On 2020-09-04 17:50, Steve Wilson wrote:> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 2020-09-02 11:00, Steve Wilson wrote: >>> jlarkin@highlandsniptechnology.com wrote: >>> >>>> On Wed, 2 Sep 2020 07:36:28 -0400, Phil Hobbs >>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>>> Another Tap on the voltage divider. >>>>> >> >>>> Even a 10M input, $20 DVM is a decent picoammeter. >>> >>> Assuming 200 mV on the lowest scale, I get 20 nanoamperes full scale. >>> >> >> Which at 4.5 digits gets you 1 pA per count.> > As with most instruments, probably should not use DVMs below 10% of full > scale. That works out to 2nA.Based on what? Is there something in your basic delta-sigma ADC that becomes unreliable below 10% of full scale? Do tell. There are all sorts of better instruments for measuring picoamps, including my Keithley 410 from 1960, which has a 100 fA FS range, and my Keithley 610C from 1968, with a 10 fA FS range. I imagine that in the last 50 years or so people have done better. ;) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
typically stupid ED article
Started by ●September 1, 2020
Reply by ●September 5, 20202020-09-05
Reply by ●September 6, 20202020-09-06
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:> On 2020-09-04 17:50, Steve Wilson wrote: >> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> On 2020-09-02 11:00, Steve Wilson wrote: >>>> jlarkin@highlandsniptechnology.com wrote: >>>> >>>>> On Wed, 2 Sep 2020 07:36:28 -0400, Phil Hobbs >>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>>>> Another Tap on the voltage divider. >>>>>> >>> >>>>> Even a 10M input, $20 DVM is a decent picoammeter. >>>> >>>> Assuming 200 mV on the lowest scale, I get 20 nanoamperes full >>>> scale. >>>> >>> >>> Which at 4.5 digits gets you 1 pA per count. > >> >> As with most instruments, probably should not use DVMs below 10% of >> full scale. That works out to 2nA. > > Based on what? Is there something in your basic delta-sigma ADC that > becomes unreliable below 10% of full scale? Do tell.Yes. The meter may be accurate, but other effects could play a major role in causing innacurate measurements. Most DVMs had decade scales, for example, 200V, 20V, 2V, 200mV, so when you got below 10% of full scale, then you downrange. The only problem is when you are at 200mV, there is no place to downrange to. Readings below 10% could be subject to noise, dc effects, poor shielding, oscillation in external sources, and other sources of error. In this case, I would use readings below 10% of full scale as relative readings and not absolute calibrated readings. In addition, readings above 10% could be subject to similar problems, so care is always needed.> There are all sorts of better instruments for measuring picoamps, > including my Keithley 410 from 1960, which has a 100 fA FS range, and > my Keithley 610C from 1968, with a 10 fA FS range.I have the 610C also. It has an analog meter. These generally became unreliable below 10% of full scale due to stiction, meter balance, stray magnetic fields and other effects. Metrology is a tricky business.> I imagine that in the last 50 years or so people have done better. ;) > > Cheers > > Phil Hobbs
Reply by ●September 6, 20202020-09-06
On Sat, 5 Sep 2020 22:45:21 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 2020-09-04 17:50, Steve Wilson wrote: >> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> On 2020-09-02 11:00, Steve Wilson wrote: >>>> jlarkin@highlandsniptechnology.com wrote: >>>> >>>>> On Wed, 2 Sep 2020 07:36:28 -0400, Phil Hobbs >>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>>>> Another Tap on the voltage divider. >>>>>> >>> >>>>> Even a 10M input, $20 DVM is a decent picoammeter. >>>> >>>> Assuming 200 mV on the lowest scale, I get 20 nanoamperes full scale. >>>> >>> >>> Which at 4.5 digits gets you 1 pA per count. > >> >> As with most instruments, probably should not use DVMs below 10% of full >> scale. That works out to 2nA. > >Based on what? Is there something in your basic delta-sigma ADC that >becomes unreliable below 10% of full scale? Do tell.Where did that 10% rule come from? Makes no sense to me.> >There are all sorts of better instruments for measuring picoamps, >including my Keithley 410 from 1960, which has a 100 fA FS range, and my >Keithley 610C from 1968, with a 10 fA FS range. > >I imagine that in the last 50 years or so people have done better. ;) > >Cheers > >Phil HobbsMy 610C is older than all of my kids but still works geat: https://www.dropbox.com/s/lo8xsx2x07b4zy4/Welwyn_1G_Keithley.jpg?raw=1 The first time I played with one of the old discrete mosfets in a TO-18 can, as a follower, I disconnected the gate and was shocked that the source voltage just hung there. -- John Larkin Highland Technology, Inc Science teaches us to doubt. Claude Bernard
Reply by ●September 6, 20202020-09-06
jlarkin@highlandsniptechnology.com wrote:> On Sat, 5 Sep 2020 22:45:21 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >>On 2020-09-04 17:50, Steve Wilson wrote: >>> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>>> On 2020-09-02 11:00, Steve Wilson wrote: >>>>> jlarkin@highlandsniptechnology.com wrote: >>>>> >>>>>> On Wed, 2 Sep 2020 07:36:28 -0400, Phil Hobbs >>>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>>>>> Another Tap on the voltage divider. >>>>>>> >>>> >>>>>> Even a 10M input, $20 DVM is a decent picoammeter. >>>>> >>>>> Assuming 200 mV on the lowest scale, I get 20 nanoamperes full >>>>> scale. >>>>> >>>> >>>> Which at 4.5 digits gets you 1 pA per count. >> >>> >>> As with most instruments, probably should not use DVMs below 10% of >>> full scale. That works out to 2nA. >> >>Based on what? Is there something in your basic delta-sigma ADC that >>becomes unreliable below 10% of full scale? Do tell. > > Where did that 10% rule come from? Makes no sense to me.with decade scales, downrange below 10%
Reply by ●September 6, 20202020-09-06
Steve Wilson <spam@me.com> wrote:> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 2020-09-04 17:50, Steve Wilson wrote: >>> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>>> On 2020-09-02 11:00, Steve Wilson wrote: >>>>> jlarkin@highlandsniptechnology.com wrote: >>>>> >>>>>> On Wed, 2 Sep 2020 07:36:28 -0400, Phil Hobbs >>>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>>>>> Another Tap on the voltage divider. >>>>>>> >>>> >>>>>> Even a 10M input, $20 DVM is a decent picoammeter. >>>>> >>>>> Assuming 200 mV on the lowest scale, I get 20 nanoamperes full >>>>> scale. >>>>> >>>> >>>> Which at 4.5 digits gets you 1 pA per count. >> >>> >>> As with most instruments, probably should not use DVMs below 10% of >>> full scale. That works out to 2nA. >> >> Based on what? Is there something in your basic delta-sigma ADC that >> becomes unreliable below 10% of full scale? Do tell. > > Yes. The meter may be accurate, but other effects could play a major > role in causing innacurate measurements.Please see the Keithley Low Level Measurements Handbook, 7th Edition, at https://download.tek.com/document/LowLevelHandbook_7Ed.pdf
Reply by ●September 6, 20202020-09-06
On Tuesday, September 1, 2020 at 11:11:28 AM UTC-4, jla...@highlandsniptechnology.com wrote:> https://www.electronicdesign.com/technologies/test-measurement/whitepaper/21140081/warning-your-dmm-is-discharging-your-battery-cell?utm_source=EG+ED+Today&utm_medium=email&utm_campaign=CPS200828010&o_eid=7322A4702401H9R&rdx.ident%5Bpull%5D=omeda%7C7322A4702401H9R&oly_enc_id=7322A4702401H9R > > It's sad how bad the electronics press has become. > > > > -- > > John Larkin Highland Technology, Inc > > Science teaches us to doubt. > > Claude BernardThe carbon dust thing was a new one on me. Maybe he's talking about some ultra-low power batteries or something. Best part is he kept the article short.
Reply by ●September 6, 20202020-09-06
On 2020-09-04 17:50, Steve Wilson wrote:> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 2020-09-02 11:00, Steve Wilson wrote: >>> jlarkin@highlandsniptechnology.com wrote: >>> >>>> On Wed, 2 Sep 2020 07:36:28 -0400, Phil Hobbs >>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>>> Another Tap on the voltage divider. >>>>> >> >>>> Even a 10M input, $20 DVM is a decent picoammeter. >>> >>> Assuming 200 mV on the lowest scale, I get 20 nanoamperes full scale. >>> >> >> Which at 4.5 digits gets you 1 pA per count.> As with most instruments, probably should not use DVMs below 10% of full > scale. That works out to 2nA.So if I'm at some customer's site and need to measure some leakage current, it's not okay to use the handy picoammeter that I have in my tool bag? In the lab I have instruments that go down to the low femtoamps--three Keithleys and an HP 4145B--that I'd certainly prefer for picoamp measurements. However, they don't go in my tool bag when I'm on the road. (I could cook up such an instrument from stuff I often do take with me, e.g. a polystyrene cap and a CMOS op amp, but it would take a little time.) It's fairly rare IME to need picoamp measurements accurate to more than a couple of significant figures anyway. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●September 6, 20202020-09-06
On 2020-09-06 11:39, jlarkin@highlandsniptechnology.com wrote:> On Sat, 5 Sep 2020 22:45:21 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 2020-09-04 17:50, Steve Wilson wrote: >>> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>>> On 2020-09-02 11:00, Steve Wilson wrote: >>>>> jlarkin@highlandsniptechnology.com wrote: >>>>> >>>>>> On Wed, 2 Sep 2020 07:36:28 -0400, Phil Hobbs >>>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>>>>> Another Tap on the voltage divider. >>>>>>> >>>> >>>>>> Even a 10M input, $20 DVM is a decent picoammeter. >>>>> >>>>> Assuming 200 mV on the lowest scale, I get 20 nanoamperes full scale. >>>>> >>>> >>>> Which at 4.5 digits gets you 1 pA per count. >> >>> >>> As with most instruments, probably should not use DVMs below 10% of full >>> scale. That works out to 2nA. >> >> Based on what? Is there something in your basic delta-sigma ADC that >> becomes unreliable below 10% of full scale? Do tell. > > Where did that 10% rule come from? Makes no sense to me. > > >> >> There are all sorts of better instruments for measuring picoamps, >> including my Keithley 410 from 1960, which has a 100 fA FS range, and my >> Keithley 610C from 1968, with a 10 fA FS range. >> >> I imagine that in the last 50 years or so people have done better. ;)> > > My 610C is older than all of my kids but still works geat: > > https://www.dropbox.com/s/lo8xsx2x07b4zy4/Welwyn_1G_Keithley.jpg?raw=1 > > The first time I played with one of the old discrete mosfets in a > TO-18 can, as a follower, I disconnected the gate and was shocked that > the source voltage just hung there. > > >The ones with the BeCu shorting springs? I still have a dozen or so of those--3N163 and 2N4mumblemumble. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●September 6, 20202020-09-06
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:> On 2020-09-04 17:50, Steve Wilson wrote: >> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> On 2020-09-02 11:00, Steve Wilson wrote: >>>> jlarkin@highlandsniptechnology.com wrote: >>>> >>>>> On Wed, 2 Sep 2020 07:36:28 -0400, Phil Hobbs >>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>>>> Another Tap on the voltage divider. >>>>>> >>> >>>>> Even a 10M input, $20 DVM is a decent picoammeter. >>>> >>>> Assuming 200 mV on the lowest scale, I get 20 nanoamperes full >>>> scale. >>>> >>> >>> Which at 4.5 digits gets you 1 pA per count. > >> As with most instruments, probably should not use DVMs below 10% of >> full scale. That works out to 2nA. > > So if I'm at some customer's site and need to measure some leakage > current, it's not okay to use the handy picoammeter that I have in my > tool bag?Of course. If you have an instrument that measures picoamps, then use it.> In the lab I have instruments that go down to the low femtoamps--three > Keithleys and an HP 4145B--that I'd certainly prefer for picoamp > measurements. However, they don't go in my tool bag when I'm on the > road. (I could cook up such an instrument from stuff I often do take > with me, e.g. a polystyrene cap and a CMOS op amp, but it would take a > little time.) > > It's fairly rare IME to need picoamp measurements accurate to more > than a couple of significant figures anyway.It depends on what you are doing. Keithley gives good examples of limitations in low level measurements.> Cheers > > Phil Hobbs
Reply by ●September 8, 20202020-09-08
jlarkin@highlandsniptechnology.com wrote:> > Even a 10M input, $20 DVM is a decent picoammeter. >Works nice with old meters. But digitally calibrated "10M" DVM may have 11M input resistance, for such DVM to get any accuracy one would have to calibrate it and multiply result by correcton factor, not so nice. Also, significant part of "10M" may be input leakage. I have nice cheap Chinse meter, lowest range is 10mV with four digit resolution. Using it I can measure voltage of a termocouple and see difference when termocouple is on table and when it is on floor level. One gets funny results measuring voltage on well discharged polyester capacitor. There is substantial drift, indicating that most of input "resistance" is in fact leakage. So, while quite sensitive this DVM needs external resistor to measure low currents. -- Waldek Hebisch
