on the osciloscope I don't get get sine wave reduce
    in amplitude, what is wrong????
    Please help me I need this to a project, and when I studied
    electronics class a looong time ago, I only did with and 741

    PS: I've cheked the pads of the 358 and its all ok

    Thanks I advanced
     
  2. CFoley1064

    CFoley1064 Guest

    Subject: LM358 voltage converter
    Hi, Ricardo. The LM358 is a good choice here, if you don't mind a couple of
    millivolts offset, and not being able to get within more than a couple of
    millivolts of GND. Given that...

    1) Check your pinouts:

    LM358 Pinout
    1. Out 1
    2. Inverting Input 1 (-)
    3. Non-invering input 1 (+)
    4. GND
    5. Non-inverting input 2 (+)
    6. INverting input 2 (-)
    7. Out 2
    8. Vcc

    2) Recheck your wiring

    3) Make sure you have the positive side of the input signal connected to the
    voltage divider, and the negative connected to GND

    4) Make sure there are no "sneak paths" for current to GND or other voltage
    problems. Look at the

    5) Check your wiring

    If none of these work, you may have a dud LM358. You're on the right track,
    and once you work out the problem, you should be OK. (By the way, set up the
    other LM358 as a voltage follower off of the output signal of the first one, if
    you're not using it for anything else. By the way, if you're using a PC ADC
    card, your signal probably has to be GND referenced. That may also be a source
    of problems.)

    Good luck
    Chris
     
  3. CFoley1064

    CFoley1064 Guest

    Subject: LM358 voltage converter


    Hi, Ricardo. The LM358 is a good choice here, if you don't mind a couple
    ofymillivolts offset, and not being able to get within more than a couple
    ofymillivolts of GND. Given that...

    1) Check your pinouts:

    LM358 Pinout
    1. Out 1
    2. Inverting Input 1 (-)
    3. Non-invering input 1 (+)
    4. GND
    5. Non-inverting input 2 (+)
    6. INverting input 2 (-)
    7. Out 2
    8. Vcc

    2) Recheck your wiring

    3) Make sure you have the positive side of the input signal connected to the
    voltage divider, and the negative connected to GND

    4) Make sure there are no "sneak paths" for current to GND or other voltage
    problems.

    5) Check your wiring

    If none of these work, you may have a dud LM358. You're on the right track,
    and once you work out the problem, you should be OK. (By the way, set up the
    other LM358 as a voltage follower off of the output signal of the first one, if
    you're not using it for anything else. By the way, if you're using a PC ADC
    card, your signal probably has to be GND referenced. That may also be a source
    of problems. See if your circuit works if it's not connected to the ADC.)

    Good luck
    Chris
     

  4. Greetings.

    Although the LM358 claims to be able to swing the output all the way to
    ground level, this is not the whole picture. For output voltages less than
    around one diode drop the output cannot sink much current at all (look at
    the graph titled "Output Characteristics Current Sinking" in the datasheet).
    IIRC the device is limited to sinking about the same amount of current as a
    roughly 10k ohm pull down resistor. This is obviously pitiful when the
    voltage gets too low.

    On method of substantially improving its current sinking capability at low
    output voltage would be to place a low value pull down resistor directly on
    the output pin to ground. Something like 470 ohms might be a good choice in
    your application.

    You say you are inputting 0-20V sine wave into the circuit with your
    function generator. Did make sure to include a large DC offset on your sine
    wave so that the input voltage (to the resistive divider) only swings from
    0-20V? Ordinary sine waves of course swing both positive and negative, and
    the op-amp will not to be able to properly handle the negative voltage
    excursion. This could certainly cause unanticipated op-amp output.

    Also, what frequency is your input signal. Hopefully your signal is well
    under one megahertz to avoid problems with the op-amps's gain-bandwidth
    product and maximum slew rate capability.

    What kind of load do you have hooked up to the output? Op-amps in general
    don't usually like having significantly capacitive loads hooked directly to
    their outputs. In these cases a simple resistor in series with the
    capacitor will often help. The resistor doesn't usually need to be too
    large, something like 100 ohms is probably adequate in many instances. I
    mention this since some ADCs have significantly capacitive inputs. So if
    the load is more than say a few picofarads try adding that 100 ohm or so
    output resistor in series.

    Otherwise your basic circuit concept looks fine and doing what you propose
    should work.
     
  5. Fred Stevens

    Fred Stevens Guest


    Ricardo,

    Are you talking about AC signals or DC voltages? You are running your
    op-amp with a single polarity supply and your function generator test
    producing the sine will pull the + input of the op amp below its
    negative supply which is at ground. Please clarify so that we can help
    you.

    Fred.
     
  6. Jamie

    Jamie Guest

    test the voltage on the + input.
    it should be what your looking for at the output.
     
  7. Hi There and many thanks to all of you that gave me a little of your
    time, yes the circuit is allright now, in fact the problem was with
    the function generator it did not supply any dc offset so the voltage
    was from -20 to +20 suppid me, and as I'm working with low freq 0 to
    10 Hz I 've read the data sheet, I think I'll not have any further
    problem. Many thanks again.

    By the way this is a common problem in Portugal, we all learn
    theorical electronics and stuff but when we get to the real world,
    these kind of things happen. Thanks again

    R. Fernandes
     
  8. Fred Stevens

    Fred Stevens Guest

    By the way this is a common problem in Portugal, we all learn

    I don't think it has anything to do with Portugal, it is a worldwide
    phenomenon (not really a problem) - we all need a period of
    "apprenticeship
    training" after doing the theoretical stuff at university to help the
    ideas sink in.

    Fred.
     
  9. Soeren

    Soeren Guest

    Hi Ricardo,

    I just wonder... Why use anything more than the potential divider ?
    K.I.S.S.
     
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