Thermocouple Type J circuit design using a AD594

[PHILTHOM]

Hi,

I am looking for information to help get an output of 0-10V for a corresponding temperature range of 35-100 deg C from utilising a Analog Devices AD594 in conjunction with a Type J thermocouple.

(For info - I had previously looked at the use of an instrumentation amplifier but was steered away from this)

I have searched the forums here and cannot find information on:-

i) Offseting - @ 35 degC the output is equal to 0V?
ii) Increase in gain - Rated output of AD594 is 10mV/degC output, to 153.846mV/degC required (100-35/10-0)?

I have also tried application notes for this device and cannot seem to find the answer.

Any advice you can provide would be greatly appreciated

 

[jayanth.devarayanadurga]

Mention max current at AD594 o/p if temperature is 100 deg C. A non-inverting Op-Amp has to be used to amplify AD594 voltage and make it 5V i.e., if temperature is 100 deg C adc input should be 5V.

 

[PHILTHOM]

Jayanth,

Thanks for the reply - I think I am understanding this now.

At 35 deg C the output from the AD594 will be 0.35V (10mv/degC) - hence this is the offset voltage required against the output of the AD594 in the Non-Inv Op-Amp with an amplification of 15.3846V/V.

Does this sound correct?

Thanks,

Phil

 

[jayanth.devarayanadurga]

If you do like that you will get wrong values for temperatures below 35 deg C. If temp is 27 deg C you will get wrong value.

If AD594 voltage for 0 deg C is 0V (say) and for 35 deg C = 50mV (say) and for 100 deg C = 250 mV (say) then Op Amp should amplify 250 mV to 5V. Your adc will then measure temp between 0 deg C to 100 deg C. Zip and post your project files and proteus file.

Just mention what voltage you get at AD594 o/p if temp is 100 deg C. Just measure temp when thermocouple is immeresed in boiling water.

 

[PHILTHOM]

Thanks for that but the temperature range to be measured is 35 - 100 deg C (For this range i have to provide a t35=0V to t100=10V), temperatures outside this range are not required to be measured.

This is theoretical design only - I do not have components available.

Thanks for your pointers.

Regards,

Phil

 

[jayanth.devarayanadurga]

t35=0V to t100=10V

t100 = 10V is beyond adc i/p range.

why do you worry about adc i/p vaoltage for out of range values. You can use a condition in code to just find and display temp for range 35 to 100 deg C.

 

[PHILTHOM]

t100 = 10V is beyond adc i/p range.

why do you worry about adc i/p vaoltage for out of range values. You can use a condition in code to just find and display temp for range 35 to 100 deg C.

Can you clarify this point please?

 

[jayanth.devarayanadurga]

Using a if((temp >=35) && (temp <= 100)){ //display temp} will do the work.