Multiplexer Crosstalk Issue

Discussion in 'General Electronics Chat' started by lmcgill, Jun 8, 2011.

  1. lmcgill

    Thread Starter New Member

    Dec 7, 2010
    3
    0
    Ok, so I'm about to give up here. I've been tinkering with this same issue for a while now, so maybe someone here might be able to help.

    Overview of the project:

    I would like to read temperatures through 4 different thermocouples. I intend to do this by multiplexing the 4 Tc signals to a Tc signal conditioner. I am using a 74HC4052 for my thermocouple and an AD8495 for my Tc conditioner. The brains of my project is an AVR ATMega32. I will be using the ADC on the AVR to read the output of the Tc conditioner.

    Temperature reading procedure:

    My thought was to pole each Tc every 0.25s. So for each polling, I would set the appropriate MUX selction bits, delay to allow the MUX output to settle, start an ADC conversion, wait for ADC conversion to complete, convert ADC value to temperature, and finally, print the temperature to my LCD.

    Problem:

    If I do this polling repeatedly to one single Tc port (that is, never switch the state of the MUX), I am able to read an accurate temperature on the port. The issue occours, however, when I begin to cycle through the 4 Tc ports.

    First note, that there is open thermocouple detection on the Tc conditioner IC. This means, when there is not Tc plugged into the Tc jack on the circuit board, the output of the Tc conditioner IC is railed to Vcc (+5VDC). So if I pole only one Tc as described above, and I unplug the Tc from the jack, I see ~600F on my LCD (which is expected because the ADC sees around 5V).

    If I pole all of the Tc ports, and plug a Tc into the first jack (and leave the other 3 empty) I would expect to see

    T1: 72F
    T2: 593.59F
    T3: 593.59F
    T4: 593.59F

    What I do see is

    T1: 225F
    T2: 114F
    T3: 186F
    T4: 259F

    Any ideas?

    Attached are the cutsheet for the MUX, Tc conditioner IC, and my schematic (part A & B). Let me know if you'd like to see my code. Thanks for any help..
     
  2. lmcgill

    Thread Starter New Member

    Dec 7, 2010
    3
    0
    So no ideas, I guess?
     
  3. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,348
    Hello,

    The signals from the TC will be to small to pass the MUX.
    The MUX will produce to much noise.
    Also the TC will need special connectors and leads to have no extra junction voltage errors.

    Bertus
     
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