>> I think it's mostly mechanical instability combined with fairly large
>> internal E fields....
>> A bit of strategically placed foil inside would probably help a lot.
>If the box in question has been around the block a few times, it might
>have a shield missing; I've seen factory-fresh meters that
>were too sensitive to nearby items, and on calling the factory,
was advised to reseat the shield. �
>"What, there's supposed to be a shield there?" �I replied.
;)
Cheers
Phil Hobbs
Reply by whit3rd●January 10, 20162016-01-10
On Sunday, January 10, 2016 at 5:51:19 AM UTC-8, Phil Hobbs wrote:
> On 01/09/2016 08:45 PM, John Larkin wrote:
> <snip>
> > I don't think the mosfets are the problem on the lowest ranges.
> I think it's mostly mechanical instability combined with fairly large
> internal E fields....
> A bit of strategically placed foil inside would probably help a lot.
If the box in question has been around the block a few times, it might
have a shield missing; I've seen factory-fresh meters that
were too sensitive to nearby items, and on calling the factory,
was advised to reseat the shield.
"What, there's supposed to be a shield there?" I replied.
Reply by John Larkin●January 10, 20162016-01-10
On Sun, 10 Jan 2016 04:30:08 -0800 (PST), Phil Hobbs
<pcdhobbs@gmail.com> wrote:
>>> Well, the full scale range is switchable from 10 femtoamps to 300
>>> milliamps, so the correct answer is "yes".
>
>>The discussion was to find ways to improve the drift on the 10 fA range.
>
>>Would your customers let you get away with such an answer?
>
>I don't recall ever having been asked such a stupid and hostile question by a customer.
>
>Cheers
>
>Phil Hobbs
Especially one that expects you to work for free.
--
John Larkin Highland Technology, Inc
lunatic fringe electronics
Reply by John Larkin●January 10, 20162016-01-10
On Sun, 10 Jan 2016 08:51:12 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
>On 01/09/2016 08:45 PM, John Larkin wrote:
><snip>
>> I don't think the mosfets are the problem on the lowest ranges. The
>> problem seems to be random charge transfers and maybe leakages.
>>
>> At heart, the mosfets are the front-end of a voltmeter. On
>> voltage-input ranges, even with the multiplier cranked all the way up,
>> it's stable. So the fets aren't drifting. They might be leaking, but I
>> doubt that. A little pressure on the side of the box, bending the
>> metal a tiny bit, makes the 10fA range go wild. Mosfets aren't doing
>> that.
>
>I think it's mostly mechanical instability combined with fairly large
>internal E fields. The power supplies are +- 55V or something, and all
>the resistors are hanging in midair. If you've got 100V gradients
>running around, with a node capacitance of 5 pF and a typical separation
>of an inch, you can get 10-fA currents from a velocity of
>
> ~ 25 mm * 10 fA / (100V*5pF) = 0.5 microns per second.
>
>That's an average, of course, so in real life the motions would have to
>be much bigger. Still, it makes sense that flexing the front panel
>(where the switch is mounted) by a millimetre or so might make it go nuts.
>
>A bit of strategically placed foil inside would probably help a lot.
>
>
>> My home-made pA meter was more stable, maybe because there wasn't so
>> much stuff inside to make e-fields.
>
>The combination of a 100-fA FS homebrew box and a 610C is a pretty good
>solution. I like your pluggable thing, which is nice and flexible. For
>current measurements, making the loops and nodes smaller would make it
>faster and more stable, so I'd lean towards a fixed 1T resistor and a
>metal box meself.
>
>Cheers
>
>Phil Hobbs
I built that before I got the Keithley, and still use it now and then.
I have an aluminum chassis box that drops over the top to shield Z1
and Z2 when needed.
I was looking for a low-leakage diode. The PAD1 is sold as such, but
it's actually a jfet with s and d shorted. It has lots of series r,
lots of shunt c, guarantees 1 pA, and it's expensive.
Turns out that small transistors can be excellent low-leakage diodes.
The c-b junction of BFT25 is amazing: sub-pF capacitance, good forward
conduction, leakage hard to measure, 10 fA maybe.
One could charge a film capacitor from diode leakage, for hours or
days, and probably resolve attoamps.
--
John Larkin Highland Technology, Inc
lunatic fringe electronics
Reply by Phil Hobbs●January 10, 20162016-01-10
Well, when you get round to selling your building, you can probably afford to store a couple more warts, even. ;)
Cheers
Phil
Reply by John Larkin●January 10, 20162016-01-10
On Sat, 09 Jan 2016 23:14:51 -0500, Martin Riddle
<martin_ridd@verizon.net> wrote:
>On Sat, 09 Jan 2016 19:57:45 -0800, John Larkin
><jjlarkin@highlandtechnology.com> wrote:
>
>>On 9 Jan 2016 19:01:19 -0800, Winfield Hill <hill@rowland.harvard.edu>
>>wrote:
>>
>>>John Larkin wrote...
>>>>
>>>> My home-made pA meter was more stable, maybe because
>>>> there wasn't so much stuff inside to make e-fields.
>>>
>>> And, no range-changing stuff, right?
>>
>>Sure it changes ranges. By plugging different resistors, on Pomona
>>plugs, into one set of jacks.
>>
>>https://dl.dropboxusercontent.com/u/53724080/Gear/Z260_pA/99A260A1.JPG
>>
>>https://dl.dropboxusercontent.com/u/53724080/Gear/Z260_pA/99A260A3.JPG
>>
>>https://dl.dropboxusercontent.com/u/53724080/Gear/Z260_pA/99S260A.JPG
>>
>>I have Z1 plugin resistors from 1K to 1T.
>>
>>The lexan was added after I discovered that those binding posts are
>>super leaky.
>
>Hang on to that Wall wart. THey don't make transformer warts anymore,
>just noisy swithers. ;)
>
>Cheers
We have a big box that must have 100 old warts in it. We really should
throw some of that junk away, but you never know when you'll need a 17
volt AC wart.
--
John Larkin Highland Technology, Inc
lunatic fringe electronics
Reply by Phil Hobbs●January 10, 20162016-01-10
On 01/09/2016 08:45 PM, John Larkin wrote:
<snip>
> I don't think the mosfets are the problem on the lowest ranges. The
> problem seems to be random charge transfers and maybe leakages.
>
> At heart, the mosfets are the front-end of a voltmeter. On
> voltage-input ranges, even with the multiplier cranked all the way up,
> it's stable. So the fets aren't drifting. They might be leaking, but I
> doubt that. A little pressure on the side of the box, bending the
> metal a tiny bit, makes the 10fA range go wild. Mosfets aren't doing
> that.
I think it's mostly mechanical instability combined with fairly large
internal E fields. The power supplies are +- 55V or something, and all
the resistors are hanging in midair. If you've got 100V gradients
running around, with a node capacitance of 5 pF and a typical separation
of an inch, you can get 10-fA currents from a velocity of
~ 25 mm * 10 fA / (100V*5pF) = 0.5 microns per second.
That's an average, of course, so in real life the motions would have to
be much bigger. Still, it makes sense that flexing the front panel
(where the switch is mounted) by a millimetre or so might make it go nuts.
A bit of strategically placed foil inside would probably help a lot.
> My home-made pA meter was more stable, maybe because there wasn't so
> much stuff inside to make e-fields.
The combination of a 100-fA FS homebrew box and a 610C is a pretty good
solution. I like your pluggable thing, which is nice and flexible. For
current measurements, making the loops and nodes smaller would make it
faster and more stable, so I'd lean towards a fixed 1T resistor and a
metal box meself.
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
Reply by Phil Hobbs●January 10, 20162016-01-10
>> Well, the full scale range is switchable from 10 femtoamps to 300
>> milliamps, so the correct answer is "yes".
>The discussion was to find ways to improve the drift on the 10 fA range.
>Would your customers let you get away with such an answer?
I don't recall ever having been asked such a stupid and hostile question by a customer.
Cheers
Phil Hobbs
Reply by Martin Riddle●January 10, 20162016-01-10
On Sat, 09 Jan 2016 19:57:45 -0800, John Larkin
<jjlarkin@highlandtechnology.com> wrote:
Hang on to that Wall wart. THey don't make transformer warts anymore,
just noisy swithers. ;)
Cheers
Reply by John Larkin●January 9, 20162016-01-09
On 9 Jan 2016 19:01:19 -0800, Winfield Hill <hill@rowland.harvard.edu>
wrote:
>John Larkin wrote...
>>
>> My home-made pA meter was more stable, maybe because
>> there wasn't so much stuff inside to make e-fields.
>
> And, no range-changing stuff, right?