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  1. #1
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    Low Pass filter making problem

    Hi, I'm trying to read temperature with a thermocouple for which i have made a amplifying circuit after which i feed the amplified voltage to a Low-pass filter to remove any noise. To be very frank i don't know how to design a LPF but after seeing from some applications i have used one which i have shown in attached diagram. This LPF is itself introducing huge noise. The output voltage is'nt stable after this like it's jumping anywhere between 674mV to 698mV at a particular fixed input voltage. If I remove this LPF the output is very stable. But I think I may need this LPF because I have to use this circuit in noisy industrial conditions.

    Please tell me what mistake I'm making.

    Thanks

    •   Alt24th November 2009, 14:38

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  2. #2
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    Re: Low Pass filter making problem

    Hi GARG29,

    1.) Use any universal opamp but not LM324
    2.) Use dual supply (if available) instead of single supply (as mentioned in the doc)
    3.) Single supply works only if the output of the first amplifying stage in the figure has a dc component of app. 1/2 Vdd. I suppose this is not the case for your application.
    4.) The R,C components yield a 3-dB-corner frequency of app. ω=2000 1/rad.
    Does this satisfy your requirements?

    Further questions?


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