typically stupid ED article...

[John Larkin]

On Wed, 2 Sep 2020 17:11:28 +0100, Martin Brown
<\'\'\'newspam\'\'\'@nonad.co.uk> wrote:

On 01/09/2020 16:48, Phil Hobbs wrote:
On 2020-09-01 11:11, jlarkin@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.

Good golly, Miss Molly, I\'m sure glad he put in that table.  Ohm\'s law
makes my head hurt.

Helps to use up more real estate in the article.

We should go back to old school potentiometer measurements of voltage -
then no current flows at all when you have balanced the bridge.

Galvos were slow and not very sensitive. A decent DVM will leak
picoamps on a low voltage range, and not take 30 minutes of your time
to make a measurement.

I charged a 2.2u film cap to 5 volts and plugged that into my Fluke.
I\'ll watch that for a while.

There are cheap opamps with fA input currents.

 

[Chris]

On 09/02/20 17:33, Tom Gardner wrote:

On 02/09/20 17:11, Martin Brown wrote:
On 01/09/2020 16:48, Phil Hobbs wrote:
On 2020-09-01 11:11, jlarkin@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.

Good golly, Miss Molly, I\'m sure glad he put in that table. Ohm\'s
law makes my head hurt.

Helps to use up more real estate in the article.

We should go back to old school potentiometer measurements of voltage
- then no current flows at all when you have balanced the bridge.

Literally, in my case. NiFe + Weston cell + metre rule + resistance wire.
Ah, them\'s were the days.


If he was complaining about the load that a Model 7 Avo puts on the
battery under test which ISTR was 500R/volt then he might have a
point. They were virtually indestructible though. I still have a
mostly working one. The ohm\'s range is shot but current and voltage
are fine.

I remember a story told at GPO/BT Martlesham Labs.

They had an Avo salesman and a (newfangled) DMM salesman demonstrate
the virtues of their equipment. It was neck-and-neck at the end of
the formal evaluation.

Then the DMM salesman threw the DMM across the room, walked over and
picked it up, and made another measurement.

I don\'t know whether or not he was grinning.

Drop an AVO once and the plastic movement frame breaks. The General
Electric (uk) was a much better meter imho. AVO 8\'s looked obsolete by
the 1970s and the cramped scale was a pain...

Chris

 

[Joe Gwinn]

On Tue, 01 Sep 2020 17:38:46 -0700, John Larkin
<jlarkin@highland_atwork_technology.com> wrote:

[snip]

We did a few products using a Zynq, with dual-core 600 MHz ARM CPUs,
running the Linux that comes with the Xilinx tools. Just a little
futzing with priorities got a critical task to run with worst-case
timeouts in the ballpark of 20 us.

\"Any OS is an RTOS if it\'s fast enough.\"

Well, no. Realtime does not mean really fast, it means the the
latency to do <long list> is *predictable* and short enough for the
task at hand, so you can draw timeline that will be followed well
enough. As discussed earlier, by favorite example is a rotary cement
kiln.

I lived through the days of the Hard RT versus Soft RT wars, and the
problem was that in systems of any size, hard RT was too fragile to
live. So, we would partition the system into domains. The largest
was non-RT, a smaller part was soft RT (VxWorks), and a very small
parte was hard RT (now often done in FPGAs).

Joe Gwinn

 

[server]

On Wed, 02 Sep 2020 11:24:20 -0700, John Larkin
<jlarkin@highland_atwork_technology.com> wrote:

On Wed, 2 Sep 2020 17:11:28 +0100, Martin Brown
\'\'\'newspam\'\'\'@nonad.co.uk> wrote:

On 01/09/2020 16:48, Phil Hobbs wrote:
On 2020-09-01 11:11, jlarkin@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.

Good golly, Miss Molly, I\'m sure glad he put in that table.  Ohm\'s law
makes my head hurt.

Helps to use up more real estate in the article.

We should go back to old school potentiometer measurements of voltage -
then no current flows at all when you have balanced the bridge.

Galvos were slow and not very sensitive. A decent DVM will leak
picoamps on a low voltage range, and not take 30 minutes of your time
to make a measurement.

I charged a 2.2u film cap to 5 volts and plugged that into my Fluke.
I\'ll watch that for a while.

So far, the voltage is increasing. Looks like about 10 pA equivalent.



--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

[Phil Hobbs]

On 2020-09-02 12:11, Martin Brown wrote:

On 01/09/2020 16:48, Phil Hobbs wrote:
On 2020-09-01 11:11, jlarkin@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.

Good golly, Miss Molly, I\'m sure glad he put in that table.  Ohm\'s law
makes my head hurt.

Helps to use up more real estate in the article.

We should go back to old school potentiometer measurements of voltage -
then no current flows at all when you have balanced the bridge.

I am a bit surprised they use a 10M shunt though. Reliable precision
100M and 1G resistors are relatively common and have been for ages.

The guy works at _Keysight_.  \"How are the mighty fallen.\"  Or maybe
it\'s their customers.

Formerly Agilent, formerly HP - do they change company name each time
there is a body to be buried?

Every time they want to hive off their test equipment business in favour
of the Junk of the Month--first it was computers and printers, then
biomed instruments. (They ruined their computer and printer
business--I\'ve no idea whether the biomed stuff is still any good.)

If he was complaining about the load that a Model 7 Avo puts on the
battery under test which ISTR was 500R/volt then he might have a point.
They were virtually indestructible though. I still have a mostly working
one. The ohm\'s range is shot but current and voltage are fine.

I have a beautiful Model 8 Mk IV that came from the collection of a guy
who also collected Rolls-Royces. It\'s in perfect condition and has not
a trace of stickiness in the meter movement. That one was the best of
the classical Universal Avometers. I also had a Model 16 that was more
modern-looking but was not in the same class at all. You had to thump
it every time you wanted to make a measurement, like a cheap barometer.
I eventually tossed it.

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

 

[Phil Hobbs]

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.

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

 

[server]

On Fri, 4 Sep 2020 09:38:52 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-09-02 12:11, Martin Brown wrote:
On 01/09/2020 16:48, Phil Hobbs wrote:
On 2020-09-01 11:11, jlarkin@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.

Good golly, Miss Molly, I\'m sure glad he put in that table.  Ohm\'s law
makes my head hurt.

Helps to use up more real estate in the article.

We should go back to old school potentiometer measurements of voltage -
then no current flows at all when you have balanced the bridge.

I am a bit surprised they use a 10M shunt though. Reliable precision
100M and 1G resistors are relatively common and have been for ages.

The guy works at _Keysight_.  \"How are the mighty fallen.\"  Or maybe
it\'s their customers.

Formerly Agilent, formerly HP - do they change company name each time
there is a body to be buried?

Every time they want to hive off their test equipment business in favour
of the Junk of the Month--first it was computers and printers, then
biomed instruments. (They ruined their computer and printer
business--I\'ve no idea whether the biomed stuff is still any good.)

Agilent bought Varian and promptly killed the NMR and FTMS groups, who
were my best customers at the time.

If he was complaining about the load that a Model 7 Avo puts on the
battery under test which ISTR was 500R/volt then he might have a point.
They were virtually indestructible though. I still have a mostly working
one. The ohm\'s range is shot but current and voltage are fine.

I have a beautiful Model 8 Mk IV that came from the collection of a guy
who also collected Rolls-Royces. It\'s in perfect condition and has not
a trace of stickiness in the meter movement. That one was the best of
the classical Universal Avometers. I also had a Model 16 that was more
modern-looking but was not in the same class at all. You had to thump
it every time you wanted to make a measurement, like a cheap barometer.
I eventually tossed it.

My Fluke 8845 seems to output +10 or 15 pA on the 10 volt range.



--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

[Steve Wilson]

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.

Cheers

Phil Hobbs

As with most instruments, probably should not use DVMs below 10% of full
scale. That works out to 2nA.

Got to find a more sensitive DVM with higher input impedance. I recall
that some switch out the divider on the lowest range and use the
impedance of the input stage for measuring current. An old Fluke DVM did
this, and changed the scale to read in nanoSiemens, which was pretty
impressive at the time.

 

[Steve Wilson]

Steve Wilson <spam@me.com> wrote:

Got to find a more sensitive DVM with higher input impedance. I recall
that some switch out the divider on the lowest range and use the
impedance of the input stage for measuring current. An old Fluke DVM did
this, and changed the scale to read in nanoSiemens, which was pretty
impressive at the time.

The DVM was the Fluke 8020A. Here\'s the datasheet:

https://stevenjohnson.com/pics/fluke-8020a-op-guide.gif

 

[Chris]

On 09/04/20 23:08, Steve Wilson wrote:

Steve Wilson<spam@me.com> wrote:

Got to find a more sensitive DVM with higher input impedance. I recall
that some switch out the divider on the lowest range and use the
impedance of the input stage for measuring current. An old Fluke DVM did
this, and changed the scale to read in nanoSiemens, which was pretty
impressive at the time.

The DVM was the Fluke 8020A. Here\'s the datasheet:

https://stevenjohnson.com/pics/fluke-8020a-op-guide.gif

Still have one of the Fluke 80-xx series and use it regularly. Handy
size for the bench and just refuses to die. 5 ukp at a car boot decades
aga...

Chris

 

[Phil Hobbs]

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

 

[Steve Wilson]

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

 

[server]

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 Hobbs

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.



--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

[Steve Wilson]

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%

 

[Steve Wilson]

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

 

[server]

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 Bernard

The 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.

 

[Phil Hobbs]

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

 

[Phil Hobbs]

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

 

[Steve Wilson]

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

 

[server]

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