Driving 18 Thermistors in parallel
Is the ADC time-multiplexed?
If so then time multiplex the feed excitation to the thermistors as well.
In sequence apply excitation to the specific thermistor, wait a bit, measure the thermistor with the ADC then move to the next one.
This would also have the advantage of reducing the power dissipation in the thermistor to minimize the error due to self-heating.