Miguel Garcia Peris

Dear all,
I've been facing recently some issues measuring with my Picoammeter 6487. I've been doing some tests with a ~3MOhm resistor and I'll describe below what I'm seeing:

-- When I apply a 10mV voltage difference to the resistor, I read a current of 3nA (20 nA range). If I start to slowly increase the voltage, current increases slowly but gets stuck at ~19 nA and doesn't autorange to 200 nA range. If I change manually, I can read the appropriate measurement. If I repeat the process but starting from a larger voltage and a smaller resistance, this also happens in the 2 and 200 uA range. So it seems that 20 nA and 2 and 200 uA ranges do not autorange to the next range when current rises, even if the autorange option is selected.

-- Futhermore, when I get to this stuck value, measurements in different current ranges are not consistent. If I apply a voltage difference of 100 mV to the 3MOhm resistor, I find the following current values in the different ranges:

                                     Range                    Measurement
                                      20 nA                     19 nA (stuck value)
                                     200 nA                    33 nA (proper value)
                                       2 um                       0.033 uA (proper value)
                                      20 um                     2.9 um (wrong value)
                                    200 um                     2.9 um (wrong value)
                                      2  mA                      0.29 mA (wrong value)
                                     20 mA                      0.29 mA (wrong value)

Does anybody know what could ba happening?
Best
Miguel