WB update: 1V output

[bcroe at juno.com]

On Wed, 14 Mar 2001 16:36:28 -0500 rr <RRauscher at nni.com> writes:
> 
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Someone asked about getting a 0V to 1V output (like a normal
OX sensor) from the EGOR output; Barry also mentioned that 
the signal ran in the reverse direction from normal.  This can
be fixed with another amplifier, but changing a couple of
resistors in the 11-6-2000 circuit will accomplish the same
thing

U3A (LM324) is a differential amp which responds to voltage
across VR3 (284 ohms).  Since the RATIO of R9 (22K) to R12
(100K) at the negative input is the same as the RATIO of
R10 (22K) to R11 (100K) at the positive input, any voltage
common to both sides of VR3 will be cancelled at the output

First I would correct the direction of the signal.  Just reverse
the connections of R9 and R10 to VR3

Next adjust to 1 volt output swing.  The Web chart 7-30-00 
says the swing is from -2.214V to +0.52V, a total of 2.734V.
We want to reduce that to 1V, which can be done by increasing
R9 and R10 by 2.734V/1V, to 60.15K

At this point I need to point out that the gain of the amp was
100K/22K, or 4.545:1.  If the output swing was 2.743V, the
input swing at VR3 would be 0.601V.  This voltage will
only give 2.118 ma current change in a 284 ohm resistor, 
but the chart  says the current varies from -3.22 ma to 
+0.75 ma, or 3.97 ma change.  To get 3.97 ma for 0.601V, 
I am going to assume VR3 is really 151.6 ohms for the rest
of these calculations

Finally the swing needs to be adjusted to 0 to 1V relative to 
the ECU actual ground.  This can be done by changing the
connection of the bottom of R11 from the virtual ground (4
volts) to a different value.  The virtual ground doesn't figure
in this value, since it gets canceled out.  A bit of ohms law
shows R11 needs to connect to 0.811V above actual gnd

Instead of looking for a 0.811V battery or another pot, I'd
suggest using 2 resistors that have the Thevenin equivalent
of 100k ohms in parallel (like R11) and 0.811V output as 
an unloaded voltage divider.  So R11 is replaced with a
111.3K resistor from U3A  + to actual ground, and a 
986.4K resistor from U3A + to 8 volts.  Yes, I know you'll
have to stick together several standard resistors to get
such values; you decide how accurate you want to be

So there is your 0 to 1V out.  Now I have questions.  First
how does the free-air calibration work; just what gets 
adjusted?  How were the A/F ratios established for the
7-30-00 measurements of 10.62:1 to 17:1?  Should we 
extrapolate to the sensors' 22:1 range capability at higher
Ip + currents (if so the above need rework)?  Bruce Roe
 
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