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Multiplexing analog resistance signal?

Discussion in 'Electronic Components' started by Mickey, Nov 15, 2003.

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  1. Mickey

    Mickey Guest

    Hello.

    I'm interested in how can resistance signals (like Pt100) be
    multiplexed?

    I mean how can I make a analog switch to switch 16 RTDs
    to just one output? Three wire Pt100 connection.

    For analog voltages and currents amalog multiplexers like
    4051 can be used but how about resistance (around 100
    Ohms)? (analog multiplexer chips have pretty big resistance)
     
  2. LameDuck

    LameDuck Guest

    Not entirely sure what you're up to, but I'd
    venture to suggest that the individual sensors
    be buffered by inexpensive opamps. The opamps
    might also employ outputs that could be digit-
    ally switched into a high-impedance state; in
    turn giving the mux effect you're looking for...
     
  3. Mickey

    Mickey Guest

    Not entirely sure what you're up to

    I'm interested in how to switch 8 or 16 Pt100 temperature sensors
    to one, so to select one sensor at a time to sample and make A/D
    conversion on it.
     
  4. Guest

    The Fluke data logger (Hydra 2625) that I've used simply has a bunch
    of relays in it. It is essentially a nice digital voltmeter with
    a 2 wire, 21 position switch (made out of relays) on the input leads.

    You can't take readings very fast with it - you might be able to read
    each RTD once a second with 16 connected - but the relays add very
    little resistance (a fraction of an ohm) to the input signal.

    If you decide to do this, get relays that are made for "dry contact"
    (very low current) service, instead of power relays. Power relays
    have contacts that are designed to be cleaned by arcs; "dry current"
    relays don't depend on arcs for the contacts to work.
    If the resistance of the multiplexer chip is fairly constant, you can
    measure it for each channel and then subtract it out of your reading.
    But the chip's resistance will probably vary with temperature enough
    to make it hard to correct for, unless you add a 17th RTD for "chip
    temperature".

    Matt Roberds
     
  5. Tim Wescott

    Tim Wescott Guest

    How much control do you have over the circuit? Most circuits for these
    sensors drive a controlled current through the sensor and look at the
    voltage. With your three-wire sensor you'd also get half a wheatstone
    connection to the sensor, so in theory you can null out the resistance
    effects of the sensor leads by calculating the drop in the paired sensor
    wire and subtracting it out.

    So there's two possibilities:

    1: pretend that your switches are matched well enough that the 3-wire
    circuit will correct for switch resistance, hook it up and go. When that
    doesn't work, use option 2.

    2: Go ahead and drive your constant current through the sensor using one
    pair of switches. Monitor the return voltages through another three
    switches (it's a three-wire circuit, remember?). If you can find a
    well-explained 3-wire circuit out there you should be able to see where to
    put the switches.
     
  6. Mickey

    Mickey Guest

    The Fluke data logger (Hydra 2625) that I've used simply has a bunch
    Thanks Matt. We have in a company where I work analog multiplexer for
    32 channels, done with relays, but the relays are for Ex (Explosively)
    dangerous enviroments, and it is a pretty large box, so I thought it could
    be made not so large for not difficult enviroments. I also thought it could
    be made without relays with something more suffisticated, but I guess
    not relays and thats that.

    Thanks.
    Time is not critical.
    Thanks for this suggestion.
    The resistances aren't paired (or simmilar betveen contacts) so it is
    going to be impossible to use analog multiplexer chips.
    I did think of converting resistance signal to voltage and then the
    analog multiplexing chips resistance wouldn't interfere much. But how
    to easily convert resistance to voltage for 16 resistances (3 wire)?
     
  7. Mickey

    Mickey Guest

    How much control do you have over the circuit?

    I don't know what you ment but this, all the control I want I guess.
    If the resistance of wires is the same for all 3 wires, and yes that is the
    point of using 3 wires for measuring resistance.
    Well, pretending works pretty badly in engineering stuff and I think that
    Murphy's law can apply here in case lets say when you have 2 possibilities
    for doing something, one is slightly easier it is probably the other one
    that
    is the right one :)))
    In this case it would be a shoot in the dark.
    This idea isnot bad, I'll give it some thought. Thanks.
     
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