voltage modification....

Discussion in 'The Projects Forum' started by CrackJack, Aug 7, 2009.

  1. CrackJack

    Thread Starter Active Member

    Aug 7, 2009
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    Hello All,
    I am a student from EE and I want to have develop a circuit (last stage of my circuit) which would clamp from 5V to 0V..
    the current circuit consists of a vtg follower, the o/p of which is connected to RC filter and then to an instrumentation amp... However, the instrumentation amp produces an o/p of 5V when the i/p to the vtg follower is 0V... I want to design the o/p stage such that the o/p is 0V when the i/p is 0v and the o/p is 5V when the i/p is around 100 mV..

    the i/p to vtg follower is a noise pattern which has a max magnitude of 130mV...
    i want to amplify this signal so that I can send it to micro-controller circuit...

    Can someone please help me.....

    plzzzzz:(
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Can you post up a schematic of your circuit?
     
  3. CrackJack

    Thread Starter Active Member

    Aug 7, 2009
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    yea, sure...

    i use B2Spice.. i knw it sucks.. but.. tats the s/w that I use for simulation..

    i have attached the schematic.. in the schematic, node V7 represents the i/p sensor which has an AM wave pattern...(basically AC noise with a freq of 60hz)

    I am taking the o/p from Vm1... The 1st Op-amp is the vtg follower, followed by RC ckt, and the next 3 op-amps form the instrumentation amplr...

    Can you please suggest if I am gng wrong anywhere?
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    Your noise source appears to be single ended. That makes the instrumentation amp more complication than necessary. You only need a gain of 50 after your filter which is simple with any op amp.

    Your circuit is running with a single supply. If your input signal goes below ground, you will lose that portion, as the op amps can't swing negative. You will either need a negative supply, or another op amp buffering a voltage divider to create a virtual ground. TI has a nice app note on the technique.
     
  5. CrackJack

    Thread Starter Active Member

    Aug 7, 2009
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    Hi,
    Thanks a lot for the suggestion, i really appreciate it... Yea, as you said, I did try having a negative supply for the op-amp.. but it took my signal high in negative value ( i dnt rem exactly what the value was) even when no i/p was connected to it... hence, I removed the negative supply....

    What can I do to avoid the complexities of the op-amp?? i have the a single source i/p...

    Can you please tell me what should I search to get the app note from TI??

    thanks once again...
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    Try this link - http://focus.ti.com/lit/an/slyt155/slyt155.pdf

    It's for audio circuits, but the principle is the same.

    Your signal does not have a common mode voltage to reject or a difference voltage to amplify. A single op amp configured as a follower with gain should work for this application.
     
  7. CrackJack

    Thread Starter Active Member

    Aug 7, 2009
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    hii.. thanks a lot for the link...

    as you said, i do have a diff vtg(since my other end of instrumentation amp is gnd) so, theoritically, it should amplify the difference which is (i/p-0) = i/p..

    right??

    thanks again for the reply...
     
  8. CrackJack

    Thread Starter Active Member

    Aug 7, 2009
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    Hi Super Moderator,
    I have a basic problem... the o/p of the vtg follower gets level shifted to a negative value wrt '0'.. i dont know if i am able to express properly, but when I see on the oscilloscope, the center line around which the signal oscillates gets shifted down by few hundreds of mV...
    This causes the whole signal to be level shifted...

    I cant understand why is this happening!!!:(
    I have connected both, +Vcc and -Vss to the op-amp...
     
  9. beenthere

    Retired Moderator

    Apr 20, 2004
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    One guess about the level shift is some characteristic of the op amp. If it is leaky at the input, an offset voltage will follow. The op amp has no gain, so you may see the leakage as a DC level on the 30 M resistor (odd value). Any reason for R10 to be so large?

    What kind of op amp are you using?
     
  10. CrackJack

    Thread Starter Active Member

    Aug 7, 2009
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    Hi,
    Thanks for such an immediate response....

    I am using such a high value resistor to make sure all of the i/p(High i/p Impedance) is being supplied to the op-amp...

    A high i/p resistance connected to gnd will make sure that there is no leakage of the signal... I manually tested for this value.. Like it was purely a trial and error basis...

    I am using a LM 741 op-amp.. simple one..
     
  11. CrackJack

    Thread Starter Active Member

    Aug 7, 2009
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    hi,
    I tried connecting a pot to pins 1-5... and the vtg level shifted a bit...
    :)
    however... i would like to have a smooth waveform... the AM wave in the signal is still present... how can I remove that?? I tried using an envelope detector.. but could not get to decide the R and C values..

    i tried using a 47uF and 400Ω resistor...

    Am I going correctly?? Can Someone please help me?
     
  12. beenthere

    Retired Moderator

    Apr 20, 2004
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    Some of your problem originates with the 741 op amp. The design is 30 year sold and is pretty lame by modern standards. Other people have their favorites, but I tend to use OPA134/2134 op amps. Besides a much faster slew rate, you get 10^13th input impedance. A 741 has a 200K input impedance. Your 30 M resistor is doing no good with a 741.

    What is the input signal source?
     
  13. CrackJack

    Thread Starter Active Member

    Aug 7, 2009
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    hi,
    Thanks again... the i/p is an aluminum foil... it acts as a pressure sensor... basically it picks up noise, and then whenever a pressure is applied on it, the amplitude of this noise increases, the frequency of the signal being 55-60Hz(yes, it picks up AC)... i want to detect this change in the pressure and then send it to a microcontroller ckt...
    I do not have OPA134/2134 OP-Amps.. what can I do to replace these op-amps, yet have the same functionality??
     
  14. beenthere

    Retired Moderator

    Apr 20, 2004
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    Rather than a low pass filter, think about a notch filter to eliminate the 60 Hz component - www.national.com/ms/LB/LB-5.pdf

    I order parts from Digi-Key or Mauser Electronics. Don't expect a local store to carry good op amps. For your application, any FET input op amp should be okay, even an old CA3140.

    You may be able to get results using the 741's, but don't try to help the input impedance - they just don't have much. Try placing your foil sensor under a box to see if the signal goes to zero.
     
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