> Bob Blick has a circuit on an SX forum, I happened across it the other da=
y.

Presumably you mean this one

           http://www.piclist.com/techref/io/sensor/amps.htm

      Info only: Comment on this by Roman Black indicates it is
'quite some years old'.

The arrangement can be useful, but circuits like this can give you
unexpectedly bad results if implemented without understanding their
limitations.

It is important to note that circuits of this sort require
increasingly good "common mode rejection" by the opamp as the ratio of
sense voltage to high side voltage increases and that errors in the
divider components also become increasingly critical as the divide
ratio increases.

For circuits with small sense voltages, either due to low sensed
currents or high division ratios the op amp choice may become
important.

For accuracy achievable with given components.

Memory suggests (and it's not utterly intuitive) that a very rough
rule of thumb is that you can expect a error about N times worse than
the tolerance of your divider components.
"Select on test" can help this greatly. It's been a wee while since I
worked through this myself (so left as an exercise for the student)  -
there may be eg a factor of 2 either way in that. Also, components
that cluster tightly around their nominal values (as often happens)
may yield better accuracies than expected.

If I'm right (ask the student), in  Bob's circuit the divide ratio is
11:1 so 1% untrimmed resistors in the divider *may* typically produce
a 10% error.

Student please run figures and report back :-)


   Russell
--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.