Proper Mixture.

[tom cloud]

>> A simple and effective indicator for O2 sensor is a DPM (digital
>> panel meter) from Hosfelt Electronics.  It is $14.95, their part
>> number 39-165 (the part no. on the data says PM-128).  It uses an
>> Intersil ICL7106 DMM chip with a fairly standard wiring.  There is
>> one problem -- they have connected the supply voltage common with
>> the input common, which can cause ground loop problems  

 [snip]

>This is a good meter but I am pretty sure the power supply ground and
>the measuring circuit ground can not be the same. Thus can not run on
>car voltage and still measure o2 volt which has the car gnd as common.
>I have also used a 9 volt battery.  That's why they are cheap.
>
>	frank
>> 
>

I use these meters in a class I teach where the students build projects,
and I have just finished scoping out the dilemma.  The DPM in question
uses the identical circuit recommended in the Intersil databook.  I
was wrong!  The common and battery negative are not connected, but the
reference voltage is derived from batt + with a voltage divider going
to the DPM common, which is the "IN LO".

Summary:  the inputs are differential, but cannot go past either
voltage rail (no problem -- the O2 sensor has one lead at ~ 0 and the
other never exceeds ~ 1 volt).  The ICL-7106 is spec'ed at 15 volts
max, so the DPM "should" be OK being powered directly from the car system
and should not have any problems with noise being induced into the
sensor readings (certainly not enough so's one would notice).

Frank .. if you've actually checked this out, would like to know.

thanks,
tom cloud