op amp question

Discussion in 'General Electronics Chat' started by tpny, Oct 2, 2012.

  1. tpny

    Thread Starter Member

    May 6, 2012
    216
    0
    why does this pull my Vin to ground at the inverting input (Vin = 1V, voltmeter reading at inverting input = 0V).

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    5. [FONT=Arial]Vin --+-----|- \       |[/FONT]
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    but this doesn't (Vin = 1V, voltmeter reading at inverting input = 1V).
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    2. [FONT=Arial]                        1k[/FONT]
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    7. [FONT=Arial]Vin ---/\/\/\--+------|- \       |[/FONT]
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  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The 1st opamp is a follower. Its input is the (+) input of the opamp, not the (-) input which connects to the output of the opamp.
    Its output voltage is exactly the same as its (+) input voltage.

    The 2nd opamp is an inverter. If the input is +1V then the output is -1V. If the input is -1V then the output is +1V.

    The resistor values in the 2nd circuit are too low at only 1k ohms. Use 10k or 100k ohms instead.
     
  3. tpny

    Thread Starter Member

    May 6, 2012
    216
    0
    ok thanks!

    1. So for the follower, I don't need a ground reference anywhere? (inverting input tied to output, Vin tied to noninverting input..)

    2. For the inverter, why is 1k too small? (My Vin is a sensor signal with output range 0~1V and internal impedance is about 7 ohms.) I chose 1k because I was trying to achieve higher snr (to prevent noise getting amplified too, output of this inverter is then fed to 2nd inverter with gain of 5).
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Correct but the middle of the positive and negative power supply is ground.

    If the input voltage is 15V then the output current of the opamp in the feedback resistor is trying to be 15mA PLUS load current. Most opamps cannot deliver an output current so high.

    But now you say your input voltage is only 1V then the current is only 1mA which is fine.
     
  5. tpny

    Thread Starter Member

    May 6, 2012
    216
    0
    OK I see. Thanks!

    In preparation for adc input:

    1. A sensor has 1V signal and I need 5x amplification, is it better to use 2 inverters (with gains of 1 and 5) or 1 non-inverter (with gain of 5)?

    2. A differnt sensor has a 5V signal and I don't need amplification, can I (a) tie it straight to adc input, (b) feed it thru 1 follower, or (c) feed it thru 2 unity gain inverters
     
  6. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    A single non-inverting opamp can easily have a gain of 5.
    The 5V sensor can feed directly into the 5V ADC if its output never goes higher than 5V.

    The low impedance sensor might damage the input of the opamp or ADC if the opamp or ADC has its power supply turned off but the sensor has voltage.
     
  7. tpny

    Thread Starter Member

    May 6, 2012
    216
    0
    Do you mean if there is no resistor in front of an opamp input, the current entering into the opamp is not limited, thusly there might be damage?

    What about opamp inputs being high impedance by definition, would that limit the incoming current irrespective of the resistor size in front of it?
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    Simply read the absolute maximum allowed input voltage of the opamp you are using on its datasheet. Most opamps have it as its power supply voltage so if its power supply is turned off then it should not have an input signal level greater than zero.
     
  9. tpny

    Thread Starter Member

    May 6, 2012
    216
    0
    given the choice of putting a resistor between Vin and opamp input and direct feed, is it always better to use direct feed?

    More specifically does adding a resistor create more noise (noise converted to voltage by resistor), lower input impedance (resistor size becomes new impedance)?

    In that case a follower is always better than 2 unity gain inverting opamps in sequence? Thank you!
     
  10. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Yes, the resistors in an inverting opamp circuit add noise from the resistors.
    A follower does not have those noisy resistors.

    Metal film resistors produce less noise than carbon film resistors.
     
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