Reading an O2 sensor

Mike Palmer Mpalmer at
Fri May 6 15:24:53 GMT 1994

>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?
>PS I have no idea what impedance is.

In simplest terms, impedance is a measure of how much current a
load will draw from a source. The source itself has a certain 
output impedance. When load impedance becomes too low, it attempts
to draw alot of current from the source. The output impedance of the
source becomes in effect a dropping resistor and the voltage drops
across this resistance by Ohm's law. The idea is that a device
measuring the voltage at any point in a circuit be of such high
input impedance that it draws neglible current. In contrast,
a current measuring device (like an ammeter) is best if it
mimics a short circuit - i.e. it adds no impedance of it's own to the

An O2 sensor is basically a little source with really high output 
impedance. This means that in an open circuit condition, the output
of the O2 sensor is what the sensor actually produced. However, 
as soon as a load is put on the sensor (i.e. a meter, A/D converter
etc), the voltage quickly drops due to the current drawn by this device
producing a voltage drop across the output impedance.

It is exceedingly simple to buffer an O2 sensor with an operational
amplifier configured as a voltage follower: (see cheesy ascii 
					  |-------------/\/\/\/\--------- bias voltage (450 mV)
					  |             1 megohm
					  |    |\       resistor or higher
	 O2 sensor--------*----|+ \
						   |    \____________________A/D converter
						   |    /    |
					  |----|- /      |
					  |    |/        |
					  |              |

Op-amps have *very* high impedances (i.e. the LF256 J-FET op-amp
has 10^12 ohms of input impedance) and can easily drive the input
of an A/D converter.

This may be simplifying things a bit *too* much since many O2 sensors
must actually have a small bias voltage applied to them (as in the
case of GM sensors: the ECM applies a small bias voltage of 450mV
through a high impedance. When the voltage read back from the
O2 sensor is not this bias, the ECM decides the O2 sensor has
reached operating temperature, it's internal imedance has fallen a bit
and it can now overcome the bias voltage and drive the line itself).
Adding this part would be simple though...(see above)

Most digital voltmeters have immensely high input impedances. These
units should be fine for measuring an O2 sensor 


- Mike 

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