Simultaneously Sampled ADCs over SPI

Discussion in 'Embedded Systems and Microcontrollers' started by Defuzzification, Mar 21, 2015.

  1. Defuzzification

    Thread Starter New Member

    Nov 6, 2014
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    I'm trying to find a way to read 4 analog signals simultaneously. The only interface available from the MCU is an SPI bus (as in CLK, MISO, MOSI, and CS).

    The analog signals are 0 to 5V. I need 8-bit resolution or better. Speed is not a concern.

    I've been searching through all kinds of ADCs but havn't found anything suitable. I'm open to using some kind of converter or even a second MCU between the ADC(s) and SPI to get it working.

    Does anybody have any ideas?
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    Hi,
    If speed is not important perhaps this option would be OK.
    Use a sample and hold pulse on each analog channel simultaneously and then ADC the values.

    http://www.ti.com/product/lf398-n
     
  3. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    2,908
    2,169
    Another idea (and maybe too much work if you already have a MCU selected for the project) if you want a second MCU for the ADC(s) and SPI.
    The PIC24H has Simultaneous Sample Select (SIMSAM) for 4 channels at 10 bits.
    http://ww1.microchip.com/downloads/en/DeviceDoc/70225B.pdf

    The devices are is 3.3 volts so to use one you would have to scale the analog signals into the proper ADC range. It has PPS so the SPI can be assigned to
    5volt tolerant pins.
    http://ww1.microchip.com/downloads/en/DeviceDoc/70293G.pdf
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    What is the max limit on "simultaneously"?
     
  5. Defuzzification

    Thread Starter New Member

    Nov 6, 2014
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    The sensor I'm using has 4 outputs and it counts by incrementing by 20mV. Once the first pin is incremented all the way to 5V, it goes back to zero and increments the next pin. So basically it can count up to a 32 bit number. The data sheet recommends simultaneous sampling so I don't read (for example) 4.8V on pin 1 and have it turn over before I read pin 2. There's no reliable way to know how fast it will count so I'm not sure how "simultaneous" it needs to be...
     
  6. Defuzzification

    Thread Starter New Member

    Nov 6, 2014
    4
    0
    This seems like the best option so far. Thanks for the advice
     
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