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buffering and multiplexing 8-ch 16-bit ADC

Discussion in 'Electronic Design' started by Glenn, Jun 27, 2004.

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  1. Glenn

    Glenn Guest

    I am currently using the ADS8344 8-ch 16-bit ADC connected
    to an Atmel AVR microcontroller to interface various types
    of pots, bend sensors, touch sensors, etc. and using 0-5V.

    Would like to know what the best way is to increase this to
    16 channels? What multiplexer chips would you recommend to
    switch two sets of 8 channels?

    Where would a unity gain buffer and low-pass RC filter be
    placed - in between the MUX and the ADC, or on each input
    channel of the MUX? Any recommended buffer chips?

    And just wondering -- does there not exist MUX chips with
    on-board unity gain buffers?

    Any suggestions would be much appreciated -- alternate
    approaches welcome.

  2. Its probably going to be easier to add another ADC.
  3. Do you need 16bit resolution for all your inputs?, for example the
    If not then how about using an AVR with an 8 channel 10bit ADC, even
    the ATtiny26 has that.

    The problem with a single RC filter after the mux is that the ADC
    input needs more time to stabalise after switching. But if you don't
    have to scan too fast then that saves you 15 RC filters. There are
    pros and cons to both approaches.

    Dave :)
  4. Tim Wescott

    Tim Wescott Guest

    And about the same board space, too.
  5. Glenn

    Glenn Guest

    Do you need 16bit resolution for all your inputs?, for example the
    I do need the 16-bit resolution for the 16 external
    channels, and will be using the on-board 10-bit ADCs already
    for less critical stuff. Even for the pots, we need a good
    resolution as often the pots are physically using only a
    smaller portion of their full range.

    The sampling needs to be kept as quick as possible - I will
    be polling the uC at around 60hz and need to return all the
    sampled AD channels with a minimum of latency. Also, I want
    to be able to oversample certain channels if necessary,
    without blowing the 60hz sampling period.

    I see what you mean about the RC filter after the MUX - I
    wouldn't be able to filter as much compared to having 16
    seperate RC filters.

    What about the opamp buffer? Would this go between the MUX
    and the ADC, and how would that affect the scanning /
    crosstalk, as opposed to having 16 seperate buffers?

    A second ADC could work but would just cost a bit more. For
    some applications it would be fine, for others, it might be
    better to add lots of multiplexers, and in the future, I may
    need even more channels. I will probably use both
    approaches at different times - any idea what multiplexer
    might be suitable?

    Thanks for all your help,

  6. Rich Grise

    Rich Grise Guest

    Don't put anything between the mux and the ADC. Buffer each
    signal individually. To 'splain, just think if two adjacent
    signals were, say, 4.5v apart? Your buffer would have to
    slew that, and then pick up on the signal itself. Buffer
    before you select.

    But truly, it sounds like a second DAC is the way to go,
    especially given your sampling requirements.
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