Quote Originally Posted by mat janssen
To read the real value you have do swap the wires and measure again.
When it is not possible to swap the wires just add an opamp wired as a one time inverting amplifier.
Hi Mat,

classical op amps can do an inverting or non-inverting amps, just by connecting "+" input to ground or to the signal input ... ( good thing with a Pic, no ???? )
Consider an inverting amp ( G = -1 R's = 100k ) and simply connect the "+" input via a 100//100 k resistor to ground or signal.

of course a negative supply for the AOP is needed ...

that is used in R/C systems to trim gain and reverse on the Tx...
I look for the scheme ... as oooooold as I .

from Elektor July/Aug 1983 p 7-62


Alain