threshold voltage shift and hysteresis. Detail see the us patent5235333, 5675340, 5006853.
And the paper, T.L.Tewksbury "Characterization, Modeling, and Miniminzation of Transient Threshold Voltage Shift in MOSFET's" IEEE Jssc, MAR. 1994 .
T.L.Tewksbury "The Effects of Oxide Traps on the Large-Signal Transient Response of Analog MOS Circuits" Apr. 1989.IEEE JSSC.
Added after 5 minutes:
But if your ADC less than 10bit , you don't care the threshold voltage shift effect. This effect like 1/f noise. use the Pmos as input MOS can reduce this effect.
Added after 6 minutes:
limited the input voltage can reduce this effect also, and I have applied to patent this method.