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

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Discussion Overview

The discussion revolves around the performance and accuracy of an electrometer used to measure current in response to a voltage sweep. Participants explore the differences in current readings at various amplification settings and consider potential issues with the instrument's specifications and measurement conditions.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant describes the electrometer's operational amplification settings and the observed differences in current response between two settings during a voltage sweep.
  • Another participant questions the specifications of the instrument, suggesting that the difference in current readings may be within acceptable limits based on the instrument's accuracy.
  • A later reply clarifies that the specification of 0.25% refers to the reading and not the full range, indicating that this could affect the interpretation of the results.
  • Participants note that the line-shape of the measurements differs and that the sign of the signal changes at specific voltage points, which may contribute to the discrepancies observed.
  • One participant suggests that performing the voltage sweep at a much slower rate could yield different results and recommends redoing the curves under those conditions.

Areas of Agreement / Disagreement

Participants express differing views on whether the observed differences in current readings are significant or within the expected range of error for the electrometer. There is no consensus on whether the instrument is malfunctioning or if the discrepancies are due to measurement conditions.

Contextual Notes

Participants mention specific measurement conditions, such as the sweep rate and the configuration of the sample, which may influence the results. The discussion also highlights the importance of understanding the specifications of the electrometer in interpreting the data.

Hyo X
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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?

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

Keithley617-Amps-specs.png
 
... and, 0.25 % of 2 nA full-scale is what?
 
Bystander said:
... 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.
 
Hyo X said:
voltage sweep at a slow rate (100 mHz)
"... 2.5 s on pA scales."
 
As bystander pointed out, sweep much much slower and redo the curves
 

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