# 75..125C temp sensing with <0.5C accuracy (T-type thermocouple)

Document created by analog-archivist on Feb 23, 2016
Version 1Show Document

### Q

Q1. Our research team need to measure the temperature profile on the surface of
a chip using multiple thermocouples (4x at least, at different positions). For
our experiments, we assume having T-type thermocouples with a fast response
time and accuracy better than 0.5°C. We then need to amplify the signal coming
from all the thermocouples and send a structured packet to the PC for
temperature trace analysis, via USB.

For this reason, we need a thermocouple amplifier that is able to provide
accurate amplification of the signal. We came across the AD594/5 IC, but from
the specs we realize that it guarantees only 1°C of accuracy, and this is too
low for us.

Do you have an IC in your portfolio that guarantees 0.5°C of accuracy or even
better (e.g., 0.1°C)?

Q2. How could we properly calibrate the AD8495 to deal with T-type
thermocouples?

### A

Q1. Something close to that is likely possible with the AD8495 and some
calibration. The "Initial Error" spec is due to amplifier offset voltage and
can be calibrated out with a 1 point temperature calibration, and then AD8495C
specifies < 0.1% gain error. But also Type T thermocouples themselves are quite
non-linear. You would definitely need linearity compensation to get anywhere
near 0.5°C. We can provide a linearity compensation polynomial or table if
needed.

The best solution is more likely something like the ADT7320 and the AD7793 in
this article :

Q2. In the narrow range, you still see some nonlinearity. For very high
accuracy, I still don't think linearity is a very good assumption. If you use
AD8495 with a type T thermocouple and you assume the output is linear between
75°C and 125°C, you end up with a maximum of about 2.8mV output error vs the
straight line. That's a little bit more than 0.5°C.

Here is the graph and the table.

The table:

The polynomial calculated by Excel's best-fit line over this range was :

The 23°C ± 5°C ambient temperature rejection error was negligible, calculated
to change the output voltage by ±0.1mV. Also, the stated Polynomial Error takes
that ambient temp error into account.

Chances are the look-up table is computationally cheaper than the polynomial in
this case.

I would stress once again the importance of an accurate one-point temperature
calibration to remove the Initial Accuracy error. Any error in that calibration
measurement will persist as residual offset.