Measuring high resistance

Just connect 100V to the "DC IN" BNC.

A simplified version of this would be just a high-value resistor and an opamp follower. I think someone here noted an opamp that's even better than the LMC6001 for leakage.

Digikey has 1G resistors. That will get you a volt per nanoamp, so you could resolve maybe 1 pA with a little zero checking. I have a set of Pomona-mounted resistors from 1K to 1T, as the "Z1".

Electrostatic and hum pickup is a problem with these measurements. I can pin the needle on the Keithley from 10 feet away, shuffling feet on the carpet. My little pA tester box has an aluminum cover, a chassis box, that I plop over the top when measuring fA levels.

On a sunny day (Sat, 15 Nov 2014 09:08:40 -0800) it happened John Larkin wrote in :

Yes, but that is not what I meant,

These JFETS BF245 are also like that, but to measure current you could perhaps use one of these transconductance opamp?

It is funny, just a while back I had a discussion about current in sci.physics. So many electrons per unit of time passing by;

Voltage is a very vague thing, and not much under our control. Any motion of charged particles will change the charge distribution (electrons, positrons, what not), I then mentioned an electron could even fall under the influence of gravity (maybe near a black hole for example) and there would be a current[1]. Voltage (Volt) is derived from current:

I like BJTs because basically those are current amplifiers,.

I have something like this with a darlington in use to amplify some photocell current to adjust the brightness of a display in an old alarm clock.

Want more gain, add an other transistor, temperature leakage is the limit probably.

there's no need for a low impedance, the voltage sourece you draw has

neither

I wrote GPIB stuff in the 1980s as well. Not that bad if you understand GPIB; worst part is dealing with all the manufacturers (stuff) that made GPIB stuff and didn't understand it well.

Back to your issue, perhaps a TIA to convert the leakage current into a voltage and fed that to the on-chip ADC? The 100 V(dc) would have to be decently quiet.

?-)

So you create 100VDC with a DC-DC converter and run it into the highest value resistor you can reasonably buy (say 10M- and it should be rated to more than 100V) and then amplify that with a low Ib op-amp to whatever your micro wants to see. Obviously you add some series resistance (1M is probably just fine) on the op-amp input so if the test electrodes are shorted you don't screw up the op-amp.

Best regards, Spehro Pefhany

Jfets have a lot of gate current, especially if you don't keep the drain voltage low. My old Keithley electrometer uses discrete small-signal mosfets. Some cmos opamps are awfully good; it's tedious to measure their input current.

I have a cheat-card always in view on my desk. Hex values. My own cell phone number. Some physical constants, including 1 fA = 6250 electrons/sec.

On a sunny day (Mon, 17 Nov 2014 11:50:21 -0800) it happened John Larkin wrote in :

I have a sticker with my own number on the phone, I knwo it by now though. I can feel them electrons with the back of my hand.

There's a Chinese version of the hand-cranked type too, at only 3% of the Grainger sticker price. That's right - not 30% - 3%.

Where does the money go, I wonder?

RL At

That actually looks pretty decent, except for the cheesy test probes.

Best regards, Spehro Pefhany

What I did was use a 100 vdc supply thru a 10megohm resistor to the dut. I used a DMM to read the 100volts and the volts across the dut. Calculate the unknown R.

Remember to correct for the DVM input resistance!