Resistive sensor reading ADC
If you have a reference voltage of (say) 3V, the range your input will see is: -


3V * 10k/13k = 2.307V (sensor at 3kohms)
3V * 10k/70k = 0.428V (sensor at 60kohms)


If you used a 50uA current excitation and a grounded sensor, the range your input will see is: -


50e-6 * 3k = 0.15V (sensor at 3kohms)
50e-6 * 60k = 3.0V (sensor at 60kohms)


With a current excitation the percentage of your 3V reference range used is 95%.
With voltage and resistor excitation you only get 63% of the range.

If your reference voltage is lower or higher, the above "range" statements are still true.



Here is an example. An ADC input is directly connected to the sensor. The sensor is fed with 50uA via the PNP transistor. The 50uA is measured across "R" and compared with "V" by the op-amp. The op-amp keeps the current through R at a value that generates a voltage "V" across it. Values could be R=10k and V=0.5V or R=20k and V=1V. Op-amp should be chosen that has close-to-either-rail IO performance like an AD8605 (used many times by me in this same configuration for strain gauge excitation).

Here's a quick DC simulation for 60k and 3k loads: -



Transistor used is BC547C or BC847C for those with good eye-sight.

Note that due to small base currents in the transistor, the current isn't 50uA but 49.846uA with 60kohm load and 49.849uA when loaded at 3kohms. Note also the voltages across the sensor - 2.991V on 60k load and 149.5mV on 3k load.