One more comment on this, do not null/zero out this offset. The applied AC voltage does not add algebraically to this offset, it adds as RSS - Root of Sum of Squares. So, hypothecially, if you get 0.200 mV with the inputs shorted, and then apply 10.000 mV, the display will be sqrt(10^2 + 0.2^2) = 10.002 mV, not 10.200 mV. If you had "nulled" out the offset, then applied the 10.000 mV, the display would read 9.800 mV, a large error.