amplification problem

Discussion in 'The Projects Forum' started by mah, Oct 2, 2013.

  1. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
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    i want to amplify dc mv aignal from irradiation sensor using ua741 but i have single supply so i connected the negative supply to ground and i used virtual ground , but when i connect the sensor to the input of op amp and measure the signal when connected it gave .5 volt however it should be 5.5 mv when disconnected , i attached the connection drawing. please help
     
  2. LvW

    Active Member

    Jun 13, 2013
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    Are you aware that the dc reference Vcc/2 appears at the output as a dc level of G*Vcc/2 ? G is the non-inv. gain (1+R2/R1).
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    If Audioguru were still here he would promptly tell you that the 741 opamp is out of date.
    Use a more modern rail-to-rail single supply opamp.
     
  4. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
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    if Vcc/2 appears in the output what will be the total output of the op amp?
     
  5. LvW

    Active Member

    Jun 13, 2013
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    Please read my post again: The output is Vcc/2 multilplied with G.

    It is normal practice for single-supply operation that the dc gain G(dc) is set to unity.
    Only in this case the output dc voltage is identical to the reference voltage applied at the pos. input terminal.
    Mostly, this is accomplished using an input capacitor in series with R1.
    However, this does not work for dc signals.
    Question: As you "want to amplify a dc mV signal" - do you intend to use a non-inverting or an inverting amplifier?
    You should better specify your requirements.
     
  6. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
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    i need to use non inverting terminal , will it make difference ?
    i know that i used inverting terminal here ,because of the virtual ground .
     
  7. LvW

    Active Member

    Jun 13, 2013
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    In principle, you can use single supply operation (using virtual ground) for both schemes: inverting and non-inverting.
    However, I repeat: What are your requirements in detail? In particular, what is the polarity of the dc input signal (pos, neg, both) and what gain is required?
     
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,006
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    Give a complete schematic with
    - resistor values,
    - power supply voltage
    - expected input voltage range delivered to the op amp from your sensor.
     
  9. WBahn

    Moderator

    Mar 31, 2012
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    The information you are providing makes little sense. You talk about what it does when the signal is connected and the indicate that this doesn't match your expectations of what it should do when disconnected.

    How are we supposed to make heads or tails out of what you are saying?

    As already pointed out, the 741 is a poor choice for this application. Not only because it is ancient, but because it is not intended for single supply operation and can only get within 2V or 3V of the supply rails. So you have to play games like providing a reference voltage. This is fine, but it has consequences. In particular, your mV scale input signal needs to be referenced to this same voltage, which it almost certainly is not.

    Instead of discussing the particulars of one specific possible solution that you have latched onto, let's take a step back and talk about what you NEED in terms of the PROBLEM you are trying to solve. As others have already requested, this means telling us about the sensor you are using (what kind of signal does it provide, what is it's output impedance, it is a two-terminal device, etc) and what kind of signal you need once all is said and done.
     
    mah likes this.
  10. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
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    I have irradiation sensor and its output is ranging from 0 to 10 mv . it is two terminals positive and negative i need to amplify it to get 0- 10 volt using single supply op amp and it is preferred to be non inverting , so would you propose newer and suitable op amp.
     
  11. WBahn

    Moderator

    Mar 31, 2012
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    Do you have a make and model of the sensor. I'd like to look at its specs, if possible.
     
  12. Neil Groves

    Member

    Sep 14, 2011
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    What happened to Audioguru?
     
  13. #12

    Expert

    Nov 30, 2010
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    Answered in PM.
     
  14. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
    2
    http://eppleylab.com/

    • Sensitivity: approx. 9 µV/Wm-2.
    • Impedance: approx. 650 Ohms.
    • Temperature Dependence: ±1% over ambient temperature range -20 to +40°C
    • (temperature compensation of sensitivity can be supplied over other ranges at additional charge).
    • Linearity: ±0.5% from 0 to 2800 Wm-2.
    • Response time: 1 second (1/e signal).
    • Cosine:
      • ±1% from normalization 0-70° zenith angle;
      • ±3% 70-80° zenith angle.
    • Mechanical Vibration: tested up to 20 g's without damage.
    • Calibration: integrating hemisphere.
    • Size: 5.75 inch diameter, 3.75 inches high.
    • Weight: 7 pounds.
    • Orientation: Performance is not affected by orientation or tilt.
    [​IMG]
     
  15. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
    2
    IS LM358 a good choice and how to connect it to perform as op-amp and differential amplifier , like traditional or it will need advanced settings?
     
    Last edited: Oct 5, 2013
  16. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Are you saying it has a differential output? I think you're going to need an instrumentation amp.
     
  17. WBahn

    Moderator

    Mar 31, 2012
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    Which model? The best I can tell is Model PSP.

    I was hoping their site had some reference designs, but they don't.

    This appears to be a thermopile that can probably be modeled as an ideal voltage source in series with a 650Ω resistor.

    A quick Google search for "thermopile amplifier circuit" turned up the following:

    http://www.analog.com/en/content/using_thermopile_sensor_in_ir_digital_thermometers/fca.html

    I'd suggest starting there.
     
  18. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
    2
    ok, i need a name of a newer single supply op amp
     
  19. WBahn

    Moderator

    Mar 31, 2012
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    What's wrong with the opamp in the link I provided?
     
  20. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
    2
    It's perfect but SMD i need through hole mounting and i think it's a little expensive for my application which generate mv signal .
     
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