Electronics > Metrology

Measuring nanoamps and below like a Ninja

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They shows more 100fA after 15minute hold at 0V. After change 0V to 7V it also shows hundred femtoamps... I've test 10 pcs or so...

Today i did first capacitance reference and accuracy tests. There is 1nF reference based on K71-7 Polystyrol capacitor.

I use "NJ100S"(it is low quality, low precision LCR) for transfer unit of capacitance from my standard to reference on 50 Hz test frequency.
And compare reference with resistance standard P4030.

First test is P4030(1000.041MOhm) at 1.00005V = 1.000009nA.
Keysight B2985A shows 1.000059nA, it means B2985A error is +0.005%.

Second test is 1nF reference(1.004426nF) with 9.95594E-01V/s ramp = expected current is 1.00000nA.
Keysight B2985A shows 1.000544nA, it means error is (+0.053% - +0.005%) = +0.048% trough resistance and capacitance method.

Results not an ideal... but when current drop down to pA ranges that error is acceptable.

And finally, these is my toys:

The dielectric absorption makes the capacitance to go down when the frequency goes up.
The measurement at 50Hz is somewhat faster than the use with the ramp.

So part of the difference can be due to the DA of the PS cap. The usual tabulated amount of DA for PS is at some 0.05%, which may be a coincidence because of the different time scales and not all PS caps are the same.


--- Quote from: Kleinstein on May 22, 2022, 08:26:50 pm ---The dielectric absorption makes the capacitance to go down when the frequency goes up.
--- End quote ---

You're definitely right!!!
When i transfer unit of capacitance on 1kHz frequency error be much higher, around 0.1%.

I do experiment's with transfer unit though "charge method", but still not ready to share final report. That transfer method can help increase precision.
Anyway.... I'm think, 50Hz transfer with 0.05% accuracy it is acceptable.

0.05 % accuracy for 1 nA looks already quite good. This is especialy knowing that there a known effect to cause a bias in that direction.


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