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  1. #1
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    Input RC filter design for analog pin of ADC

    I am implementing ADC filter for AD7705 ( Sigma Delta ADC - Analog Devices Chip ). Input to ADC is constant DC voltage. So, what will be cut-off frequency I should consider to implement RC filter at analog input.

    If we are designing RC filter, then by assuming C value ( typ 0.1uF ) and cut-off frequency above which we are trying to block signals (ripples) , we can calculate R value. Is this a approach which we consider for filter design? Am I correct in my consideration.

    So for DC input at ADC pin, what should be ideal or practical cut-off frequency I should consider for designing RC filter.

    Thank you in advance.

    •   Alt4th May 2015, 13:09

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  2. #2
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    Re: input filter design for analog pin of ADC

    Hi,

    we can not answer this.

    IF input signal is DC it NEVER changes. But i´m sure it does. And you have to decide what changes in value up to what frequency you want to see, and what frequency makes no sense.
    You could either decide in frequency or timeconstant.

    If you want to measure room temperature you maybe want to see changes with a time constant of 5 minutes.
    If you want to measure and regulate water temperature in a coffeemachine you need a time constant of less than a second.

    Or see it the other way:
    If you want to build a voltmeter with a display... then keep in mind that the value needs about 5 x timeconstant to reach its final value (within 1% error)

    So if you want to read the value after 1 second you need a time constant of less than 200ms.

    ***
    If all this doesn´t help you, then tell us more about your application.

    Klaus



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