EGO Output

[Walter Petermann]

Wayne.MacDonald at zurich.com.au wrote:
> 
> Is it OK to connect the EGO sensor directly to the A/D input of a 68HC11
> processor or do I need some filtering/conditioning circuitry ?.

You should protect the A/D input from voltage spikes 
on the EGO input. The highest frequency I've seen an EGO
work at is about 3-4Hz. You could use a low pass rc 
combination with a cutoff frequency of 30Hz. Then diodes
to vcc and ground of the uP. This will protect the A/D
and also filter any noise coming in.

You should also have a biasing circuit on the EGO input.
This would consist of a reference voltage (.45-.5 or 
whatever voltage you have chosen as the 'reference')
connected to the EGO input with a resistor of around 1Meg.
This circuit will provide a voltage to the A/D when the
EGO is cold, very high impedance and not generating
any voltage. The uP then will know when the EGO is
starting to work by noting when the voltage changes from
your set reference.

There are a couple of ways that I know of, of reading the
EGO voltage.

i) The EGO voltage is fed to two comparators. 
     One has a threshold of reference + ~10 to 20mv with a 
     high output when the EGO voltage is higher than the
     above. The other a threshold of ref - 10 to 20mv with
     a high output when the EGO voltage goes below this
     amount.
     The outputs of the comparators are fed directly to two
     uP digital inputs. This has the advantage of
     freing up an A/D.

ii) After biasing and filtering the EGO is fed directly to
     an A/D input. If the A/D is 8 bits this will give you
     about 20mv resolution (based on 0-1v and A/D ref of 
     5v). It would be nice to have 20mv equal to 3 bits
     or 10 bit A/D.How many bits does the 68HC11 A/D have?

 regards
  Walt