What's wrong with my electrometer (meas. current)?

[Hyo X]

I have an electrometer that measures current and ouputs a voltage. It has a range of operational amplifications, for example 1E-2 A/V to 1E-3 to ... to 1E-11 to 1E-12 A/V.

I applied a voltage sweep at a slow rate (100 mHz) and measured the current response. The sample is expected to have some resistance and some capacitance. The applied voltage, the sample configuration, and the sweep rate are all identical for red and blue. The only difference is the electrometer amplification setting.
The blue curve is the 1E-9 A/V setting, and the red curve is the 1E-10 A/V setting. There are two sweeps for each setting.
Why are they so different? is my electrometer busted?

 

[Bystander]

30-50 pA in 1 nA is 3-5% ----- what are specs on the instrument?

 

[Hyo X]

30-50 pA in 1 nA is 3-5% ----- what are specs on the instrument?

These are the Specs from the Spec Sheet.
I suppose for these runs I am switching between the 3rd and the 4th rows when I switch amplification settings.

 

[Bystander]

... and, 0.25 % of 2 nA full-scale is what?

 

[Hyo X]

... and, 0.25 % of 2 nA full-scale is what?

Well, the spec says 0.25% rdg, which means 0.25% of reading, not full range. Part of the issue is the different line-shape. And that the sign of the signal is actually different for example at x=0.64 Volts.
If i use these specs, at x=0.64 V,
Red: 1.06E-11 +/- 1.7E-13
Blue: -5.5E-12 +/- 1.4E-14

These values are different by 1.5E-11 A.
Even if specs were 0.25% of full range, that is 0.25% of 2E-9 A or 5E-12, which is 1/3 of the difference between these values.

 

[Bystander]

voltage sweep at a slow rate (100 mHz)

"... 2.5 s on pA scales."

 

[meBigGuy]

As bystander pointed out, sweep much much slower and redo the curves