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relative temperature measurment using ad8495


i need to measure a sample temperature and the relative temperature using 2 thermocouples and two ad849.

the way i connect the thermocouples is as follow : 

in the sample measurement it quite stait ahead the - of the thermocouple goes to - of the amp and the + of thermocouple goes to + input of the amp.

in relative measurement the + of the first thermocouple is shorted to the + of second themocouple so the - input of thermocouple A goes  to - of the second amp and the - of thermocouple B goes to + input of second amp. in case that both themocouples are in same temperature i should have the absolute temperature in first amp and 0 or almost 0 in second amp.

what i got is that the absolute is wrong and the relative temperature has great errors

what should i do?

  • Hello Idan,

    Thank you for explaining your circuit. Just to make sure I understand it though, could you please provide a schematic? Please include any other connections that you have around the AD8495.

    Could you also describe what you mean when you say the output is wrong? What are you reading out of the AD8495? Are you already trying to sense a temperature through the thermocouples? It would help a lot if you could provide some data for what you are measuring.



  • thanks krisf

    i made a block diagram which describe what i mean

    as I said there are two k type thermocouples which both positive tips are connected . in first amp I measure the absolute temp so the thermocouple A is connected to amp A in the as usually. in second amp I measure the relative temperature between thermocouple B and thermocouple A , i.e  delta_temp = thermocouple_B  - thermocouple_A . for example if both thermocouples are in same temperature , I should measure in amp A the environment temperature and in amp B i should measure 0 deg C temperature or something close to 0 deg C. in fact what I measure is that in amp A  the environment temperature with 1 or 2 degC error and in amp B I measure a temperature of 27 degC no matter what is the environment temperature ( I checked it in two different environments: room temp of 25 degC and the in ice about 0-1 degC). when I placed  thermocouple A in ice and thermocouple B in room temp i got  0 degC reading in amp A and 54 degC in amp B.   

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