Using AD8253 in classic scope-type DAQ front end. Both inputs have 1 Mohm to ground. In an AC-coupled mode, the amp Ib develops offset voltage across the input resistor. Data sheet specs indicate 5 nA typical, 50 nA max over temp, further supported by Figures 8, 9, 18 and 19. I am seeing consistent 30-50 nA = 30-50 mV on tens of samples with all my other circuitry isolated. Any hints?