AN 769: Intel FPGA Remote Temperature Sensing Diode Implementation Guide

ID 683139
Date 4/06/2022

3.1. Ideality Factor (η-Factor) Mismatch

When you perform junction temperature measurement by using an external temperature diode, the accuracy of the temperature measurement depends on the characteristics of the external diode. The ideality factor is a parameter of a remote diode that measures the deviation of the diode from its ideal behavior.

You can usually find the ideality factor in the data sheet from the diode manufacturer. Different external temperature diodes give you different values because of the different design and process technologies they use.

Ideality mismatch can cause a significant temperature measurement error. To avoid the significant error, Intel recommends that you select a temperature sensing chip that features a configurable ideality factor. You can change the ideality factor value in the chip to eliminate the mismatch error.

Ideality Factor Contribution to Temperature Measurement Error

This example shows how ideality factor contributes to the temperature measurement error. In the example, the calculation shows the ideality mismatch causing a significant temperature measurement error.

Equation 4. Ideality Factor Relationship to Measured Temperature


  • ηTSC—ideality factor of the temperature sensing chip
  • TTSC—temperature read by the temperature sensing chip
  • ηRTD—ideality factor of the remote temperature diode
  • TRTD—temperature at the remote temperature diode

The following steps estimate temperature measurement (TTSC) by the temperature sensing chip, given the following values:

  • Ideality factor of the temperature sensor (ηTSC) is 1.005
  • Ideality factor of the remote temperature diode (ηRTD) is 1.03
  • Actual temperature at the remote temperature diode (TRTD) is 80°C
  1. Convert the TRTD of 80°C to Kelvin: 80 + 273.15 = 353.15 K.
  2. Apply Equation 4. The calculated temperature by the temperature sensing chip is .
  3. Convert the calculated value to Celsius: TTSC = 344.57 K – 273.15 K = 71.43°C

The temperature error (TE) caused by the ideality mismatch:

TE = 71.43°C – 80.0°C = –8.57°C