How are femto-amps measured?
I saw the other day a Keithley 2636B SMU which has 0.1fA resolution @
10uV in a "normal" Keithley DMM box size. Keithley also have some
systems with external amps that go down even further, to tens of
atto-Amp regions. Even the old HP resolves down to 1fA.
It made me curious.
Dear list: What's the basic principle of such measurements?
With modern digital readout meters, this can be very misleading in terms
of actual useable capability. In this case, using the specified highest
sensitivity 100 pA range, and six digits of digital resolution, gives
E-10 A/E6 or E-16 A, which is 100 aA for the last digit. But, looking at
the noise and accuracy specs shows that it is swamped by these numbers.
The noise can be reduced greatly by sufficient averaging over long
enough time. The same issues are encountered in voltage measurement.
For good info on making very small current measurements, investigate
electrometer technology of the good old days, especially from Keithley.
Back in the 1960s - 1980s, electrometers typically used special tubes or
MOSFETs that had bias currents in the fA region, along with high
resistances that topped out at around E11 to E12 ohms (which is still
about the limit of practicality for R).
Nowadays there are opamps available with bias current in the fA region
at room temperature, but noise is still a limiting factor. For better
absolute accuracy and drift performance needed for modern digital
meters, the ranging resistors can be lower - probably E10 ohms or less,
since the meters can measure and resolve much smaller voltages.
Ed
A simplest model is RC integration.
http://electronicdesign.com/test-amp-measurement/whats-all-femtoampere-stuff-anyhow
----- Original Message -----
Subject: [volt-nuts] How are femto-amps measured?
I saw the other day a Keithley 2636B SMU which has 0.1fA resolution @
10uV in a "normal" Keithley DMM box size. Keithley also have some
systems with external amps that go down even further, to tens of
atto-Amp regions. Even the old HP resolves down to 1fA.
It made me curious.
Dear list: What's the basic principle of such measurements?
I'll only add two things. You can believe Keithley's specs for equipment
made prior to their acquisition by Danaher in 2010 or so. You also have to
make these measurements under controlled conditions; for example, but using
a Triax connected Faraday cage, not ever touching what you are measuring
before you measure it, etc. As Ed mentioned, there is good info out there
by Keithley on the subject for guidance.
Russ
On Fri, Jun 5, 2015 at 11:46 AM, Ed Breya eb@telight.com wrote:
With modern digital readout meters, this can be very misleading in terms
of actual useable capability. In this case, using the specified highest
sensitivity 100 pA range, and six digits of digital resolution, gives E-10
A/E6 or E-16 A, which is 100 aA for the last digit. But, looking at the
noise and accuracy specs shows that it is swamped by these numbers. The
noise can be reduced greatly by sufficient averaging over long enough time.
The same issues are encountered in voltage measurement.
For good info on making very small current measurements, investigate
electrometer technology of the good old days, especially from Keithley.
Back in the 1960s - 1980s, electrometers typically used special tubes or
MOSFETs that had bias currents in the fA region, along with high
resistances that topped out at around E11 to E12 ohms (which is still about
the limit of practicality for R).
Nowadays there are opamps available with bias current in the fA region at
room temperature, but noise is still a limiting factor. For better absolute
accuracy and drift performance needed for modern digital meters, the
ranging resistors can be lower - probably E10 ohms or less, since the
meters can measure and resolve much smaller voltages.
Ed
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A simpler representation.
http://www.edn.com/design/other/4381004/Femtoamp-fA-measurements
Subject: Re: [volt-nuts] How are femto-amps measured?
A simplest model is RC integration.
http://electronicdesign.com/test-amp-measurement/whats-all-femtoampere-stuff-anyhow
in the Flintstones era you would have a bird count the electrons as they
flow down the wire.
On Fri, Jun 5, 2015 at 10:23 AM, MARVIN marvin.gozum@comcast.net wrote:
A simpler representation.
http://www.edn.com/design/other/4381004/Femtoamp-fA-measurements
Subject: Re: [volt-nuts] How are femto-amps measured?
A simplest model is RC integration.
http://electronicdesign.com/test-amp-measurement/whats-all-femtoampere-stuff-anyhow
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--
John Phillips
As described, the pico-femto amp meters are "pass-through", current
goes in AND out, at 10uV burden voltage. They are explicitly not shunt
and as far as I understand, can't be trans-impedance amplifiers
either.
The very lowest leakage OP-amp I know of, LMP7721 is rated 3 fA typ,
with a specified limit of ±20 fA at room temperature, ie much higher
than the sub-femto.
The best candidate discreet DIFETI know of, LS320, has a specified
input leakage resistance of 100Gohm which would be equivalent to a
gate current of 10fA at 10uV, ie numbers in the right ballpark. Still
that does not explain the current pass through.
So still it makes me wonder how they do it. Even the old HP 4140B
(model fell out of my last post) has a specified resolution to one fA.
The only possibility I can think of is to use selected FETs and let
the test current flow through D-S and control the gate voltage to the
specified D-S voltage drop and correlate/calibrate test current to
gate voltage.
I have a Keithley 6485 and guess that out of pure curiosity I'll have
to make a tear down at some point :-)
Any thoughts?
Jan Fredriksson jan@41hz.com writes:
As described, the pico-femto amp meters are "pass-through", current
goes in AND out, at 10uV burden voltage. They are explicitly not shunt
and as far as I understand, can't be trans-impedance amplifiers
either.
The very lowest leakage OP-amp I know of, LMP7721 is rated 3 fA typ,
with a specified limit of ±20 fA at room temperature, ie much higher
than the sub-femto.
The best candidate discreet DIFETI know of, LS320, has a specified
input leakage resistance of 100Gohm which would be equivalent to a
gate current of 10fA at 10uV, ie numbers in the right ballpark. Still
that does not explain the current pass through.
So still it makes me wonder how they do it. Even the old HP 4140B
(model fell out of my last post) has a specified resolution to one fA.
The only possibility I can think of is to use selected FETs and let
the test current flow through D-S and control the gate voltage to the
specified D-S voltage drop and correlate/calibrate test current to
gate voltage.
I have a Keithley 6485 and guess that out of pure curiosity I'll have
to make a tear down at some point :-)
Any thoughts?
The 6517A electrometer just uses a commodity opamp, LPC661 I
think. Selected, presumably. Most mosfet input opamps are extremely high
impedance but would take a very long time to test on a production line
so are not specified to their true potential. The LMP7721 you mention is
just better tested I expect.
In use the bias current can be measured and compensated for, leaving the
current noise which is much smaller (though still objectionable and is
what limits the measurement precision and time).
The 6517A has a resolution of 10aA IIRC but it is very noisy and drifty
on the last two digits unless you use a lot of averaging and long
measurement intervals.
Anecdotally a 2n7000 gate leaks "electrons per second".
It would be fun to try this (configure as follower and leave the gate
open circuit after charging it to a suitable bias voltage).
--
John Devereux
Yes, the input DC bias current can be compensated for, as long as it
doesn't change too quickly, or isn't too big to make the input amplifier
circuits run out of dynamic range. In a digital meter environment, all
sorts of auto-zeroing and multi-sloping things and digital signal
processing can be going on to correct for various device and circuit
limitations, which would have been very difficult or impossible in the
old analog machines.
Even in analog, some correction can be implemented fairly easily. A lot
of the old Keithley electrometers have controls called "zero
suppression," which can effectively offset the bias current, or even
much larger amounts up to hundreds of full-spans, for certain
applications. It's not quite the same as having true zero bias current -
you still have to be aware of the effects and the compensation settings,
depending on the measurement.
I modified my Keithley 417 by replacing the original zero suppression
last digit fine adjust pot with a ten turn helipot and kilodial knob,
which provides higher resolution. I can dial in the 17 fA bias current
offset directly. When I eventually get around to washing and silicone
coating the electrometer tube, I expect to get it down to around 2-5 fA.
My Kethley 410 electrometer doesn't have zero suppression - just a
"zero" knob to set the input offset voltage over a small range to zero
the meter. I added a bias nulling circuit which adds a small variable
offset voltage superimposed on the output signal to the highest feedback
resistor (E11 ohms) , with an optocoupler in PV mode. The effective bias
current can be set to zero on the last two or three most sensitive
ranges. On the other ranges, it doesn't matter since the effect is so small.
Ed