I connect all voltage inputs together with analogue ground to measure the zero point (Pico ADC-20 data logger). When I use range +/- 1250 mV the raedings are : ch1 0.010-0.007 mV, ch2 0.000-0.005 mV and etc. to ch8. When the range is +/-2500 mV, the values are around the -2 mV ( ch1 -1.874 mV, ch2 -2.050 mV , ch3... ).
Is it a normal situation ?