Dear Michal,
welcome to our forum and thank you for your interest in our ADS1118.
Actually I am not aware of any comment in the ADS1118 datasheet about the device having integrated open/shorted input detection.
We talk about open sensor detection in the thermocouple application section. What we do there is to connect pull-up and -down resistors to the thermocouple. Those bias the thermocouple to mid-supply. In addition they serve to help detect an open sensor condition. In case one of the thermocouple leads would be disconnected, the resistors will pull the inputs of the ADC to opposite supply rails. Means the positive input to AVDD and the negative input to AVSS. Consequently the ADC would read a full scale value which will be an indication that something with your thermocouple is wrong.
A better device to use might be our brand new ADS1120. The device will release to market in three weeks. You can find the preliminary datasheet on ti.com. Samples would be available in case you want to evaluate it.
The ADS1120 is perfectly suited to measure both thermocouples and NTC's/RTD's. It integrates a high input impedance PGA, thus an external buffer might not be required. The device also integrates precision excitation current sources which could be used to excite the NTC.
Regards,