O2 Sensors

[Brian Scott Haskett]

Can someone who is electrically-inclined please explain how O2 sensors 
are read?  I understand the output of the sensors (basically an s-curve 
that is an on/off switch, between 0 and 1 volt), but do you read this 
voltage?  I have heard that due to something (high-impedance?), you 
cannot read this voltage with a normal voltmeter or you will ruin the 
sensor.  Can someone explain WHY this is and what the correct way to read 
the sensor is?  Also, how will we read the sensor for our EFI system?  
Will a regular A/D board be able to do it?

thanks-
Brian

PS I have no idea what impedance is.