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Low bias current opamps

J

Jeroen

Jan 1, 1970
0
I was playing today with a little ionization chamber amplifier,
which is an exercise in high-impedance design. My usual business
revolves around wideband RF amplifiers, so this is unfamiliar
territory for me.

I selected an LPC661 opamp which has a specified maximum Ib of
4pA, but with a typical value stated to be 2fA. That's a very
large difference, which, I guess, is motivated by the need to
minimize testing time. It takes a while to measure fA
currents with some precision.

It appears I got lucky: Its measured Ib comes out at an amazing
170aA! I'm impressed. I hadn't yet noticed some opamps had gotten
*that* good.

Jeroen Belleman
 
I was playing today with a little ionization chamber amplifier,
which is an exercise in high-impedance design. My usual business
revolves around wideband RF amplifiers, so this is unfamiliar
territory for me.

I selected an LPC661 opamp which has a specified maximum Ib of
4pA, but with a typical value stated to be 2fA. That's a very
large difference, which, I guess, is motivated by the need to
minimize testing time. It takes a while to measure fA
currents with some precision.

It appears I got lucky: Its measured Ib comes out at an amazing
170aA! I'm impressed. I hadn't yet noticed some opamps had gotten
*that* good.

Jeroen Belleman

Impressive.

You can make it even better--Pease wrote an ap note on nulling the
LPC661's Ib by varying / bootstrapping the supplies.
 
F

Fred Bartoli

Jan 1, 1970
0
Jeroen a écrit :
I was playing today with a little ionization chamber amplifier,
which is an exercise in high-impedance design. My usual business
revolves around wideband RF amplifiers, so this is unfamiliar
territory for me.

I selected an LPC661 opamp which has a specified maximum Ib of
4pA, but with a typical value stated to be 2fA. That's a very
large difference, which, I guess, is motivated by the need to
minimize testing time. It takes a while to measure fA
currents with some precision.

It appears I got lucky: Its measured Ib comes out at an amazing
170aA! I'm impressed. I hadn't yet noticed some opamps had gotten
*that* good.

When I did my 200pV/rtHz preamplifer with some huge Interfet IF3602 I
was very pleasantly surprised the IG was under one pA at 20mA Id, for it
to be spec'ed at 0.5nA max in off state which is generally more
favorable due to the lack of impact ionization. (Ok, I run them a real
low VDS).
 
M

miso

Jan 1, 1970
0
I was playing today with a little ionization chamber amplifier,
which is an exercise in high-impedance design. My usual business
revolves around wideband RF amplifiers, so this is unfamiliar
territory for me.

I selected an LPC661 opamp which has a specified maximum Ib of
4pA, but with a typical value stated to be 2fA. That's a very
large difference, which, I guess, is motivated by the need to
minimize testing time. It takes a while to measure fA
currents with some precision.

It appears I got lucky: Its measured Ib comes out at an amazing
170aA! I'm impressed. I hadn't yet noticed some opamps had gotten
*that* good.

Jeroen Belleman

Yes to minimizing test time, leakage in the DUT board, handler, etc.
plus guard-band. The parts also tend to get charged sliding on the
handler, so these kinds of measurements are really tough in production.

It only has 1000v ESD. I assume the protection structure is minimal.
 
J

Jure Newsgroups

Jan 1, 1970
0
Jeroen said:
I was playing today with a little ionization chamber amplifier,
which is an exercise in high-impedance design. My usual business
revolves around wideband RF amplifiers, so this is unfamiliar
territory for me.

I selected an LPC661 opamp which has a specified maximum Ib of
4pA, but with a typical value stated to be 2fA. That's a very
large difference, which, I guess, is motivated by the need to
minimize testing time. It takes a while to measure fA
currents with some precision.

It appears I got lucky: Its measured Ib comes out at an amazing
170aA! I'm impressed. I hadn't yet noticed some opamps had gotten
*that* good.

Jeroen Belleman

for low bias : how about the LMP7721 ?
"the industry's lowest guaranteed input bias current precision amplifier.
The ultra low input bias current is 3 fA, with a guaranteed limit of ±20 fA
at 25°C and ±900 fA at 85°C."
http://www.ti.com/product/lmp7721

the classic is the LMC6001 http://www.ti.com/product/lmc6001



also I guess you may have seen ,"what's all this teflon stuff" by Bob Pease.
http://portal.national.com/rap/Story/0,1562,4,00.html

http://portal.national.com/rap/Story/0,1562,5,00.html


Jure Z.
 
J

Jeroen Belleman

Jan 1, 1970
0
How did you measure that?

I made it an integrator with a 10pF feedback capacitance. The
output drifts slowly and linearly at about 17uV/s. It drifted
a mere volt since yesterday afternoon! There is a plot of the
voltage vs. time curve at <http://cern.ch/jeroen/tmp/chamber.gif>.
The steeper part near the start is where I put something slightly
radioactive nearby.

Jeroen Belleman
 
J

Jeroen Belleman

Jan 1, 1970
0
for low bias : how about the LMP7721 ?
"the industry's lowest guaranteed input bias current precision amplifier.
The ultra low input bias current is 3 fA, with a guaranteed limit of ±20 fA
at 25°C and ±900 fA at 85°C."
http://www.ti.com/product/lmp7721

the classic is the LMC6001 http://www.ti.com/product/lmc6001

Yes, I looked at that one before I discovered the LPC661.
I decided to give it shot, with the result as described.
An LMP7721 is ten times the price, supposedly because of
the cost of testing it for that low Ib.

Yes, of course.

Jeroen Belleman
 
J

John Devereux

Jan 1, 1970
0
Jeroen said:
I was playing today with a little ionization chamber amplifier,
which is an exercise in high-impedance design. My usual business
revolves around wideband RF amplifiers, so this is unfamiliar
territory for me.

I selected an LPC661 opamp which has a specified maximum Ib of
4pA, but with a typical value stated to be 2fA. That's a very
large difference, which, I guess, is motivated by the need to
minimize testing time. It takes a while to measure fA
currents with some precision.

It appears I got lucky: Its measured Ib comes out at an amazing
170aA! I'm impressed. I hadn't yet noticed some opamps had gotten
*that* good.

I think that chip is what is used as the front end in current Keithley
electrometers.
 
J

Jon Kirwan

Jan 1, 1970
0
<snip>
I selected an LPC661 opamp which has a specified maximum Ib of
4pA, but with a typical value stated to be 2fA. That's a very
large difference, which, I guess, is motivated by the need to
minimize testing time. It takes a while to measure fA
currents with some precision.

An interesting note also appears on page 9, second column, of
the LPC661 datasheet. It says:

"When one wishes to take advantage of the ultra-low
bias current of the LPC661, typically less than
0.04 pA, it is essential to have an excellent layout."

Note that here it seems to provide yet another kind of spec:
a "typically less than" spec of 40fA.

So "max" is 4pA, "typical" is 2fA, and "typically less than"
is 40fA. ;)
It appears I got lucky: Its measured Ib comes out at an amazing
170aA! I'm impressed. I hadn't yet noticed some opamps had gotten
*that* good.

Did you take note of their comment, "the leakage of the
capacitor and socket must be taken into account?"

Jon
 
J

Jeroen Belleman

Jan 1, 1970
0
An interesting note also appears on page 9, second column, of
the LPC661 datasheet. It says:

"When one wishes to take advantage of the ultra-low
bias current of the LPC661, typically less than
0.04 pA, it is essential to have an excellent layout."

Note that here it seems to provide yet another kind of spec:
a "typically less than" spec of 40fA.

So "max" is 4pA, "typical" is 2fA, and "typically less than"
is 40fA. ;)


Did you take note of their comment, "the leakage of the
capacitor and socket must be taken into account?"

Jon

Yes, the input node is in the air. In fact, the whole package
is sitting on top of the bypass caps. It's just a quick
lash-up to see what could be done. The capacitor is a 1206
NP0 ceramic. If it had been significantly leaky, I would have
seen a hint of an exponential curve. Over the time scale I
have been taking data (~20 hours), there is no sign of that
yet.

Jeroen Belleman
 
U

Uwe Hercksen

Jan 1, 1970
0
Jeroen said:
I made it an integrator with a 10pF feedback capacitance. The
output drifts slowly and linearly at about 17uV/s. It drifted
a mere volt since yesterday afternoon! There is a plot of the
voltage vs. time curve at <http://cern.ch/jeroen/tmp/chamber.gif>.
The steeper part near the start is where I put something slightly
radioactive nearby.

Hello,

but what about the 10pF feedback capacitance and its leakage current?
Are you sure that the output drift is caused by the bias current only
and is not influenced by the leakage current of the capacitor?

Bye
 
S

Spehro Pefhany

Jan 1, 1970
0
I made it an integrator with a 10pF feedback capacitance. The
output drifts slowly and linearly at about 17uV/s. It drifted
a mere volt since yesterday afternoon! There is a plot of the
voltage vs. time curve at <http://cern.ch/jeroen/tmp/chamber.gif>.
The steeper part near the start is where I put something slightly
radioactive nearby.

Jeroen Belleman

Interesting that the data sheet shows the part in "hermetic package"
to be more than 10x worse leakage (but no hermetic packages are shown,
just DIP and SOIC).
 
J

Jon Kirwan

Jan 1, 1970
0
Yes, the input node is in the air. In fact, the whole package
is sitting on top of the bypass caps. It's just a quick
lash-up to see what could be done. The capacitor is a 1206
NP0 ceramic. If it had been significantly leaky, I would have
seen a hint of an exponential curve. Over the time scale I
have been taking data (~20 hours), there is no sign of that
yet.

Jeroen Belleman

If the leakage were voltage-dependent, yes. Is there any
mechanism by which there might be leakage that is fixed
rather than dependent on applied voltage across it? (I can't
think of one. So I think I agree with you. But the question
has to be asked. What happens if the capacitor is left in
free air unattached? I'm thinking about the outside
environment in which all this is contained. And again, I
can't think of any reason to imagine different from what you
said.)

Jon
 
M

miso

Jan 1, 1970
0
Interesting that the data sheet shows the part in "hermetic package"
to be more than 10x worse leakage (but no hermetic packages are shown,
just DIP and SOIC).

What page are you on?

The bold face limits are over temperature, so the military ("M") part
has a higher leakage limit over temp. [100pA versus 4pA.]

What I find confusing is the Ib limit on datasheet page 2. The
industrial temp range devices will have no leakage higher than 4pA over
temperature. But the military part can leak as much as 20pA at 25 deg C.
 
S

Spehro Pefhany

Jan 1, 1970
0
What page are you on?

http://www.ti.com/lit/ds/symlink/lpc661.pdf

Upper right graph on PDF page 5 (numbered page 4). "Input Bias Current
vs. Temperature".
The bold face limits are over temperature, so the military ("M") part
has a higher leakage limit over temp. [100pA versus 4pA.]

What I find confusing is the Ib limit on datasheet page 2. The
industrial temp range devices will have no leakage higher than 4pA over
temperature. But the military part can leak as much as 20pA at 25 deg C.

Strange- maybe that limit is actually tested? I see datasheets from
as far back as 2001 that are exactly the same regarding the
above-mentioned two points.


Best regards,
Spehro Pefhany
 
M

miso

Jan 1, 1970
0
What page are you on?

http://www.ti.com/lit/ds/symlink/lpc661.pdf

Upper right graph on PDF page 5 (numbered page 4). "Input Bias Current
vs. Temperature".
The bold face limits are over temperature, so the military ("M") part
has a higher leakage limit over temp. [100pA versus 4pA.]

What I find confusing is the Ib limit on datasheet page 2. The
industrial temp range devices will have no leakage higher than 4pA over
temperature. But the military part can leak as much as 20pA at 25 deg C.

Strange- maybe that limit is actually tested? I see datasheets from
as far back as 2001 that are exactly the same regarding the
above-mentioned two points.


Best regards,
Spehro Pefhany

Well as you know, only the electrical test limits (in theory) can be
trusted. The curves are guidelines. The story I was always told about
electrical limits is the customer gets the right to return a part for a
replacement if the part fails electricals. Since nobody does incoming
inspection these days, that means a lot of crap gets shipped if the
vendor has poor quality. [Note the manufacturers flow usually has a QA
test for each lot on a sample basis to insure the test hardware wasn't
fubar. QA test is probably over temperature. ]

Anyway, I see your point and the datasheet doesn't make sense. I can't
think of anything in a ceramic package that would cause it to leak more
than plastic. I assume they don't put carbon black in this plastic
package, but that could make it worse than ceramic.

Some manufacturers put goop over the chip prior to the plastic going
around the leadframe. I assume that goop has high resistivity.
 
J

josephkk

Jan 1, 1970
0
I was playing today with a little ionization chamber amplifier,
which is an exercise in high-impedance design. My usual business
revolves around wideband RF amplifiers, so this is unfamiliar
territory for me.

I selected an LPC661 opamp which has a specified maximum Ib of
4pA, but with a typical value stated to be 2fA. That's a very
large difference, which, I guess, is motivated by the need to
minimize testing time. It takes a while to measure fA
currents with some precision.

It appears I got lucky: Its measured Ib comes out at an amazing
170aA! I'm impressed. I hadn't yet noticed some opamps had gotten
*that* good.

Jeroen Belleman

Cool. I had not noticed that they had gotten that good either, but it did
not surprise me. How is the offset voltage?

?-)
 
On 11/8/2012 6:56 AM, Spehro Pefhany wrote:
On Thu, 08 Nov 2012 08:45:54 +0100, Jeroen Belleman
I made it an integrator with a 10pF feedback capacitance. The
output drifts slowly and linearly at about 17uV/s. It drifted
a mere volt since yesterday afternoon! There is a plot of the
voltage vs. time curve at <http://cern.ch/jeroen/tmp/chamber.gif>.
The steeper part near the start is where I put something slightly
radioactive nearby.
Jeroen Belleman
Interesting that the data sheet shows the part in "hermetic package"
to be more than 10x worse leakage (but no hermetic packages are shown,
just DIP and SOIC).
What page are you on?

Upper right graph on PDF page 5 (numbered page 4). "Input Bias Current
vs. Temperature".
The bold face limits are over temperature, so the military ("M") part
has a higher leakage limit over temp. [100pA versus 4pA.]
What I find confusing is the Ib limit on datasheet page 2. The
industrial temp range devices will have no leakage higher than 4pA over
temperature. But the military part can leak as much as 20pA at 25 deg C.
Strange- maybe that limit is actually tested?  I see datasheets from
as far back as 2001 that are exactly the same regarding the
above-mentioned two points.
Best regards,
Spehro Pefhany

Well as you know, only the electrical test limits (in theory) can be
trusted. The curves are guidelines. The story I was always told about
electrical limits is the customer gets the right to return a part for a
replacement if the part fails electricals. Since nobody does incoming
inspection these days, that means a lot of crap gets shipped if the
vendor has poor quality. [Note the manufacturers flow usually has a QA
test for each lot on a sample basis to insure the test hardware wasn't
fubar. QA test is probably over temperature. ]

Anyway, I see your point and the datasheet doesn't make sense. I can't
think of anything in a ceramic package that would cause it to leak more
than plastic.

Plastic: 1e16 ohms. Ceramic: 1e14 ohms. Pease says so.
I assume they don't put carbon black in this plastic
package, but that could make it worse than ceramic.

Some manufacturers put goop over the chip prior to the plastic going
around the leadframe. I assume that goop has high resistivity.

James Arthur
 
M

miso

Jan 1, 1970
0
Plastic: 1e16 ohms. Ceramic: 1e14 ohms. Pease says so.
James Arthur

I found a spec for ceramic, which runs 1e14 to 1e16 ohm meter. Did Pease
drop the units?


In a ceramic package, you generally solder the die bottom to the metal
inside the package. [I think they call it a scrub. I've never that step
done.] Now since the resistance is volumetric, the metal on the inside
of the package certainly has a decent contact to the ceramic. Actually
it approaches the ideal metal plate used in the typical description of
how you define volumetric resistivity.

In a plastic package, the part sits on a paddle in the leadframe, which
can be isolated. But it also has contact to the plastic. Visualizing
this, it seems like both packages have significant contact to the
insulating material.

But it is possible in a plastic package to put the chip on "glass
beads." That floats the substrate contact. I don't know if that scheme
is possible for ceramic packages. Obviously if we had a packaging
engineer on the list it would be far better than someone who has just
had chips packaged.

I haven't found a spec on the plastic resistivity.
 
G

Gerhard Hoffmann

Jan 1, 1970
0
Am 10.11.2012 01:36, schrieb miso:
But it is possible in a plastic package to put the chip on "glass
beads." That floats the substrate contact. I don't know if that scheme
is possible for ceramic packages. Obviously if we had a packaging
engineer on the list it would be far better than someone who has just
had chips packaged.

I haven't found a spec on the plastic resistivity.

The sealing glass in those cheap non-sidebrazed ceramic Eproms produced
so much water during fritting that it already was corrosive.

regards, Gerhard
 
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