When I use AD4110-1 RTD function (excitation current).
May I use internal reference for RTD excitation current?
Or It must be to use external 2.5V reference?
An external 2.5V reference is required when excitation currents are used as it is used to generate the excitation currents through 25kohm internal resistors. It is also suggested/recommended to use the external reference as your reference voltage so that the RTD measurement is ratiometric, meaning any errors in voltage reference (accuracy or temperature drift) will not affect the measured resistance. It is not recommended to use an internal reference as your measurement will not be ratiometric.