Thanks,
Test what? the cap? Discharge the cap with the 1T, and see it's exponential... You want to measure the opamp bias current too? The biggest R I have is 1 Gig. (1pA is 1mV which is easy on a good DMM.)
George H.
100 mv and resistor into an op amp with capacitor in feedback makes a ramp. Ramp feeds second capacitor into test node.
Current through C2 is (C1 * 0.1)/(C2 *R)
A capacitance ratio of 10uf/100 pF would give you five orders of magnitude in addition to the small-voltage-large-resistor current. The 'burden' of the load/test capacitor complicates the formula, but not by much.
Probably you want to integrate for a second or more, so a ramp that goes 30V in three seconds would be... convenient.
When I was a kid, I had 40m and 20m dipole antennas strung across our house, with coax fed down to my room. I'd always disconnect the antennas when I wasn't using it. If there was any storm activity you could hear the arcing across the PL259 connectors. At night it could get really loud.
The millivolt scale of a 10-Mohm meter reads picoamps directly.
Cheers
Phil Hobbs
Even cheap DVMs are pretty good at that.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
I like the 0.001 nS conductance mode on the Fluke 87 handheld multimeters.
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Thanks,
- Win
Why not dump the switches and just measure dV/dt?
Vdd | .---. |DUT| Vdd '---' | | ||--' 2n7002 / BSS138 | ||
With the bootstrap, LTSpice figures this guy ramps at just a tad (2%) under 1mV/S with 10aA input.
+12V +12V | | .---. '--| 2n5484 |DUT| .--|Vout | | --- | --- 10pF \|2n3904 | |---+--22k---
Um, 10e-18/10e-12 = microvolts per second, sorry. (fixed)
Nice circuit. The Vgs tempco will limit measuring really low currents.
The inherent time constant of an aA circuit makes these measurements really tedious.
I don't know what the leakage of the fet might be; it could be aAs.
I played with a 2N7000 some time back:
As you can barely see, the gate is open. The fet is halfway turned on, so the LED is half bright. It takes hours for the LED brightness to change visibly. Gate leakage must be pretty low, very rough guess 5 aA, a few thousand electrons per second maybe.
One can tweak the LED current by touching the source or the drain with a tiny insulated screwdriver, then touch the gate, to pump the current up or down.
But given the abstract, arguably useless, challenge to get the lowest possible current resolution, my switched cap thing is probably the best way to do it.
A variation on the vibrating reed electrometer would be fun, continuous readout with no lost charge.
I wonder how much charge just floats around in the air, ionization somehow.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
A 2n5484 could be trouble, one we measured at 1mA showed -Vgs approaching zero above 2mA, AoE III Figure 3.21. Table 3.5 measurements show -Vgs = 0.73 volts for 2n5484 at 1mA, and suggest a 2n5485 or 5486 to help the MOSFET. At any rate, bench measurements may be necessary.
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Thanks,
- Win
I made a set of open-gate measurements some time ago, with a 5-digit meter measuring drain current. It was amazingly stable, but with occasional jumps, in the wrong direction for drain-gate currents. Cosmic Rays?
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Thanks,
- Win
I have the advantage of specifying LTSpice(tm) brand JFETs; accept no substitutes. :-)
Yes, that's nicely spotted. Lowering the current sink value would be another approach to ensure the MOSFET at least some reasonable Vds.
I was trying to keep Vds low to minimize the potential idg leakage. In retrospect, it might make more sense to center Vg between Vd and Vs, geometry-weighted for the MOSFET, to quasi-cancel any idg with an offsetting igs.
Cheers, James Arthur
On a sunny day (30 Jun 2019 11:10:29 -0700) it happened Winfield Hill wrote in :
Philips used this system in their TVs to hold the volume and picture settings, they used a neon bulb as switch was lighted va a high value resistor from some high negative or positive voltage to charge / discharge the capacitor, Units were is a mall seeled plasict housing with the MOSFET and neon in it, preventing cables and their noise and leakage to change the C voltage
Those things held their value for weeks, have not tried any longer.
Was posted here by me maybe > 10 years ago, is likely patented.
Something like this: + user buttons | up -----------------|----- | | | | --- | | |
+100V 0 0 | |---- | |--R 10M--|---(00)----|| to volume, contrast, brightness, colr etc controls, | | === | | one such a sealed unit for each. down | | [ ] | | |_______| | | | | |_______________ |______| | /// Clever idea that neon, good insulator when not ignited.It is old hat.
An updated version could use two Vgs LEDs in photovoltaic mode, one up, one down.
Cheers, James Arthur
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