Load Cell Amplifier AD620
As on any instrumentation amplifier (INA), of which the AD620 is an example, you can't just leave the REF pin (pin 5) floating. The output voltage is relative to REF, and you need to measure the difference in voltage between the two pins.

Also, although the AD620 can operate at low voltage (5V supply), it is not a rail-to-rail device, either on input or output. The +IN, &#150;IN, REF and OUTPUT pins all need to be away from the supply rails by anywhere from 1.1 to 1.9 volts.
In other words, you won't get a 5V output swing from a device powered with 5V.

Look at Figure 38 in the datasheet (I added a link to your question). It shows one recommended way to connect the output of the AD620 directly to an ADC. Note that the voltage divider establishes a 1V usable output range while using a power supply of 5V.