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Phil Hobbs
Guest

Thu Dec 27, 2018 5:45 am   



Apropos of the 'Digital Multimeters' thread, I've just been running into
the problem of getting enough current resolution on my DMMs. The
dedicated current ranges tend to have resolutions in the tens of
nanoamps or worse, which isn't adequate.

Of course the classical trick is to use the lowest DCV range for current
sensing. Most DMMs have a 10 Mohm input resistance on their voltage
ranges, so a 4.5-digit meter on its hundreds-of-millivolts range has a
current resolution of

10 uV/10 Mohm = 1 pA, far better than on their dedicated current ranges.

That works perfectly well, but it's a mild pain with photodiodes, which
are sensitive to burden voltage. You can put in a parallel resistance
to keep the burden voltage low enough that the photodiode's forward
conduction doesn't cause measurement errors. (In practice, 100-200 mV
won't cause significant errors with a silicon photodiode in most cases.)

My Keithley 177A has nice low-current ranges, and I also have their
models 610C, 602, and 410 analogue picoammeters; I can also easily slap
together all sorts of custom current amps.

In testing products for patent lawsuits, however, it's super helpful to
have instruments from well-known manufacturers that just read what
you're trying to measure directly. Today I was measuring nanoamp level
currents from green-filtered photodiodes, and it would have been more
convenient to have an actual current range with resolution down to 100
pA or 1 nA.

Suggestions?

Thanks

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

Mikko OH2HVJ
Guest

Thu Dec 27, 2018 9:45 am   



Phil Hobbs <pcdhSpamMeSenseless_at_electrooptical.net> writes:

Quote:
In testing products for patent lawsuits, however, it's super helpful
to have instruments from well-known manufacturers that just read what
you're trying to measure directly. Today I was measuring nanoamp
level currents from green-filtered photodiodes, and it would have been
more convenient to have an actual current range with resolution down
to 100 pA or 1 nA.

Suggestions?


Keysight 34465A ? I think 1uA range has 6.5 digits. I don't know about
stability of the last digits, though.

--
mikko

Robert Lacoste
Guest

Thu Dec 27, 2018 10:45 am   



"Phil Hobbs" wrote in message news:q01l3u$fdt$1_at_dont-email.me...
Quote:
Apropos of the 'Digital Multimeters' thread, I've just been running into
the problem of getting enough current resolution on my DMMs. The dedicated
current ranges tend to have resolutions in the tens of nanoamps or worse,
which isn't adequate.


Hi Phil,
Look at Tektronix/Keithley DMM7510. Not exactly cheap, but 1pA resolution
with maximum 15mV burden voltage.
Yours,
Robert Lacoste
www.alciom.com

Winfield Hill
Guest

Thu Dec 27, 2018 8:45 pm   



Phil Hobbs wrote...
Quote:

Most DMMs have a 10 Mohm input resistance on their voltage
ranges, so a 4.5-digit meter on its hundreds-of-millivolts
range has a current resolution of ...


Most of my DVMs have infinite impedance on the 200mV range,
and many have infinite impedance on the V range. So you
can use 100M or 1G sense resistors.

You could always throw in a battery-powered opamp.


--
Thanks,
- Win

John Larkin
Guest

Thu Dec 27, 2018 9:45 pm   



On 27 Dec 2018 11:05:21 -0800, Winfield Hill
<hill_at_rowland.harvard.edu> wrote:

Quote:
Phil Hobbs wrote...

Most DMMs have a 10 Mohm input resistance on their voltage
ranges, so a 4.5-digit meter on its hundreds-of-millivolts
range has a current resolution of ...

Most of my DVMs have infinite impedance on the 200mV range,
and many have infinite impedance on the V range. So you
can use 100M or 1G sense resistors.

You could always throw in a battery-powered opamp.


I keep around a set of Pomona dual banana plugs with added parts:
short, 1K, 1M, 1G, 1T ohms. My old Keithley electrometer does pA but
has one side grounded, so sometimes I want to float a shunt and a DVM
to measure current.

I think Phil prefers a single, traceable instrument.



--

John Larkin Highland Technology, Inc trk

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com


Guest

Thu Dec 27, 2018 9:45 pm   



John Larkin <jjlarkin_at_highlandtechnology.com> wrote in
news:5vca2e949q4jtu6mc152usrsgtukumi6ff_at_4ax.com:

Quote:
On 27 Dec 2018 11:05:21 -0800, Winfield Hill
hill_at_rowland.harvard.edu> wrote:

Phil Hobbs wrote...

Most DMMs have a 10 Mohm input resistance on their voltage
ranges, so a 4.5-digit meter on its hundreds-of-millivolts
range has a current resolution of ...

Most of my DVMs have infinite impedance on the 200mV range,
and many have infinite impedance on the V range. So you
can use 100M or 1G sense resistors.

You could always throw in a battery-powered opamp.

I keep around a set of Pomona dual banana plugs with added parts:
short, 1K, 1M, 1G, 1T ohms. My old Keithley electrometer does pA
but
has one side grounded, so sometimes I want to float a shunt and a
DVM
to measure current.

I think Phil prefers a single, traceable instrument.




Precision current shunt, with or without calibration papers. And a
precise voltmeter. Job done.

Safer for the meter anyway.

Phil Hobbs
Guest

Thu Dec 27, 2018 11:45 pm   



On 12/27/18 2:05 PM, Winfield Hill wrote:
Quote:
Phil Hobbs wrote...

Most DMMs have a 10 Mohm input resistance on their voltage
ranges, so a 4.5-digit meter on its hundreds-of-millivolts
range has a current resolution of ...

Most of my DVMs have infinite impedance on the 200mV range,
and many have infinite impedance on the V range. So you
can use 100M or 1G sense resistors.

You could always throw in a battery-powered opamp.


Yeah, it isn't hard to finesse with a parallel resistor or a slow TIA,
as you say. Or I could use the analogue output of the Keithley 610C,
602, or 410, all of which I have.

It's just easier to explain to a jury that I used an ammeter to measure
amps. The case is about automatic control of display backlights, so I'm
using green-filtered photodiodes (intended for lux measurement) to
measure both the ambient and display brightness. That way it's really
apples-to-apples. I already have to explain to them how the
photocurrent and brightness correlate.

There's a lot of money at stake, so it's worth doing it right. (I
ordered another Keithley 177 for $60, and will check its calibration
before using it.) That will reduce the amount of plugging and
unplugging I have to do.

I thought about some of the higher-end meters suggested by the assembled
multitude, but I already have a couple of HP 3456A 6.5 digit DMMs for
voltage and resistance, so all I'd really gain would be a current input
and a fancy display.

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


Guest

Fri Dec 28, 2018 5:45 am   



Maybe you need a uCurrent…

https://www.eevblog.com/product/ucurrentgold/

Steve

Phil Hobbs
Guest

Fri Dec 28, 2018 5:45 am   



On 12/27/18 11:16 PM, sroberts6328_at_gmail.com wrote:
Quote:
Maybe you need a uCurrent…

https://www.eevblog.com/product/ucurrentgold/

Steve


That's just a simple TIA optimized for super low speed. It's useful and
all, but in this instance it's one more obstacle a jury has to overcome
to understand a (very compelling) line of evidence in what is to them a
very unfamiliar field. A name-brand ammeter measuring amps in the
normal way is the ticket.

Thanks

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


Guest

Fri Dec 28, 2018 7:45 am   



whit3rd <whit3rd_at_gmail.com> wrote in
news:d51e236c-a5e2-402e-9749-90a63e17f942_at_googlegroups.com:

Quote:
On Wednesday, December 26, 2018 at 8:40:34 PM UTC-8, Phil Hobbs
wrote:
Apropos of the 'Digital Multimeters' thread, I've just been
running into the problem of getting enough current resolution on
my DMMs. ...

convenient to have an actual current range with resolution down
to 100 pA or 1 nA.

I've been happy using Keithley 480 (digital, but has analog
pickoff); the more modern 485 might be suitable as well. Unlike
a DMM, it's usually not tied up with other tasks when I need a
small-current measurement.


He said "resolution" (in the title), which to me would mean
"precise accuracy".

"small current" is a different animal when simple environmental
factors can easily change a reading. That is why one usually will
see a task specific meter made just for reading very tiny flows
accurately.

I do not think that any simple multimeters would do well in those
'ranges' as far as accuracy goes.

Lab bench meters likely can with the right setup.

There are actual 'picoammeter' instruments out there. The one I
used was analog. I think it had a taught band movement in it.

whit3rd
Guest

Fri Dec 28, 2018 7:45 am   



On Wednesday, December 26, 2018 at 8:40:34 PM UTC-8, Phil Hobbs wrote:
Quote:
Apropos of the 'Digital Multimeters' thread, I've just been running into
the problem of getting enough current resolution on my DMMs. ...

convenient to have an actual current range with resolution down to 100
pA or 1 nA.


I've been happy using Keithley 480 (digital, but has analog pickoff); the
more modern 485 might be suitable as well. Unlike a DMM, it's usually
not tied up with other tasks when I need a small-current measurement.

Chris Jones
Guest

Fri Dec 28, 2018 12:45 pm   



On 28/12/2018 08:48, Phil Hobbs wrote:
Quote:
On 12/27/18 2:05 PM, Winfield Hill wrote:
Phil Hobbs wrote...

  Most DMMs have a 10 Mohm input resistance on their voltage
ranges, so a 4.5-digit meter on its hundreds-of-millivolts
range has a current resolution of ...

  Most of my DVMs have infinite impedance on the 200mV range,
  and many have infinite impedance on the V range.  So you
  can use 100M or 1G sense resistors.

  You could always throw in a battery-powered opamp.

Yeah, it isn't hard to finesse with a parallel resistor or a slow TIA,
as you say.  Or I could use the analogue output of the Keithley 610C,
602, or 410, all of which I have.

It's just easier to explain to a jury that I used an ammeter to measure
amps.  The case is about automatic control of display backlights, so I'm
using green-filtered photodiodes (intended for lux measurement) to
measure both the ambient and display brightness.  That way it's really
apples-to-apples.  I already have to explain to them how the
photocurrent and brightness correlate.


Would it not be better to obtain and use a photometer, to make it even
simpler for them?

Winfield Hill
Guest

Fri Dec 28, 2018 1:45 pm   



Phil Hobbs wrote...
Quote:

On 12/27/18 11:16 PM, Steve, sroberts6328_at_gmail.com wrote:

Maybe you need a uCurrent…

https://www.eevblog.com/product/ucurrentgold/

That's just a simple TIA optimized for super low speed. It's useful and
all, but in this instance it's one more obstacle a jury has to overcome
to understand a (very compelling) line of evidence in what is to them a
very unfamiliar field. A name-brand ammeter measuring amps in the
normal way is the ticket.


Hey, EEVblog is a widely-known brand name by now.
You can safely assert that to a jury.


--
Thanks,
- Win


Guest

Fri Dec 28, 2018 3:45 pm   



Quote:
Would it not be better to obtain and use a photometer,
to make it even simpler for them?


Not really. The patent talks about photocurrent, for one thing, and for another, using the same type of ambient light sensing photodiode for both the ambient and display brightness is a nice intuitive idea that also makes physical sense.

Besides, a 2.5 mm square PD is easy to mount right next to the ambient light sensor for the ambient reading (ensuring good tracking) and easy to tape upside-down on top of a full-brightness area on the display to measure its brightness in a repeatable way.

Photometer heads are bigger and clunkier, so that would be hard to do.

Cheers

Phil Hobbs


Guest

Fri Dec 28, 2018 3:45 pm   



Quote:
Hey, EEVblog is a widely-known brand name by now.
 You can safely assert that to a jury.


Sure, every pipe fitter and school teacher knows ol' Squeaky Dave. ;)

Cheers

Phil Hobbs

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