What some people do to read negative voltages in a +5V only system is to use
a pullup-R to 5V on the measuring-node, and apply the negative voltage through
a series-R,
Vin -- SeriesR -- + -- pullupR -- +5V
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to ADC
To do this successfully, you would have to know the output-resistance [impedance] of
the probe. It would probably also be best to use this ckt with the opAmp in there
before the ADC.