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Need to amplify 0,1-0,8mV range at least 100 times

  1. Jan 31, 2008 #1
    Hi,

    I measure the output from my circuit between 0,1 mV to 0,8 mV at 200mV setting on my digital voltmeters. How can I measure it more sensitively at least 100 times, or amplify it 100 times so that I can know the values such as 0,427mV instead of 0,4mV?
    I tried it using a differential amplifier circuit with LM308, R1=R2=1K and, R3=R4=100K, and 0-18VDC but it didn't work. Maybe something was wrong which I couldn't figure it out.

    I appreciate any suggestions either on diff amp circuit or with any other methods.
     
  2. jcsd
  3. Jan 31, 2008 #2
  4. Jan 31, 2008 #3

    berkeman

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    Staff: Mentor

    You will need to run the opamp off of split power supply rails (like +/-12V) if you want to amplify a signal that is near ground.
     
  5. Jan 31, 2008 #4

    madmike159

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    Gold Member

    You can make a non-inverting amplifier from a op amp and to resistors. The equation which gives you the gain is Gain = 1 + R(top) / R(bot), where R top and R bot are resistor values. You can find a circuit diagram on google.
     
  6. Feb 1, 2008 #5
    yes, i searched for voltmeters with more digits but they cost a lot. on the other hand i followed the link you gave to check the specs, but noticed an "amplifier for photodiode sensor" circuit, that was a direct solution so i built it with my fds100 photodiode and a red laser beam (ca. 620 nm), and it worked, w/o any power to my surprise, because i thought every opamp should work with some power. now i got the mV's in the range 26 mV to 42 mV. that is a much better resolution but i still need to try experimenting with it in different conditions.

    thank you much edmondng for your fast response.
     
  7. Feb 1, 2008 #6
    thank you much berkeman for your reminding since i ran it first on +18V with no result. i applied the split power +/-18V (with four 9V batteries) and this time it amplifeid the 1,4 mV to 1,346V that is about 1000 times though it was supposed to amplify only 100 times with 100k/1k = 100. i think i need to check things if something is still wrong. now i am working on this circuit and another photodiode amplifer circuit, whichever gives any reliable results...
     
  8. Feb 1, 2008 #7

    berkeman

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    Ouch. +/-18V is the Absolute Maximum Rating for the supplies on an LM308, and if you used fresh 9V batteries, you exceeded the +/-18V by about a volt. You should not be running parts at or near their Absolute Maximum Rating -- that's a good way to damage parts and run into other problems.

    Instead, if you are constrained to using 9V batteries for your power supplies, just run the LM308 off of +/-9V. That will give you about +/-7V of useful operating voltage for the LM308's input and output voltages (see the datasheet).
     
  9. Feb 1, 2008 #8
    i don't think you build it right. it may 'seem working' but not correctly. the output must be coming from another path. i believe all op amps need power to work

    typical supply for op amp are +-12V, +-15V, +-5V depending on supply voltage rating op amp of course.
     
  10. Feb 1, 2008 #9

    berkeman

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    Staff: Mentor

    Shining the red laser on the photodiode is generating a photocurrent, and the OP is probably just seeing the results of the photocurrent going wherever it can with the circuit unpowered (like through the I->V conversion resistor).
     
  11. Feb 2, 2008 #10
    On one pair I exceeded by half the volt and it was just below 18V on the other pair. Right... then i will run the LM308 off of +/-9V batteries on monday and see the readings again. Thank you much...
     
  12. Feb 2, 2008 #11
    i built the circuit exactly the same way as given on the datasheet. first i supplied power with +/-18V thinking that it should be fed somehow. i got the constant output +18,11V with the laser blocked or unblocked. then i removed all the batteries, and the reading was about 26mV on daylight in the room with laser beam blocked, and rose up to 46,2mV with the red laser beam. then it responded consistently each time repeated even for the in between values. on the diagram http://www.alldatasheet.com/datasheet-pdf/pdf/70230/LINER/LM308.html it was given that Vout = 10V/uA. so i think that converts micro ampers generated from the photodiode into voltages.
    thank you much...
     
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