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multiplexor Inputs

Discussion in 'Electronic Design' started by Richard, Aug 10, 2005.

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  1. Richard

    Richard Guest

    I need to come up with some ideas to take a Thermocouple chip like the AD595
    and have 16 type K- Inputs going to it. I would imagine it would be best to
    use 4 - 8 channel multiplexors, using 1 pair for each 8 channels, in order
    to switch the + and - of each thermocouple.

    I have not dealt with multiplexors yet, so do you all have any
    recommendations on which would be the best ones to start playing with. I
    also need someway to filter noise at the each channel before the
    multiplexor, to eliminate one open channel from injecting noise into other
    valid channels. Any ideas on this one? Simple caps or some pull downs?

    Thanks for any help.
     
  2. Richard

    Richard Guest

    One of the muxes I'm looking at is the Intersil 8x2 DG407DY. Using two of
    those would switch the pos and Neg of 16 channels.
    Any comments on these? Wow, that AD595 is pretty expensive. Any
    alternatives that work well?

    Thanks
     
  3. Ban

    Ban Guest

    Richard,
    I'm afraid you will need one compensation chip for each channel and also the
    required filter afterwards, and only then mux, because a mux in front will
    corrupt the signal. The charge injection will probably saturate the amp and
    the long time constants of the filter will need very long settling times.
    If you make up some kind of amplification/filtering all connections have to
    be at the same temperature ( that is why that chip is ceramic and
    expensive).
    But depending on your required accuracy you could thow away precision and
    have an instrumentation amp up front followed by a filter and you could
    subtract some temp dependent ice-point correction, that you have amplified
    identically, or probably leave that out as well. Who cares about +/-20°C
    then? So you save the AD595 as well. Go figure out.
     
  4. MK

    MK Guest

    You don't really need one compensation chip per channel - you need to
    measure the temperature of the cold (local) junctions, so if you can keep
    them the same within the limits of accuracy you want to work with then one
    sensor will do. Most small multi input instruments work like this.

    It is possible to use passive filtering, protection etc and a multiplexer
    and this can work well. You can get better performance by using
    buffer/filter amplifiers before the mpx. If you do it right you can use your
    buffers to get rid of common mode noise, filters to get rid of serial mode
    noise and then use a cheap modern mpx running off a 5V supply. Several
    modern ADCs or micros have built in temperature sensors and you can (with
    care) use one of these for the cold junction compensation.

    Linear technology have the LTC2480 with built in sensor and an app. note
    showing how to use it for a single channel thermocouple system. (The package
    is not hand built prototype friendly - I know I've just soldered 5 of them
    !).

    (I'm running a test on this now and the reference thermometer in the dry
    well calibrator says 70.02C and my PC reading out from the prototype says
    69.9C. I'm happy because that was calibrated at 25C. I know from tests on
    other prototypes that the effect of ambient temperature changes in the range
    5 to 40C is less than 0.5C.)

    www.mkesc.c.uk


    Michael Kellett
     
  5. Guest

    Hi, it's not really feasable to switch the thermocouples directly. A
    better aproach is to have each thermocouple connected to its own
    amplifier/cold jinc compensator/filter input stage, then multiplex the
    output voltages. You dont need any exotic chips, even a 741 will do at
    a pinch and a diode can be used for cold junc comp.
     
  6. Ach, ye could hae fooled me. What makes ye think ye cannae do it?
    ....grounded junctions...
    If you don't care a whit about accuracy and are using only the basest
    of base-metal thermocouples.


    Best regards,
    Spehro Pefhany
     
  7. Jon

    Jon Guest

    As others have mentioned, the problem with using a mux upstream of the
    AD595 is cold junction compensation errors. One way around this, if
    you don't need extreme accuracy is to use a temperature sensor, such as
    the AD590 to measure the cold junction temperature (assuming that it is
    roughly the same for all 16 phusical thermocuople interface points).
    You can then replace the AD595 with an ordinary OP Amp or
    Instrumentation amplifier. You can do your own cold junction
    compensation in software, using the known thermocuple characteristics.
    I have succesfully used this technique for measuring high temperatures,
    where the variation of the cold junction temperatures among
    thermocouples was negligible. One way to ensure that the temperature
    variation is small is to place a large ground plane under the
    thermocouple connections. The ground plane will tend to equalize the
    cold junction temperatures. You can place the cold junction
    temperature sensor on or above the ground plane.
     
  8. Richard

    Richard Guest

    I had planned on using Phoenix Contact connectors all in one row and have
    the muxes and AD595's right next to them pretty close. I would think the
    temp would be very, very close this way, however I could be more wrong than
    I think.

    The large ground plane sounds like a great idea.

    Most of the temps measured would be in the 200 to 1200 Degree F Range if
    that make a difference. Accuracy of 1 - 2 Degrees F is plenty good for this
    application.

    I have seen on the Net where some state to add 10k resistors to ground for
    each lead on each pair right before the mux to reduce noise, and also to
    help reduce the antenna effect from one open channel from affecting other
    channels. Does this make sense?

    Richard
     
  9. That can be quite challenging to achieve in practice especially if you
    don't want to allow a 1 hour warmup or have a housing to contend with.
    A 10°F difference can arise fairly readily, depending on what else is
    around.
    Not to "reduce noise", but it might be important where you have
    multiple instrumentation amplifiers- to establish a ground within the
    common-mode range. Most T/C's are low enough resistance that 10K will
    only affect the reading by a small amount. Commercial instruments will
    usually operate within specs for source impedance of 100 or even 1000
    ohms maximum, so the effective shunt 20K could add significant error.


    Best regards,
    Spehro Pefhany
     
  10. Richard

    Richard Guest

    Thanks for all the input. Sure gives me more to think about.

    Richard
     
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