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Help with voltage divider

  1. May 1, 2010 #1
    Hi first time poster. I'm a little stuck, through trial and error on Mentor graphics I was able to create the attached circuit. Using +&- 9V supply rails I managed to output -10mV, which I required. Now I have come round to try and work it out by hand i'm struggling to remember how! I know that the first part is basically a potential divider but how does this work with a positive and negative voltage?

    Any help greatly appreciated

    Attached Files:

  2. jcsd
  3. May 1, 2010 #2
    Treat it as a divider between 18V and 0V and then subtract 9V from the output voltage.
  4. May 1, 2010 #3
    The two supplies on the left and the two resistors can be replaced with an equivalent single supply and single resistor.

    R = 10*8.87/(10+8.87) KOhms

    V = (9+9)8.87/(10+8.87) - 9 Volts
  5. May 1, 2010 #4
    So therefore using this
    R = 10*8.87/(10+8.87) KOhms = 4.7KOhms

    V = (9+9)8.87/(10+8.87) - 9 Volts = -0.53895V

    So then is there some calculation required using the 500K and 1K utilising the -0.53895V to reach the 10mV?

    Thanks for your help so far
  6. May 1, 2010 #5
    That's an odd combination of components. What's it for?

    Vout = -0.539[10/(500 + 4.7 + 10)]

    not 10mV, but -10mV
  7. May 1, 2010 #6
    Its for an input offset voltage on the non-inverting input of an op-amp to give a zero voltage output. I spent ages designing an RMS circuit for a sound level meter and then I hit this problem. Managed to figure out what offset was required simulated that circuit but then I could remember how to hand calculate!
  8. May 1, 2010 #7


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    Staff: Mentor

    Input offset voltages vary with temperature, and from part-to-part. What opamp are you using? There may be a more reliable way to deal with the input offset voltage. Can you post your schematic?
  9. May 2, 2010 #8
    Here's a screen shot of what I've been designing. Im happy with the overall performance of the circuit, its being used within a group to create a sound level meter, and i have this working as a Type 0 with a 60dB dynamic range.

    Attached Files:

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