Forums

Low bias current opamps

Started by Jeroen November 7, 2012
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
On Nov 7, 6:25=A0pm, Jeroen <jer...@nospam.please> wrote:
> 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. -- Cheers, James Arthur
Jeroen a &#2013265929;crit :
> 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). -- Thanks, Fred.
On Thu, 08 Nov 2012 00:26:50 +0100, Jeroen <jeroen@nospam.please>
wrote:

>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
How did you measure that? -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
On 11/7/2012 3:26 PM, Jeroen wrote:
> 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.
"Jeroen" <jeroen@nospam.please> wrote in message 
news:k7eqm6$ar9$1@speranza.aioe.org...
>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 &#2013266097;20 fA at 25&#2013266096;C and &#2013266097;900 fA at 85&#2013266096;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.
On 2012-11-08 04:33, John Larkin wrote:
> On Thu, 08 Nov 2012 00:26:50 +0100, Jeroen<jeroen@nospam.please> > wrote: > >> 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 > > 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
On 2012-11-08 07:21, Jure Newsgroups wrote:
> "Jeroen"<jeroen@nospam.please> wrote in message > news:k7eqm6$ar9$1@speranza.aioe.org... >> 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 &#2013266097;20 fA > at 25&#2013266096;C and &#2013266097;900 fA at 85&#2013266096;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.
> 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
Yes, of course. Jeroen Belleman
Jeroen <jeroen@nospam.please> writes:

> 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. -- John Devereux
On Thu, 08 Nov 2012 00:26:50 +0100, Jeroen
<jeroen@nospam.please> wrote:

><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