Low signal testing

P

Paul

Guest
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
 
On Jul 17, 3:58 pm, Paul <energymo...@gmail.com> wrote:
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
Click to expand...
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
 
"Winfield Hill"
Paul <energymo...@gmail.com>


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...
Click to expand...

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
 
On Jul 17, 3:58 pm, Paul <energymo...@gmail.com> wrote:
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
Click to expand...
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 = 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?
 
On Jul 17, 3:58 pm, Paul <energymo...@gmail.com> wrote:
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
Click to expand...
Add a series resistor to limit current.

George H.
 
On Jul 17, 8:47 pm, "Phil Allison" <phi...@tpg.com.au> wrote:
"Winfield Hill"
  Paul <energymo...@gmail.com

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...

 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
Click to expand...
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.
 
On Jul 17, 12:58 pm, Paul <energymo...@gmail.com> wrote:
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
Click to expand...


Thanks for the replies and info! So far we're leaning toward the
Keithley 6514 programmable electrometer. It has <1fA noise, >200Tohm
input impedance on voltage measurements, in voltage mode measures down
to 10uV DC, in current mode measures down to 100aA DC.

Regards,
Paul
 
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