Measuring opamp output voltage (with virtual ground) on arduino
Make sure that op-amp GND (connected to its pin 4) is also connected to the Arduino GND.
Connect virtual ground to Arduino A1. Now the Arduino will be able to measure the reference voltage.
In your code the analog reading is A0 - A1. 


This will give you positive values when A0 > A1 and negative values when A0 &lt; A1.