passive infrared sensor output voltage

Hi. I have a Murata IRA-E710ST0 PIR sensor, where I've tried to attach the sensor circuit diagram and a human detection diagram. If it didn't work, here's the link:

**broken link removed**

I apply constant 5V to the drain and attach the gate to ground. When I cover the sensor so that it won't produce a signal, I get a constant voltage of about 1V from the source terminal. Is this expected? I thought that with no voltage applied to the gate, the output would be zero.

If the output voltage should be about 1V with no motion, do I need to somehow set the voltage to zero before connecting it to an op amp?

Thanks in advance for your help.

The leftmost 100KΩ in the circuit is probably the one pulling low that will bring the source voltage from your measured 1V to zero when inactive.