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Category: Hardware

Hello Team,
We are designing a system to measure the temperature using a thermocouple.
We are planning to use the IC AD8495ARMZ.
The ADC we are using is AD7193BRUZ-REEL and it can take values from 0V to 4.096V.
We need to measure the entire working range of the K Type thermocouple (From 3.15K to 713.15K).
So we are planning to give a referance voltage of 1.25V at the REF pin of the AD8495ARMZ.
But when we calculated the output voltage with the equation provided in the datasheet VOUT = (TMJ × 5 mV/°C) + VREF, the output voltage at 3K goes below 0V (to -100mV).
But the output voltage is 470mV.

Can you please suggest the voltage needed at the REF pin of the AD8495ARMZ so as to get a 0V to 4V output for the temperature range from 3.15K to 713.15K?

Attached is our schematic for the same.

Parents
• I have moved this thread here (+) Q&A - Instrumentation Amplifiers - EngineerZone (analog.com). Someone here should be able to assist you in your query.

Regards,

JC

• Hi shibinvarghese9@gmail.com

Thank you for reaching out to the team and sharing your inquiries.

To further assess your questions and be able to share the right recommendations, we would like to know more about the intended applications and design parameters.

The Intended Applications: what is the purpose of this circuit? i.e., as an oven temp sensor, temp controller, etc.?

What is the temperature range to measure? You mentioned 3.15K to 713.15K; is this regarding Kelvin? Are you planning to measure purely hot temps, cold temps, ambient room only, or both, and all three?

In your applications, where do you typically place or locate the AD8495? How about your Thermocouple? Where do you place it? i.e., to measure the temp in an oven, do you place the 8495 outside the oven and the TC inside the oven?

Are your reference junction and 8495 mounted in one PCB? and are your thermocouples connected on the dotted line?

The equation used to compute the expected output voltage is correct, and you can use this as long as the measured temp is within the range stated in Table 7 of the Datasheet.

Hence, the reference voltage used is also correct to offset the outputs during sub-zero temp measurements. However, I suggest using a buffer amplifier (AD8613 and OP777) to drive the MAX60101 (U24) output to the REF pin.

Now if the temperature to measure falls outside the range, you can use this equation for AD8495 (this is available on the Application Note): AN-1087: Thermocouple Linearization When Using the AD8494/AD8495/AD8496/AD8497 | Analog Devices

There is a look-up table on the NIST in which you can refer to the values calculated by the equation, and you can confirm if the output is aligned with the temperature.

fNIST is a millivolt-to-temperature function based on the standard lookup tables or on equations published by the National Institute of Standards and Technology (thermocouple databases can be found at http://srdata.nist.gov/its90/main).

Please let me know if this helps.

Regards,

• Hello Arsol,
Our product is a temperature monitor.
The thermocouple we are using is a  K Type thermocouple.
The temperature range we want to read is from 3.15K to 713.15K (Kelvin).

In your applications, where do you typically place or locate the AD8495? How about your Thermocouple? Where do you place it? i.e., to measure the temp in an oven, do you place the 8495 outside the oven and the TC inside the oven?
The AD8495 will be placed in the PCB located close to the thermocouple connector J4.
The TC will be placed in a place we want to measure the temperature (can be an over or chiller)

Are your reference junction and 8495 mounted in one PCB? and are your thermocouples connected on the dotted line?
Yes.

Hope everything is clear to you

The ADC we are using is AD7193BRUZ-REEL and its input should be in the range of 0V to 4V only.
Can I use a 1.25V refrance voltage to get the output voltage of the AD8495ARMZ to be in the input range of ADC for the temperature range 3.15K to 713.15K (Kelvin)?

• Thank you for getting back to me.

Regarding your question, The ADC we are using is AD7193BRUZ-REEL, and its input should be in the range of 0V to 4V only.
Can I use a 1.25V reference voltage to get the output voltage of the AD8495ARMZ to be in the input range of ADC for the temperature range of 3.15K to 713.15K (Kelvin)?

Arsol: Yes, you can use the 1.25V Reference on Single supply configuration on the temp measurement range of 3.15K (-270C) to 713.15K (439.85C). From my previous response:

Hence, the reference voltage used is also correct to offset the outputs during sub-zero temp measurements. However, I suggest using a buffer amplifier (AD8613 and OP777) to drive the MAX60101 (U24) output to the REF pin.

Now if the temperature to measure falls outside the range(as indicated on Table 7 of the Datasheet), as what are you planning on your applications(3.15K to 713.15K). You can use this equation for AD8495 (this is available on the Application Note): AN-1087: Thermocouple Linearization When Using the AD8494/AD8495/AD8496/AD8497 | Analog Devices

Hope this helps.

Regards,

• Thank you for getting back to me.

Regarding your question, The ADC we are using is AD7193BRUZ-REEL, and its input should be in the range of 0V to 4V only.
Can I use a 1.25V reference voltage to get the output voltage of the AD8495ARMZ to be in the input range of ADC for the temperature range of 3.15K to 713.15K (Kelvin)?

Arsol: Yes, you can use the 1.25V Reference on Single supply configuration on the temp measurement range of 3.15K (-270C) to 713.15K (439.85C). From my previous response:

Hence, the reference voltage used is also correct to offset the outputs during sub-zero temp measurements. However, I suggest using a buffer amplifier (AD8613 and OP777) to drive the MAX60101 (U24) output to the REF pin.

Now if the temperature to measure falls outside the range(as indicated on Table 7 of the Datasheet), as what are you planning on your applications(3.15K to 713.15K). You can use this equation for AD8495 (this is available on the Application Note): AN-1087: Thermocouple Linearization When Using the AD8494/AD8495/AD8496/AD8497 | Analog Devices

Hope this helps.

Regards,

Children
• Hello Arsol,
Is there any method to identify the open sensor condition for the thermocouple with the IC AD8495ARMZ?
Or do we need any extra circuits to achieve this functionality?