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Leakage current of a breadboard?

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solis365

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Does anyone have experience with leakage current in your typical solderless breadboard?

I have a circuit with a +/- 10V supply. I am trying to build a current sink that is around 500nA. I am able to measure the current, but there is a current mirror in the design using matched NPN transistors, and the matching at 500nA is abysmal - for 500nA input I get ~900nA output from the current mirror. The current mirror is a special topology (emitter modulated) so the error is not due to the input side having to supply both collector and base current.

The matched transistors are precision matched audio BJTs, part number LS313 from Linear Systems, they arent just 2n3904s with similar betas.

This was just a quick job to see if the circuit worked (it does), and I was wondering what parasitics might be affecting the matching. I plan to move to a carefully laid out PCB soon, with guard rings and planes around sensitive nodes.
 
Capacitance is the biggest problem with breadboards. But, I've seen some that basically had a glued piece of cardboard on the bottom side. If so, you'll have leakages depending on the moisture in the air. I used to measure leakages of a methanol film on a piece of glass in the pA range at 100V. The methanol rinse was contaminating the process and the measurement I was doing. Measuring the resistance or should I say conductance, of paper was fun.
 
I would think a solderless breadboard should have leakage current in the pA region.
 
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