@barkingsheltie
I see you are now using a different protection circuit with a 120Ω loop resistor and a 10KΩ resistor in series. That arrangement will decrease your resolution and reduce the bandwidth. However you could use the internal reference but it is not accurate at all with a -6%/10% error range, hardly useful if you are trying to make precise measurements.
I recommend that you use an accurate external reference.