TI also developed specific IC to meet NTC and RTD signal conditioning application. Such IC generally had integrated two adjustable constant current sources to excite NTC and RTD sensor. The IC also integrated high resolution ADC and associated PGA for signal conditioning. For multichannel application, the IC also integrated a MUX. The miscellaneous user interface for meeting varies requirements. A typical IC is ADS124S08. Figure 3-3 shows a typical configuration for ADS124S08 to measure temperature with a 3-wire RTD.

The internal adjustable constant current source 1 of ADS124S08 flow out reference resistor R_REF, diode, R_lead1, RTD, R_lead3 and R_bias then back to ground. The internal adjustable constant current source 2 also flow out ADS124S08, diode, R_lead2, R_lead3 and R_bias then back to ground. Here R_lead1, R_lead2 and R_lead3 are resistor of wire of RTD and has equal resistor. The impact of lead resistor was removed since the voltage produced by excited current are exactly the same under exactly same excite current and canceled. Assume the value of constant current source is I, PGA gain is G, then ADC input voltage can be: I×R_RTD×G. ADC reference can be equal voltage of R_REF. So, the ADC result can be:

Equation 3 shows that ADC result has no function with precision of constant current source. The result is only associated with reference resistor and gain of amplifier. The customer can directly get the resistor value of RTD/NTC from ADC result. Then to deduce the temperature.

The ADS124S0x Low-Power, Low-Noise, Highly Integrated, 6- and 12-Channel, 4-kSPS, 24-Bit, Delta-Sigma ADC with PGA and Voltage Reference data sheet and A Basic Guide to RTD Measurements application note show how to remove the impact of resistor of lead and error of adjustable constant current source to get better precision. Refer to the data sheet and application note for more detailed information.