typically stupid ED article

Well, no. Realtime does not mean really fast, it means the the latency to do 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

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

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.)

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

Which at 4.5 digits gets you 1 pA per count.

Cheers

Phil Hobbs

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

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

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.

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

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

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

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.

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.

Where did that 10% rule come from? Makes no sense to me.

My 610C is older than all of my kids but still works geat:

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.

with decade scales, downrange below 10%

Please see the Keithley Low Level Measurements Handbook, 7th Edition, at

The carbon dust thing was a new one on me. Maybe he's talking about some ul tra-low power batteries or something. Best part is he kept the article shor t.

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

The ones with the BeCu shorting springs? I still have a dozen or so of those--3N163 and 2N4mumblemumble.

Cheers

Phil Hobbs

Of course. If you have an instrument that measures picoamps, then use it.

It depends on what you are doing. Keithley gives good examples of limitations in low level measurements.

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.

That's not a giant burden, to get a cheap picoammeter. My handheld Flukes are really 10M. It's not hard to check.

I did a little experiment last week: charged a film cap to 5 volts and connected it to my Fluke 8845, on its 10-volt HI-Z range. The DVM slowly charged the cap, at about 15 pA.

You can get cheap fA-bias current opamps and make your own very sensitive current meter easily.

I think there are even better opamps around now.