Problem:
1. I have to make a null detector with MCU.
2. Input voltage maximum is 30V dc.
3. Current to detect is 1nA to 300mA.
4. Galvanometer resistor selected is 100ohm. As this is standard in many analog null detector I have. And it also limits the current through power supply in case two opposite are zero & other two are open. In this case entire 30V will appear across 100 ohm.
5. Current measured should be bidirectional.
6. I have attached the circuit.
7. I can use multiple IC with different ranges i.e. 1nA-500nA , 500nA - 1000nA , 1uA-500uA , 500uA-1000uA , 1mA-300mA.