I am trying to amplify the signal from a 50 mV/100 A current sensor (shunt) like the one you find here. I though it would be straightforward to amplify the input voltage with a LM328 op amp to 5 V. The circuit diagram is attached. When I read the signal with current passing through the shunt, it is as though the opamp involved at all. That is I get the same reading with or without the opamp. I can’t understand where I am going wrong. I have tried with a gain of 10 and 100. Regardless, the op amp seems to have no effect on the reading.
Here is a better schematic.
LM328 is NOT a rail-to-rail amp, it's output will only come up to VCC - 1.2V, about 3.8V with 5V supply, I have used a MCP6002 for that. Don't forget a 0.1µF bypass cap from Vdd to Vss on the amp.
BTW: What voltage is pushing that 100A?
Indeed, not rail to rail, that's the first problem.
The second is the voltage range of 0-50 mV, which is very low.
Third is the gain: 50-100x gain is a lot for a single OpAmp. You may need two of them in series for that.
Finally: you may want to look into the HX711 chip, as that's designed to read such very low voltages and may be more suitable for your application. There may be other chips that are designed for just this kind of application.