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Output range problem for AD8497

Hi. I am using AD8497 for K-Type thermocouple measurement. I have given supply of +5v and 0V. and Ref voltage of +0.5V. My input signal ranges from -2.0mV to 70mV. I get output signal only for input signal upto 40mV. Please suggest schematic for measuring the complete range of my input signal.

Below is the schmatic I have used.

Thanks and Regards,

Kumar

  • Hi Kumar,

    Are you able to increase your supply voltage?  The AD8497 has a gain of about 122.4V/V - so at a 5V supply you will be hitting the rail around a 40mV input.  You can increase the +VS supply to get more range.  Just note that the Absolute Max is +/-18V supplies,  or 36V between the supplies - I don't think you'll run into that.  

    Let me know if that helps.

    Regards,

    Jordyn

  • Hi Jordyn,

    Thanks for your reply.

       1. Actually, 5V is the maximum range that I am using in my design and I

       don't have scope to increase it. *So, can you suggest a way to measure

       using 5v supply.* Does changing the values of resistor at the input side

       will meet my requirement. I have a limitation to using the same circuit as

       I got the PCB. I can just vary the values of components.

       2. Is there any other IC that meets my requirement with 5v and same

       Package.

    Regards,

    Kumar

    On Sat, Feb 17, 2018 at 3:30 AM, jrombola <

  • Hi Kumar,

    The gain of the AD8497 is fixed and any modifications to the circuit would likely cause unnecessary errors to be introduced.  There are other in-amps where you can set the gain with an external resistor or by pin-strapping.  This would allow you to set the gain a little bit lower to be able to measure the 70mV input. 

    I have a couple questions though:

    Are you expecting the 70mV from the k-type thermocouple?

    What temperature range are you trying to measure?

    Regards,

    Jordyn

  • Hi Jordyn,

    I don't know exactly at what temperature is being measured. But the details

    I have is that a K-Type thermocouple is used which gives 70mV max.And I

    have to measure it using Thermocouple amplifier. What will happen if I

    divide the input signal and feed to the amplifier. I don't want to use

    external resistors to adjust the gain externally.

    Regards,

    Kumar

    Dear

    Thanks and Regards

    P.Kumara swamy

    Embedded Engineer

    Sigma Microsystems

    On Sat, Feb 17, 2018 at 10:22 AM, jrombola <

  • Hi Kumar,

    Can you find out what the measurement temperature range is?  Or reconfirm the 70mV max?  70mV seems very high - take a look at the k-type thermocouple table from NIST: https://srdata.nist.gov/its90/download/type_k.tab 

    The max measurement temperature of 1370degC translates to a thermocouple voltage of about 54mV.  

    Regards,

    Jordyn

  • Hi Jordyn,

    Thanks for your reply.

    I have reconfirmed and the measurement range is 70mV max. My people have

    refused to give details of Thermocouple but they mentioned that it is

    chromel -alumel type.

    I also saw the table you have mentioned. but I didn't understand how they

    get 70mV. Please suggest method to measure using AD8597 with 5v supply.

    using voltage divider at the input.

    Regards

    Kumar

    Dear

    Thanks and Regards

    P.Kumara swamy

    Embedded Engineer

    Sigma Microsystems

    On Sun, Feb 18, 2018 at 10:48 AM, jrombola <

  • Hi Kumar,

    May I know if you were able to make this circuit works and get measurements accurately? The reason Jordyn wants to confirm the 70mV is because for the standard type K-type thermocouple, the 1350°C temperature produces around 54mV as shown on this link. Is it correct to assume that you are sensing temperature higher than 1350°C using K type thermocouple? AD8497 is precision trimmed to optimize a linear operating range for K type thermocouple for measurement temperature range from -25°C to +295°C and ambient temperature of 25°C to 100°C only. For temperature outside this range, implementation of look up table or equation using a microprocessor is needed for nonlinearity correction as discussed on this article.

    Let me know if there something that we could help on your current design.

    Best regards,
    Emman