Designing with AD8495ARMZ

Hi shibinvarghese9@gmail.com ,

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.

Please share the following items:

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?

Regarding your questions:

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.

f_NIST 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,
Thank you for your reply.
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).

Please find our responses to your queries below.

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)?

Looking for your reply.

Hi shibinvarghese9@gmail.com 

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,

Hello Arsol,
Thank you for your reply.
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?


Looking for your reply.

Hello Team,
Awaiting your responses

Hi shibinvarghese9@gmail.com 

I apologize for missing your question.

 I recommend two hardware checks for the thermocouple and AD849x; to validate that either is still functional. You can use the following steps on your applications:

• The part can be configured as a standalone thermometer to confirm if AD849x is working; the thermocouple sensing functionality is disabled by shorting both AD849x inputs to the ground; the AD849x outputs the value from the onboard temperature sensor. It should measure the ambient temperature where the AD849x is located. Just note that this is only applicable for ambient temp range of -40C to 125C.

• AD849x offers open thermocouple detection. The inputs of the AD849x are PNP-type transistors, which means that the bias current always flows out of the inputs. Therefore, the input bias current drives any unconnected input high, which rails the output. Connecting the negative input to the ground through a 1 MΩ resistor causes the AD849x output to rail high in an open thermocouple condition. On this condition (open thermocouple), the output will clamp to (+Vs-0.1) value.

Regards,