My project is to turn a digital panel voltmeter (0-200mVDC) into an ammeter capable of reading current to 100A DC max. Application is to measure total current draw between my vehicle's 12VDC battery and my DC-AC inverter. I know i need to use a shunt resistor and measure voltage drop across it. Differential op-amp? From there i'm lost as to what amp to use or how to scale my voltage drop to read an equivalent value in the range i'm working with. Any help?