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Thermocouple signal amplification

Discussion in 'Electronic Design' started by apprentice_nerd, May 24, 2005.

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  1. Hi.
    I am attempting to measure temperature with a thermocouple
    connected to an AVR atmel microcontroller.
    I am wondering if a LM1458 dual general purpose operational
    amplifier can be used to amplify the signal from the thermocouple.
    I am familiar with the AN844 "Simplified Thermocouple Interfaces and
    PICmicro MCUs" from microchip however i dont have experience in
    opamps.
    Is a negative voltage to LM1458 necessary to achieve moderate acurracy
    -/+2 C?
    Is the 555 negative voltage generator circuit suitable for such an
    application?
    Can i take the amplified signal and feed it to the second amplifier in
    LM1458?
    I have made a web search for thermocouple amplification circuits
    but the majority suggest the use of specialized ICs or Opamps whose
    cost is not
    justified for amateur experiments like mine.
    Any thought or advice is going to be deeply appreciated.




    KEYWORDS: AVR PIC THERMOCOUPLE AMPLIFICATION LM1458 TEMPERATURE SENSOR
     
  2. Joerg

    Joerg Guest

    Hello Apprentice_nerd,
    Since you are dealing with signal differences in the sub-millivolt range
    this amp isn't very suitable. It has several volts in offset error. You
    either need a very precise amp or a regular one with a chopper
    stabilization (clamping away CD errors and DC drift).
    You have to look at the "compliance range" or "common mode range" in the
    data sheet of the opamp you select. It should clearly state how close
    inputs and outputs can get to the rails until things become non-linear.
    Please mind that the DC performance may not be as good when hugging the
    rails.
    What should it do there?
    In that case I guess you have to familiarize yourself with "auto-zero",
    "chopper" or "clamping" techniques to get rid of DC offset errors. The
    AVR can greatly ease that task because it can act as the "capacitor" by
    storing and subsequently subtracting the DC offset.

    Regards, Joerg
     
  3. It can be, however it's a particularly unsuitable choice with its
    large offset voltage and no TCVos specification and limited
    common-mode input range.
    The input common mode range of that part does not include ground, so
    something like that might be required.
    Don't know what circuit you're talking about, but you'd need 2 or 3V
    negative IIRC, to allow it to work with the input at ground. If you're
    going to use a crap op-amp you may as well use an LM358 then. But it
    won't be very stable with temperature.
    You /can/. Why would you want to?
    You can't afford $3 for a decent op-amp? You can hardly get a cup of
    coffee for that.


    Best regards,
    Spehro Pefhany
     
  4. Joerg

    Joerg Guest

    Hello Spehro,
    We can still get a mid size (venti?) cappucino for $2.90 ;-)

    Regards, Joerg
     
  5. [snip]

    A thermocouple is perhaps the most difficult temperature
    sensor to learn on, and no benefit unless you actually
    need to measure temperatures above about 150C. That is
    especially true when you mention a uP and single rail in
    the same sentence.

    If the range is within -20C to +120C you might find it
    easier to get going with (say) a 1% thermistor.
     


  6. I am for the moment doing some experiments on DC-amplification for low
    impedance sources. I use a DMOS switch( 5 ohm), an inductor and a
    transistor.
    I kind of buck-converter, i.e. the small dc gives a current through the
    inductor which is suddenly stopped, and the voltage spike is analyzed. The
    spike is proportional to the source (thermo-)voltage.
    The outcome of this experiment is in short:
    Better offset stability than the best op-amps.
    (About 10-50 nanovolts for hours)
    Low cost components, but a few too many maybe, are needed.
    But if you can use the functionality of the microcontroller, the number of
    components would be reduced.

    /Sven Wilhelmsson
     
  7. Sven Wilhelmsson wrote...
    Interesting. How much amplification do you get? I suppose the source
    impedance must be low, to avoid noise/offsets from charge coupling from
    the FET's gate drive. What frequency and inductor values are you using?
     
  8. John Perry

    John Perry Guest

    You've given no indication of knowing the properties of thermocouples,
    and no one else has piped up with it, so here goes.

    First, amplification is pointless without first having a
    well-established reference. The DC voltage you get from a thermocouple
    is meaningless without a known temperature to reference your reading to.

    Second, thermocouples are nonlinear with respect to temperature. You'll
    need a conversion algorithm more complex than mx+b to get anywhere near
    a real temperature reading over more than a few 10's of degrees.

    Look at http://www.omega.com/techref/thermoref.html to get a very brief
    look at reference junctions, and at
    http://www.capgo.com/Resources/Temperature/Thermocouple/Compensation.html
    for a quick look at linearization.

    Google can be a big help: google "thermocouple reference junction" and
    "thermocouple linearization" to get some real information.

    As others have said, _after_ you've got the reference junction out of
    the way, you still need an auto-zero amplifier to get an accurate
    voltage. Then you can use your AVR to get a temperature.

    Are you sure you want to get into this? Diodes and thermistors are much
    easier to deal with.

    John Perry
     
  9. Amplification in step one, the inductor, is around 100, which is the
    integration time (1 ms) over the spikewidth (10 us).
    Amplification step two is the transistor. Maximum 50.
    The overall amplification is uncertain but reasonably stable, but it is
    clear that it must be stabilized using feedback.

    Yes, the method is pointless unless source impedance is low, say below 10
    ohm. If you have Z_source > 100 Ohm, using an OP is a better way. One of
    the best low-Z OP is LT1028, and it works best at about 200 Ohm.

    In fact, the main idea is to use an inductor to transform the low source
    impedance (The inductor can of course have a secondary winding, a
    transformer), so that it matches the optimal Z for the transistor. This
    would be in the range 100-1000 Ohm.

    In my experiments I use a frequency of 1kHz. But this is not so important.
    The important thing is to make use of all the integration time:
    L/R > T
    where T is integration time and R is the resistance of the switch including
    source and coil. In my case R=5 Ohm and L>5 mH.

    Noise/offsets from charge coupling:
    This is dominant but fairly stable. I try to balance charge coupling
    somewhat.
    Yes, this might be the limiting factor besides thermal noise.
    I'm not sure about the noise mechanisms here. In switched capacitor devices
    we have kTC-noise, but what is its equivalent in "switched inductor
    devices" ?. Any ideas?

    In any case, I'm almost convinced that this technique is better than the
    best OP-amplifiers for sources below one Ohm.

    Finally, it is important to keep things thermally clamped, and to be careful
    with powerburning near the sensitive area. Any solderpoint is a thermo
    voltage sorce.

    /Sven Wilhelmsson
     
  10. Hello Joerg

    Thanks for your informative answer.

    I read in the LM1458 Datasheet that a negative voltage is recommended
    for better performance.
    I dont have any negative voltage generator at hand only some 555 and this
    schematic about producing negative voltage.
    http://www.csgnetwork.com/ne555c1.html

    Best regards.
     
  11. Hello Mr Pefhany

    Thanks for your time and your valuable suggestions.
    I dont have a negative voltage generator only some 555 and this
    http://www.csgnetwork.com/ne555c1.html

    I thought maybe its possible to further amplify the signal in case the gain
    from LM1458 is not big enough.
    I totally agree that $3 is affordable but at this moment
    i am only investigating if i can measure temperature even
    with +/- 10 acurracy.Unortunately I still have a lot of things
    to learn and i think its better to struggle with minimal budget
    than to make costly mistakes destroying IC's because of ignorance.
    Also i live in Greece and I order from digikey every 3 months because
    Greece is the most expensive country on earth and a cup of coffee is
    at 3.5 Euro!:)

    Best regards and thanks again.
     
  12. Mike Monett

    Mike Monett Guest

    apprentice_nerd wrote:

    [...]
    You might be able to find useful parts in old computer monitors, pc power
    supplies, printers, modems, telephone answering machines, clock radios,
    tv's, xerox copiers, and other things that people usually throw away.
    Usually the older designs have more usable parts.

    For example, answering machines and monitors may contain op amps such as
    LM358. Some modems may have TL074's. Look for ones that combine a modem
    with a soundcard.

    A monitor may have useful parts, such as high voltage MOSFET's like the
    IRF634 and IRF640, potentiometers, electrolytic caps, voltage regulators
    such as 7812, 7809, and 7805, and high voltage transistors.

    Usually people are happy to let you have stuff they normally throw away.
    Just let the word get around that you like these things, and pretty soon
    you will be swamped with parts:)

    Mike Monett
     
  13. Mike Monett

    Mike Monett Guest

    Also, when you start stripping old electronics, you need to find out what the
    part numbers mean. Here's some info on decoding semiconductor part numbers and
    finding data on the parts:

    "Decoding Valve, Transistor and CRT Numbers"

    http://www.bvws.org.uk/405alive/tech/valvenos6.html

    "Transistor Specifications Database"

    http://www.orpheuscomputing.com/technicians/transistor_specs.html
    http://www.orpheuscomputing.com/technicians/transistor_specifications.html

    "Transistor Database"

    http://samed.netfirms.com/htm/trn/Transistor katolog.htm
    http://www.geocities.com/saphanlex/p/tr.htm

    "Transistor Cross-Reference TCG/NTE/ECG To JEDEC and Japanese"

    http://www.flippers.com/X-Ref-ECG.html

    "BC Transistor Cross-Reference"

    http://www.gsg-asia.com/fpl.htm

    Here's a good source for datasheets. It allows partial searches, so
    you don't have to know the full part number:

    http://www.datasheetarchive.com/

    Here's another one in case the above doesn't have what you are looking for:

    http://www.datasheetcatalog.com/

    Mike Monett
     
  14. Yes, I thought it might be something like that. ;-) You may not get
    enough negative voltage using a bipolar 555 at 5V with two diode
    drops. Consider using a CMOS 555 or driving a CMOS buffer with the
    555. Or consider a 7660. You'll have to worry about exceeding the
    allowable negative input voltage on the AVR (typically something like
    -300mV).
    Okay, here's a chance to put pen to paper. Given your low accuracy
    requirements, does the open-loop gain of the LM1458 really matter?

    Calculate the gain of the closed loop amplifier given a finite gain
    op-amp, and substitute the minimum and typical open-loop gains of the
    1458. Simple algebra, and you'll get a better feel for what's
    important. Say you want the output of the op-amp to swing from 0 to
    2.5V (it won't go much higher reliable with +5/-3V supplies, IIRC)
    with a 60mV input, the closed-loop gain would be about 40.
    Is that for that mud-like stuff that most of the rest of the world
    calls "Turkish coffee"?
    No problem.


    Best regards,
    Spehro Pefhany
     
  15. Jon

    Jon Guest

    SENSOR

    The biggest problem with using non-specialized circuits for
    thermocouple voltage amplification is the lack of "cold junction
    compensation". Suppose you have a copper-constantan thermocuople. If
    you connect the constantan thermocouple wire to your circuit via a
    copper wire, you have formed a new "phantom" thermocouple in series
    with the "real" thermocouple. This "phantom' thermocoule generates a
    voltage in series with the voltage from the "real" thermocuople. The
    phantom thermocuple voltage is a function of its temperature.
    Thermocuple interface amplifiers have built in voltage generators (cold
    junction compensation) that takes care of this problem.
    ..
    ..Historical note: In the "old days", for laboratory work, the cold
    junctions were immersed in an ice water bath to ensure that they
    produced a known "phantom" voltage in series with the "real"
    thermocuple voltage, so that it could be subtracted out to get the
    correct net voltage.
    ..
    Regards,
    Jon
     
  16. That's not really a problem in this case, he's got a micro and there's
    absolutely no reason to use the old-fashioned analog methods for CJC.
    In fact if you want to design either a universal T/C front end or a
    very accurate one (with delinearization of the CJC signal) it's like
    totally the wrong way to go about it.


    Best regards,
    Spehro Pefhany
     
  17. Joerg

    Joerg Guest

    Hello Apprentice_nerd,
    That would be a charge pump. You can do that but the 470uF caps seem a
    bit large for the little current you need.

    I would look for a single supply opamp which can go to GND. Less hassle.

    Regards, Joerg
     
  18. René

    René Guest

    1st I would use e.g. a LM 335. Given the fact that you contemplate the
    1458, I guess the power supply will be 5 Volt or such - the LM335 will
    fit the ADC straight without any amplification. With 10 bit conversion
    the resolution will be adequate.

    If the opamp road must be followed - just use a virtual ground and a
    single supply. (and a slightly better opamp - please)

    success!
     
  19. If he's planning on measuring the temperature in, say, a ceramic kiln
    then that part would have a rather brief career. ;-)


    Best regards,
    Spehro Pefhany
     
  20. Fred Bartoli

    Fred Bartoli Guest

    But you can still measure the LM335 temperature with a pyrometer.
     
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