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Quick Question-Noob

Discussion in 'General Electronics Discussion' started by nanafy, Oct 13, 2013.

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  1. nanafy

    nanafy

    2
    0
    Oct 13, 2013
    Hey Guys, I am a real big noob when it comes to electronics.

    I picked up this schematic and I made it , but my piezo sensor is getting clipped like mad. I got it hooked up to an arduino board and the voltage output goes from 0 to 5v right away. ( My op amps use 5 v). I need to lower the gain and I dont know which resistors control the gain.

    http://www.maximintegrated.com/app-notes/index.mvp/id/1127

    This is the schematic I used. I just need to know which resistors will change my gain. I should of just used an instrument amplfier :( But I dont have time to redo it .
    I can see this is like a modified instrument amplifier.

    Any help would be appreciated !

    Ps. Can I just add a resistor at the end haha ( probably a dumb solution )

    Thank you guys , I am really looking forward to learning about all this stuff
     
  2. Rob_K

    Rob_K

    59
    0
    Sep 20, 2013
    I am quite the noob myself, but I did study op amps at Uni and even though off the top of my head I can't remember, I understand it is to do with the value of the resistor within the feedback loop.

    I found this. At a quick glance it does appear to address you issue.

    Hope this helps.
     
  3. Laplace

    Laplace

    1,252
    184
    Apr 4, 2010
    Question: Are your op amps the same MAX4257 as specified in the application note? If not, it could explain the large output offset. Input leakage around the sensor input circuit may also be a culprit. Cleanliness is next to good insulation in high impedance circuits such as this.

    R9, the resistor at the end, controls the gain. If you add a resistor at the end (in parallel with R9) it will lower the feedback resistance and lower the gain.
     
  4. nanafy

    nanafy

    2
    0
    Oct 13, 2013
    No they are not those op amps i have dual op amps that work off of 5v

    I do not know if that makes that big of a difference

    I can tell you which op amps I used:
    http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en010434

    MCP 6002

    So only Resistor 9 controls gain?

    How about the other two op amps, the website says those are highpass filters on the first two op amps but they look like low pass to me :/ am i wrong? (probably)
     
  5. Laplace

    Laplace

    1,252
    184
    Apr 4, 2010
    The MCP6002 seems to have the same 1 pA input bias current as the specified op amps, so that should not be a problem.

    Resistor R9 (actually the ratio of R7 & R8 to R9) controls both the AC & DC gain together. I don't understand the purpose of resistors R5 & R6. The AC gain at high frequency is controlled by the ratio of the input capacitance to the feedback capacitance. The AC gain is highest at high frequency so that is high pass filter behavior, but the DC gain is not zero.
     
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