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Low signal testing

Hi,

I'm trying to help a friend, a EE, with the best method of testing a DUT that produces anywhere from ~ 0.1V to a few volts, and current levels from ~ 0.1pA to 10pA, all at DC. The DUT has ~ 10nF capacitance, no appreciable inductance, and resistance from as low as

100 Gohms to possibly as high as 1 Tohm. He would like to rent a Keithley electrometer and picoamp meter to measure both DC voltage and current simultaneously. This could be a problem because the DUT is exceptionally sensitive to external sources. Simply connecting the meter to the DUT could cause an unacceptable current pulse. If the meter is already connected to DUT, and it's turned on, then does anyone have an idea, even a guesstimate what kind of surges the Keithley will produce the moment it's turned on?

Any ideas would be greatly appreciated! Also recommendations on meters and a data logger would help.

Regards, Paul

Reply to
Paul
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I don't know much about the pico amp meter but the electrometer has a compliance voltage setting used to charge the cable shield to minimize load capacitance. Is there an option to control this to be on/off? Be sure to set (limit) it low enough that it does not zap either the operator or the DUT and be sure to drain the open circuit voltage off the cable before connecting it to the DUT. Maybe shunt a low impedance across the inputs before making the connection and remove it before the measurement is made. al

Reply to
mickgeyver

He can use a source-measure instrument in current mode, with voltage limiting, to keep the device currents under 10pA, etc. A Keithley 6430 should be able to do that job nicely (settable down to 0.5fA on the 1pA full-scale range), and some cheaper models may be able to as well. But, oops, I see in the manual that the maximum capacitance for the 100nA and below ranges is 100pF.

For a load of 10nF, the most sensitive range that works without loop ringing is 100uA, which is far too high. But then, whew, a 10pA current only charges a 10nF load by dv/dt =3D i/C =3D 1mV/sec. At 0.1pA that's 100 seconds to go 1mV, and more than a day to go 1V. That's not a workable set of numbers. What's the story here?

Reply to
Winfield Hill

"Winfield Hill" Paul

For a load of 10nF, the most sensitive range that works without loop ringing is 100uA, which is far too high. But then, whew, a 10pA current only charges a 10nF load by dv/dt = i/C = 1mV/sec. At 0.1pA that's 100 seconds to go 1mV, and more than a day to go 1V. That's not a workable set of numbers. What's the story here?

** Bound to be an alternative energy device - look at Paul's nick.

Betcha it is a solar cell array operating from moonlight.

..... Phil

Reply to
Phil Allison

Add a series resistor to limit current.

George H.

Reply to
George Herold

Yep, I bet you've nailed it Phil. Solar cells have dramatically- lower impedances at higher currents and voltages, so applying the high- illumination parms at low-light-level conditions is a wrong approach. We need to know more about what's really going on.

Reply to
Winfield Hill

Thanks for the replies and info! So far we're leaning toward the Keithley 6514 programmable electrometer. It has 200Tohm input impedance on voltage measurements, in voltage mode measures down to 10uV DC, in current mode measures down to 100aA DC.

Regards, Paul

Reply to
Paul

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