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TMP64: using TMP6-THERMISTOR-DESIGN (4th oder polynomial) for TMP6431

Part Number: TMP64

Team,

We are currently using the TMP64 devices (TMP6431QDECRQ1 to be specific) in our project, and used the Thermistor Design Tool from the TI homepage (https://www.ti.com/tool/TMP6-THERMISTOR-DESIGN), to get a 4th order polynomial equation for this device, and calculating the temperature in degree from the ADC voltage.
-We entered our parameters on the first page (V_bias = 5V, and ADC = 14bits), and tried to verify the equation with the builtin function on sheet “4th order Polynomial TMP vs. Vdc”, to verify the resulting equation.
-However, if we entered 2.5V as Vtemp for example, we thought that we should get the nominal temperature of 25°C @ 47kOhm, to which the 47kOhm thermistor and its 47kOhm bias resistor is designed for.
But if we do this, we get 29.1°C as output temperature of the design tool.

Our question is therefore, if we missed something in the device datasheet, or the design with this Excel tool?

From the sheet “device resistance table”, we also can see that the resistance @25°C is 45.834kOhm, which is not what we expected from the datasheet.

A.

• Hi Anber,

From the information you're providing I would say that you are using the tool correctly. The 4th order polynomial from the Thermistor Design Tool is the most accurate resistance, or voltage, to temperature conversion method. I would recommend using the information from this sheet of the tool while designing your system.

To address your question on why the Vtemp at 25°C is not 2.5V, this is due to the intrinsic behavior of the TMP64. It is important to note that the TI thermistor family of devices are silicon based linear thermistors, which are different than NTC thermistors. What you are seeing is an affect of this. We have this note at the top of the 4th Order Polynomial TMP vs Vdc tab to address this difference.

"TI thermistors are silicon-based linear PTCs and are affected by the total current and voltage applied. Changing the bias current will affect the voltage to temperature relationship and may cause temperature errors outside of the specified tolerances in the datasheet."

This means that depending on the TMP6 part number and bias method, the nominal resistance will vary slightly. That is why you see a resistance of 45.834kΩ rather than 47kΩ at 25°C.

Jalen

• Hi Jalen,

thanks for your clarification. We just wanted to be sure that we did not do something wrong using the tool, and did not expect these 1-2kOhm deviations from the datasheet.

: You can mark this question as resolved, thank you!

Best regards

StGr

• Thanks for letting us know.  I'll mark it closed.