Increase voltage drop

Hi guys,
I am still working on my alternator/generator setup (see magneto interference in motors sect.) and have come against another problem. I have a 100A - 100mA shunt that I'm trying to use as an amp guage. I understand all the logic and have the code working with all the float values correctly.

I'm reading 15v dc and I have wired both sides (indipendantly) through a 1k resister to arduino input and then a 500 ohms to ground which (theroreticaly) gives me 5v signal. All is good there and multimeter is testing at 4.2v.
The problem is that I read pin A4 and calculate volts, then send it to LCD, then I read A4 and A5, subtract them and calculate to read amps (voltage drop) and send that to LCD. It seems that the resolution of the analog in is not great enough to read the tiny amount of voltage drop as I have tried both float and double and the increments are to large. I don't need it to be exact, just a little more readable than 8A chunks.

This is where I need help. I was wondering if it was posible to cross ref the two signals and somehow amplify the voltage drop in the second signal (A5). I can calibrate it with the code no worries and that should fix the resolution problems. I'm not too sure as how to do it because I'm only new to electronics and only know the basics. I am really sorry I can't post the code or draw a wire diagram for you. I only have mobile Internet because of all the rain and flooding!! Hope it stops soon!

Search for : high side current sensing

On the arduino float and double are both the same.

The Arduino (at 5V) can sense at best a 4.8mV resolution. If you ignore the LSB as noise, then that rises to 9.8mV resolution.

To increase sensitivity you can switch the analogue voltage reference. If you have the 328P on your board (most do now, only the old ones don't) then you have an internal 1.1V reference voltage you can use for the analoge voltage reference. That would increase the resolution to 1.1mV (or 2.2mV ignoring LSB), which might give you better results.

Either that or you will need to amplify your voltage drop using an op-amp. There are specific current amplifiers available for this job, such as the MAX4172 which I have used and gives good results.

Could also use an external ADC with more resolution, such as 12-bit MCP3201.

1.2mV resolution
easy to use SPI interface