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Air fuel ration sensing circuit.
Air fuel or wide band Oxygen sensors sensors measure Oxygen content by using a
differential op amp circuit to compare a control voltage against a varying voltage.
Practical operational amplifiers or op amps, draw a small current from each of their inputs
due to bias requirements and leakage. These currents
flow through the resistances connected to the inputs and produce small
voltage drops across those resistances. In A C signal applications this seldom matters.
If high-precision D C operation is required, as in A F R sensors however, these
voltage drops need to be considered. The design technique
is to try to ensure that these voltage drops are equal for both inputs, and therefore
cancel. If these voltage drops are equal and the common mode rejection ratio of the
operational amplifier is good, there will be considerable cancellation and improvement
in the A F R accuracy. If the input current into the operational amplifier are equal at
both sides, to reduce offset voltage the designer must
ensure that the D C resistance looking out of each input is also matched. This is
the reason why A F R circuits are more costly than normal O2 sensors.
The oxygen pump generates the change in voltage to compare to the control voltage
from the PCM, which balances one against the other to maintain an internal oxygen
balance inside the sensor chamber.
This A F R sensor circuit is in effect a differential amplifyer that hadles very
small amounts of current. There is ussually no dedicated chip in these circuits,
and the operations are carried out using normal dicreate components.