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pt100 and 4051

Discussion in 'Electronic Design' started by Marco Trapanese, Jan 13, 2013.

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  1. Hello,

    I want to acquire 8 pt100 in three-wires configuration. I'm going to use
    a single LTC2402 as described here:

    http://cds.linear.com/docs/Design Note/dn237f.pdf

    so I need to multiplex the 24 incoming wires.
    For example I could use three 4051.

    The Rds_on resistance is quite high but it shouldn't be a big deal
    because the current flowing is very small.

    I'm afraid about the very high delta among channels... the datasheet
    says it can reach 15 ohm when Vdd = 5V. Too much!

    Do you know any other analog multiplexer with a matched Rds_on?
    Or do you recommend a completely different design?

    I cannot use relays for this purpose.

    Thanks in advance
    Marco
     
  2. whit3rd

    whit3rd Guest

    So, your intent is to switch the excitation current in one lead of the
    RTDs but to leave the other lead's excitation current common
    between the eight units? That takes an 8-to-1 switch, then you
    use the other two 8-to-1 switches
    to probe the third wire and the common node of the
    unswitched excitation wiring.

    There's no need here (unless your voltage sense amp bias current
    draw is significant) for the switches to match in resistance.
    Only the excitation wires on the two poles of the RTDs have
    to be matched (as in all three-wire resistance schemes).

    If, rather than leaving one excitation (current-carrying) wire common,
    you wished also to switch it, the problem of switch resistance mismatch
    does show up, and the multiplexer has to grow to four-wire
    complexity (i.e. you'd use four 8-to-1 switches), even though the device
    wiring stays in the three-wire configuration.
     
  3. Il 13/01/2013 18:22, John Larkin ha scritto:

    I'm trying to fully understand your schematic.
    One channel of AD7793 reads the mux'd RTD's signals while the second
    channel measures the voltage drop across R324, that is the current
    flowing into the selected RTD.

    In this way, the Rds_on of any channel of the three 4051 is different
    doesn't matter at all?

    Thanks for your hint
    Marco
     
  4. Il 13/01/2013 19:06, whit3rd ha scritto:

    No, I was mean to use a switch for each lead. So three 8-to-1 switches.


    I will try to draw a schematic and I will post here to be sure I
    understood correctly.


    In your solution doesn't matter, but in mine does.



    Yes, now I understand my error.

    Thanks
    Marco
     
  5. Il 14/01/2013 00:01, John Larkin ha scritto:

    Right, the other two legs doesn't carry any current (ok, it's very very
    small -> negligible)


    I usually power my RTDs with 500 uA. No evidence of self-heating and the
    voltage is not very difficult to read using a 24-bit converter and a
    good layout.

    In this particular case I may rise the current over 1 mA because I can
    power each RTD for a short time. I need to get a reading every several
    minutes.

    Thanks
    Marco
     
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