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# TIDA-010019: CJC IC accuracy effect on the output

Part Number: TIDA-010019

Hi Steffen,

Could you show how to estimate the effect of CJC component accuracy when doing TC measurement?

I found two transfer functions in below material

https://www.ti.com/lit/ug/tidu271/tidu271.pdf (equation 2)

https://www.tij.co.jp/jp/lit/ug/tiduba5/tiduba5.pdf (equation 1,2)

First one seems the simplest, and the second one using RTD.

The system I am considering is like the one on TIDA-010019, specifically ADS112U04+anagol temp IC.

The user guide has result of temp error of CJC, would it directly affect the result accuracy?

For example, LMT70 has 0.1 degree offset and 0.05 degree error. Without offset calibration, would the system accuracy have additional 0.1 degree offset and 0.05 degree error? Or, would it be more complicated than that?

Best regards,

Kurumi

• Hi Kurumi,

simply speaking, you need to calculate the corresponding TC voltage from the ambient temperature, subtract this from the measured TC voltage and then calculate a temperature from this.

Same needs to be done with the error, now it gets a bit more complicated, as the TC voltage is not linear and you care about the temperature difference. If the TC and CJC is at the same temperature, it is simple, the error is only the error of the CJC (+ additional ADC offsets).

With temperature differences it gets a bit more complicated if you want to have it accurate.

Let me do a sample estimation:

Assumption: CJC temperature 25 °C and 26 °C

TC type K, voltage: 4.096 mV (100 °C temperature difference)

We now need to calculate or look up the corresponding voltage from the CJC 25 °C --> 1 mV, 26 °C --> 1.041 mV

Than we take our TC voltage reading and compensate it: 4.096 mV + 1 mV = 5.096 mV and for 26 °C it is 5.137 mV

This voltage we convert now back to a temperature reading: 124.3 °C (25 °C) and 125.317 °C (26 °C)

So you see for this example with 1 °C difference in the CJC (which would be a +- 0.5 °C error), we end up with a total error of 1.017 °C error. This means, the error is roughly the error from the CJC, but not exactly. To look closer, we would need to know the type of TC used, and the temperature range.

I hope this helps you to estimate the error for your case.

Best regards,

Steffen

• Hi Steffen,

Thank you so much for detailed explanation. Can we discuss a bit more offline? I'll send you email with more system info.

Best regards,

Kurumi