I'm building a uCurrent Gold clone (https://www.digikey.ch/reference-designs/de/sensor-solutions/current-sensing/2637) - with a digital interface (measuring the voltage directly with a microcontroller). However, I also want to automatically select the Shunt-Resistor by two solid state relay or two Mosfet's.
However, is this feasible? Is there a Mosfet with a very low Rds_on (<10mOhm), very low Gate and Drain leakage current and very low Drain-cut-off current, such that there are nearly no unwanted effects (similar to a regular switch with on resistance of ~10mOhm)?
This is what my intention looks like (out goes to input of OpAmp, pull-down on MOSFET should be 100k):
EDIT: ADDITIONAL PARAMETERS
Rds_on <10mOhm
Gate & Drain leakage current should be 0nA, since they would sophisticate results very heavily in the nA range.
Drain-cut-off current should be around 0.1uA
-> As there isn't any MOSFET with this characteristics, especially for the Gate leakage current, maybe a Photo-Transistor would help. But the Drain leakage current would still remain. I would also be satisfied with an electromechanical solution, like a relay or a reed-relay, as long as the mechanical dimensions aren't to big (Footprint of SOIC-8 would be acceptable).