tocpcs:
It appears the temperature sensor on the circuit being monitored has a diode on the grnd (looks like a shottky so I think -- lightning protection?)
And the positive is a 10k resistor.
It sounds to me that the temperature sensor arrangement has quite a low source resistance, so you can connect it directly. Can you provide a link to the datasheet for the sensor?
If it's on a long lead then you might want to include a 10K resistor between the sensor and the analog input pin, and a 0.1uF capacitor between the input pin and ground.