DMM and O2 sensor

Tom Cloud cloud at peaches.ph.utexas.edu
Mon Mar 31 17:10:44 GMT 1997


Scott, do I read you right?  Are you saying that the DMM input Z
_changed_ during the sample time?  I can't disagree with this,
but I can tell you that it shouldn't happen.  The dual-slope
integration technique used in these meters does switch the
integrator between the input and reference, but the input
should be buffered, meaning no change should be able to be
detected on the DMM input Z ????   (puzzled .......)

Just for grins, I've taken an inexpensive Fluke 73 and a 4.50 volt
source (measured with the 73).  Put a 1M resistor in series, and
the 73 reads 4.10.  Hung a Tektronix 2235 (10-X probe) onto
that.  DMM reads the same, no movement on scope trace at 2 volts
per division.  Went to AC coupled.  Have a .1 volt spike ~ every
450 mS and about 20 mV ripple (pickup from the test leads and hi-z
connection).

Based on the above test, I can say that the Fluke 73 had no
measurable input impedance change at any time.  I'd say any
DMM that had the attributes you list below is malfunctioning.


>The VOM was sampling the O2 input, and had a lower input impedance during 
>sample time that lowered the measured voltage.  While not sampling, the 
>VOM had a much higher input impedance that let the open circuit voltage 
>rise.  This difference showed up in the ECM's measured O2 voltage, as it 
>samples the O2 sensor much more often than the VOM does.

Tom Cloud <cloud at peaches.ph.utexas.edu>




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