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  1. #1
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    LTC1062 LPF values for 3KHz cut off?

    Hi,
    In page 11 the circuit "Single 5V Supply 5th Order LP Filter" is what I need.
    1. Can you please let me know What values should I put on R and the two C for a cut-off of 3KHz? I want to veryfy my calcs.
    2. Should I leave the "DC accurate output" unconnected from the next stage (audio amplifier) and take the output from pin 8?
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    •   Alt19th April 2018, 10:44

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    Re: LTC1062 LPF values for 3KHz cut off?

    Hi Neazoi,

    It's like the blind leading the blind...

    If you trust me (I wouldn't), 5k2 and 10nF compute to 3060Hz using the formula provided in the schematic, the 1/(2*pi*R*C) formula. Are you certain the right-hand C is related to the filter at all? I don't know about that IC.

    I have no idea about the second question. Page 6, section: Internal Buffer may answer that.



    •   Alt19th April 2018, 20:40

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  3. #3
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    Re: LTC1062 LPF values for 3KHz cut off?

    The clock frequency should be 100 times 3Hz or 300kHz.

    That formula in the data sheet on pg. 11 appears to be in error.
    It should be [1/(2*pi*R*C) * fc/100 * 1/1.63]
    That gives an RC value of 86.47s for a 3kHz corner frequency and a 300kHz clock.

    Thus, for a capacitor value of 10nF for each capacitor, the resistor value is 8.66k.

    I would use the DC accurate circuit as shown.
    Last edited by crutschow; 20th April 2018 at 05:25.
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    •   Alt20th April 2018, 05:15

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    Re: LTC1062 LPF values for 3KHz cut off?

    Quote Originally Posted by crutschow View Post
    The clock frequency should be 100 times 3Hz or 300kHz.

    That formula in the data sheet on pg. 11 appears to be in error.
    It should be [1/(2*pi*R*C) * fc/100 * 1/1.63]
    That gives an RC value of 86.47s for a 3kHz corner frequency and a 300kHz clock.

    Thus, for a capacitor value of 10nF for each capacitor, the resistor value is 8.66k.

    I would use the DC accurate circuit as shown.
    Right, so 10nF and 8.6k for about 3KHz cut-off. Thanks a lot!
    In the DC-accurate circuit, can I put an output coupling capacitor to DC isolate the next stage? Will this have any serious impact on the cut-off frequency?
    Professional engineering is the top, but amateur engineering is more fun.
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    Re: LTC1062 LPF values for 3KHz cut off?

    Quote Originally Posted by neazoi View Post
    ............
    In the DC-accurate circuit, can I put an output coupling capacitor to DC isolate the next stage? Will this have any serious impact on the cut-off frequency?
    It won't as long as the coupling capacitor is large enough to carry the lowest frequency of interest.

    Note that the output load impedance must be much larger than 8.66k to avoid affecting the rolloff frequency.
    Alternately make the parallel value of the input resistor and the output impedance equal to 8.66k ohm (or as close as you can).
    Last edited by crutschow; 20th April 2018 at 17:23.
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