MCP3421 - Battery Monitor

Hi! I am trying to use MCP3421 in order to monitor Battery voltage. All are fine when i use external battery with no common wiring with the main sytem. The problem begins when i am measuring the system battery. Here is the diagram Arduino does not run. Is there wiring problem?

I tried to power first the arduino and then connect battery contacts to ADC chip mcp3421 and it works fine. So why do i have to power arduino and theN connect battery to ADC Analog inputs?

Great! but i would like more resolution (16bit). Thanks anyway!!!

Are you sure you need an 18-bit A/D to measure a battery. Battery voltage is only an indicator for charge. 10 bits is probably way more than you need. What sort of battery are we talking about.

Not familiar with this MCP3421, but I assume you need a voltage divider to bring the voltage down to the 2.048volt A/D range. Leo..