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    MAX31856 how increase thermocouple ESD protection?

    MAX31856 is precise 19bit Thermocouple to Digital Converter with Linearization and have on T+ T- input protection on +-45V. The range can be increased by resistors, but it brings a number of problems.
    I need to increase resistance to ESD.
    My idea is add on T+ T- ESD protection diodes any as PESD5V0S2UQ

    My question
    1. It's a good idea or the ESD diode affects the measurement of the thermocouple
    2. If not, I can say that the thermocouple is junk
    IEC 61000-4-2; level 4 (ESD) > 15 kV (air); > 8 kV (contact)
    MIL-STD-883; class 3 (human body model) > 4 kV

    - - - Updated - - -

    one more question.

    I do not see the specification for DRDY in the datasheet.
    I can dirrenct connect 4 this output on one MCU input?

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    Re: MAX31856 how increase thermocouple ESD protection?

    Hi,

    Did you read the datasheet? The IC already includes ESD protection, but lower level.

    and have on T+ T- input protection on +-45V.
    This is the "Absolute maximum rating" not the operating common mode input voltage range.
    This means going beyond the CMIVR may cause measurement error.

    The range can be increased by resistors,
    Tell us your idea, show a schematic..then we can discuss about it.

    it brings a number of problems.
    What problems?

    It's a good idea or the ESD diode affects the measurement of the thermocouple
    WIthout schematic impossible to say.
    What exactly do you mean with "affect"?
    * continously
    * or during ESD condition

    If not, I can say that the thermocouple is junk
    It's nit the problem of the thermocouple...and maybe even not of the MAX IC...
    Maybe you just have to use another ESD protection device, or you have to adjust the schematic

    IEC 61000-4-2; level 4 (ESD) > 15 kV (air); > 8 kV (contact)
    MIL-STD-883; class 3 (human body model) > 4 kV
    Well done. Good specification that is often missed in other threads)

    I do not see the specification for DRDY in the datasheet.
    Just use the search function of your pdf reader.

    I can dirrenct connect 4 this output on one MCU input?
    Impossible to answer without knowing what microcontroller you use ... and informations on port, port setup, supply voltage...

    Klaus
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    Re: MAX31856 how increase thermocouple ESD protection?

    Another important point: you are dealing with tiny voltages and high gain amplifiers. The voltage source must only be the thermocouple tip unless you compensate using cold junction offset as at the MAX input pins. If you add other junctions to the thermocouple wiring you risk introducing voltages from other sources which will obviously result in wrong temperature readings.

    Brian.
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    Re: MAX31856 how increase thermocouple ESD protection?

    If I got it right, all intup and T+ T- too are provided internal protection diode for max continuaous current 20mA.
    Tthese diodes allow on T+ T- vlong time voltage +-45V and short time ESD 2000V .
    The first option to increase protection is add to input resistr for example R4,R5 20k , voltage lost for 20k and 20mA is 400V
    i.e. voltage protection for long time willbe 400+45V , but the impact on ESD will be marginal.
    the inserted resistors impair the thermal bond between the thermocouple end and the temperature sensor for compensation in MAX31856.

    My idea is add ESB protection diode D3.
    This diode increase ESD protection on 4kV human body modem and 15kV air, increase cuccent on long time, my estimate min by another 20mA.
    disadvantages
    - increase parasitic capacitance, I thing no problem, diode have max 200pA and on T+T- are 10nF C
    - leacage current diode have max 300nA for 5V and on thermocouple id thecmocouple voltage +735mV BIAS I thing no problem but I am not sure
    - something else ?

    DRDY - in datasheet is not described in detail electrical chracteristic of this output. I theng stnadart outpit with pulup resistor and MOSFET which pulls the output to the ground. It is not a problem to connect several of them to one output, but I'm not sure yet again.

    Click image for larger version. 

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    Re: MAX31856 how increase thermocouple ESD protection?

    DRDY - in datasheet is not described in detail electrical characteristic of this output. I think standard output with pullup resistor and MOSFET which pulls the output to the ground. It is not a problem to connect several of them to one output, but I'm not sure yet again.
    There's no indication in the datasheet that DRDY or FAULT are open drain outputs. Strict reading the DRDY waveform suggests a push pull output. I agree that open drain might be useful in multiple chip configurations, but apparently it's not designed this way. Obviously nobody can clarify the point except Maxim support.



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    Re: MAX31856 how increase thermocouple ESD protection?

    I concact Maxim Integrated and her is information from technical support.

    DRDY - is push-pull. And so, unfortunately, their direct connection is not possible.
    Using external ESD diode on T+ T- is not a problem and is recommended, Maxim Integrated use for example NUP2105L



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    Re: MAX31856 how increase thermocouple ESD protection?

    Hi,

    NUP2105L makes more sense:
    * It is a bipolar device (your device is unipolar, only)
    * less capacitance of 30pF (whereas your device has 150pF)
    * less leakage current of 1.5nA @ 24V (50nA @5V)

    Klaus
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    Re: MAX31856 how increase thermocouple ESD protection?

    You're right, but
    - bipolar device - in the thermocouple circuit is not negative voltage
    - parasitic capacotnace - all thermocouple input they are against the earth 10nF capacitor I thing +-150pF does not matter
    - less leakage current - Thermocouple Input Bias Current MAX31856 is -20 +400nA in dependence on temperature , I thing thermocouple is current source and resistence of thermocouple allows the thermocouple to be loaded with minimal tens of uA without affecting accuracy. 50nA is on 5V but on thermocouple will be significantly lower voltage under 1000mV



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    Re: MAX31856 how increase thermocouple ESD protection?

    Hi,

    - bipolar device - in the thermocouple circuit is not negative voltage
    Correct, but we are talking about ESD ... and it's voltage, which may be positive, as well as negative, thus your protection needs to be bipolar..

    - parasitic capacotnace - all thermocouple input they are against the earth 10nF capacitor I thing +-150pF does not matter
    I agree.

    - less leakage current - Thermocouple Input Bias Current MAX31856 is -20 +400nA in dependence on temperature , I thing thermocouple is current source and resistence of thermocouple allows the thermocouple to be loaded with minimal tens of uA without affecting accuracy. 50nA is on 5V but on thermocouple will be significantly lower voltage under 1000mV
    Thermocouple is a voltage source. Usually with low resistance. Thus a bit of current doesn't hurt....usually ... but the voltage signal of a thermocouple is very low...and thus even small error voltage affects accuracy.
    Btw: identical leakage current at both electrodes doesn't hurt, only the difference current.

    Klaus
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