Forums

Low bias current opamps

Started by Jeroen November 7, 2012
On 2012-11-08 12:53, Jon Kirwan wrote:
> 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
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
On Thu, 08 Nov 2012 08:45:54 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:

> >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).

Jeroen Belleman schrieb:

> 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
On 11/08/2012 09:55 AM, Uwe Hercksen wrote:
> > > Jeroen Belleman schrieb: > >> 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 >
If that were it, the curve would always be concave away from ground, (it would go as [1-exp(-t/RC)] if the resistance were linear), whereas Jeroen's crosses zero and is a very creditable straight line, suggesting that it's ordinary leakage. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
On 11/08/2012 10:05 AM, Phil Hobbs wrote:
> On 11/08/2012 09:55 AM, Uwe Hercksen wrote: >> >> >> Jeroen Belleman schrieb: >> >>> 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 >> > > If that were it, the curve would always be concave away from ground, (it > would go as [1-exp(-t/RC)] if the resistance were linear), whereas > Jeroen's crosses zero and is a very creditable straight line, suggesting > that it's ordinary leakage. > > Cheers > > Phil Hobbs >
(exponential the wrong way up, but you know what I mean--cut me some slack, we just had a very unseasonable snowstorm to go with our hurricane) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
On Thu, 08 Nov 2012 10:30:07 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>On 11/08/2012 10:05 AM, Phil Hobbs wrote: >> On 11/08/2012 09:55 AM, Uwe Hercksen wrote: >>> >>> >>> Jeroen Belleman schrieb: >>> >>>> 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 >>> >> >> If that were it, the curve would always be concave away from ground, (it >> would go as [1-exp(-t/RC)] if the resistance were linear), whereas >> Jeroen's crosses zero and is a very creditable straight line, suggesting >> that it's ordinary leakage. >> >> Cheers >> >> Phil Hobbs >> > >(exponential the wrong way up, but you know what I mean--cut me some >slack, we just had a very unseasonable snowstorm to go with our hurricane) > >Cheers > >Phil Hobbs
Got power back? -- 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 11/08/2012 11:51 AM, John Larkin wrote:
> On Thu, 08 Nov 2012 10:30:07 -0500, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 11/08/2012 10:05 AM, Phil Hobbs wrote: >>> On 11/08/2012 09:55 AM, Uwe Hercksen wrote: >>>> >>>> >>>> Jeroen Belleman schrieb: >>>> >>>>> 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 >>>> >>> >>> If that were it, the curve would always be concave away from ground, (it >>> would go as [1-exp(-t/RC)] if the resistance were linear), whereas >>> Jeroen's crosses zero and is a very creditable straight line, suggesting >>> that it's ordinary leakage. >>> >>> Cheers >>> >>> Phil Hobbs >>> >> >> (exponential the wrong way up, but you know what I mean--cut me some >> slack, we just had a very unseasonable snowstorm to go with our hurricane) >> >> Cheers >> >> Phil Hobbs > > Got power back? > >
Yes, thanks--we were only out for a day at home with the hurricane. Internet and phone (Optimum) took a week. Cell service at the lab was out for 9 days and is still spotty, but the lab power and cable stayed on, more or less. (The UPS didn't shut down the server, anyway.) Power didn't go out due to this snowstorm--not like Hallowe'en last year, we had 2 feet of snow dumped on us, with all the leaves still on the trees. We were out for a day then too, but friends of ours were out for 10 days. Of course the same friends are still out from Monday the 29th. They have boatloads of money--I wonder why they don't get a natural gas standby generator. You don't have to stand in line to get fuel for that. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
On Thu, 08 Nov 2012 13:44:10 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:

>On 2012-11-08 12:53, Jon Kirwan wrote: >> 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 > >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
On 11/8/2012 6:56 AM, Spehro Pefhany wrote:
> On Thu, 08 Nov 2012 08:45:54 +0100, Jeroen Belleman > <jeroen@nospam.please> wrote: > >> >> 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? 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.
On Thu, 08 Nov 2012 17:31:42 -0800, the renowned miso <miso@sushi.com>
wrote:

>On 11/8/2012 6:56 AM, Spehro Pefhany wrote: >> On Thu, 08 Nov 2012 08:45:54 +0100, Jeroen Belleman >> <jeroen@nospam.please> wrote: >> >>> >>> 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?
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 -- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com