Single filter for multiple analog inputs

Discussion in 'General Electronics Chat' started by Dritech, Feb 15, 2016.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Hi all,

    I need to continuously monitor 8 analog inputs using a single ADC module. An analog multiplexer IC will be used to repetitively switch between the analog inputs to the ADC (something similar to the diagram below):

    [​IMG]

    I also need to apply a band pass filter to all analog inputs. Will the filter work it I implement one at the output of the multiplexer, or do I have to implement a band pass filter for every analog input (8 in total) ?
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,042
    3,243
    You can use one at the output of the mux as long as you wait for the filter to settle before taking a reading.
    The time to settle within the resolution limits of the ADC depends upon the design and rolloff frequency of the filter.
     
  3. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Thanks for the reply. Lets assume I use an active bandpass filter as the one below:

    [​IMG]

    Is there a way to calculate the settling time for it?
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,042
    3,243
    That's not really an active filter.
    It's just an op amp with a single-pole RC high-pass input and single-pole low-pass feedback making a buffered single-pole bandpass filter.
    Generally it's only called an active filter when it has feedback to give complex poles and zeros for second-order (or higher) responses such as the Sallen-Key topology.

    I would not use the high-pass (DC block) input unless you are only converting AC signals and aren't concerned about the signal's DC value.
    Is that the case?

    You can calculate the settling time, but I'm lazy and would just simulate it since that will show the effects of the amplifier on the time. :rolleyes:
     
    Last edited: Feb 16, 2016
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,545
    1,251
    That's because you can whip this up in LTSpice faster than most people can spell their own name.

    ak
     
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