MCP6002 Gain stage issue

Discussion in 'The Projects Forum' started by BuxZED, Jun 1, 2014.

  1. BuxZED

    Thread Starter Member

    Apr 14, 2014
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    0
    cerc.JPG

    Hi Everyone,

    This is a circuit I assembled about a month ago with the help of the members from this forum(Link, special thanks to "ericgibbs").

    I recently took the oscilloscope to it and I was wondering weather anyone can explain to me what is going on.

    compared to the signal from the piezo element (top), why is the gain stage from the IC is not steady and have a strange peak after the middle
    IMG_20140520_100612.jpg

    I thought it's the negative peak from the piezo what's causing the problem, so I used a Shotky diode (Bat85) to ground the negative side but It seems to make the make the problem worse.
    IMG_20140520_100649.jpg

    This strange peak seem to affect the LED from LM3914
    IMG_20140519_093705.jpg

    any idea whats going on?
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    hi Bux,
    I assume your piezo is a high impedance device.??

    If so, remove that diode and change the input 30K to a 1meg resistor, check the result, let me know what you see.

    E
     
  3. BuxZED

    Thread Starter Member

    Apr 14, 2014
    32
    0

    22.jpg


    seem to have sorted the anomaly , but the negative peak is still missing:confused::confused:
     
  4. BuxZED

    Thread Starter Member

    Apr 14, 2014
    32
    0
    LM3914 is looking for a DC reference voltage at pin 5, is there any way I can give it a more cleaner representation of the AC peaks from the piezz, Vrms in dc perhaps?
     
  5. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    hi bux,
    The missing negative peak is due to the OPA being powered from a single supply.

    To get a rectifies signal at pin #5, you could use the 2nd unused half of the OPA as a fullwave rectifier.

    E
     
  6. BuxZED

    Thread Starter Member

    Apr 14, 2014
    32
    0
    could you kindly explain what you mean by "OPA being powered from a single supply"?

    ;)
     
  7. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    hi bux,

    You have single +3v battery source supplying the voltage/current to the LM3914 and the MCP6002.

    A double power supply would be +3v and -3v., so the OPA output could swing positive and negative.

    If you did add a +V/2 offset to pin #5 as stated in your post, when there was no input from the piezo, the LM3914, LED #5 would be lit.
    As the pin #5 AC drive signal went below zero the LED's 4 to 1 would light.

    I dont think thats what you want.? [ ie: centre Zero]

    If you full wave rectified the piezo AC signal and connected it to pin #5 [ with no +V offset] the LED's would appear brighter for a given signal.

    The downside is, knowing that you want to keep the power drain on the batteries to a minimum, would mean that you would double the battery drain, by driving the LED's on the positive and negative half cycles of the piezo AC signal.!

    E
     
    Last edited: Jun 5, 2014
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