I was going to say (and should have), you will most likely require a calibration dmm. To read nanoamps requires a balanced input, which the 8808a has. If you notice they mention 2/4 wire, 2 wire is common of all dmm's but 4 wire is balanced, nulls common mode noise which is the kicker. Currents of this magnitude can be easily influenced by just about anything, including your proximity. Any king of stray capacitance/inductance from the leads, emi etc. Best if used in a screen room, think faraday cage. Do not know any individual that has one. Well a friend does have one built to protect his backup computer and data incase of a emp bomb, but thats a subject for another day 😀
1nA through 10k gives a voltmeter reading of 0.1mV
The circuit will probably tolerate a 10k current sensing resistor better than it will tolerate the added loop of a set of DMM cables and clips and the meter gubbins.
The circuit will probably tolerate a 10k current sensing resistor better than it will tolerate the added loop of a set of DMM cables and clips and the meter gubbins.
Ahem... 4 wire (Kelvin sense) techniques are for measuring low resistance values and have nothing to do with the problems encountered when measuring low currents. Low currents may require a guard terminal (the 8808A doesn't have one), but at 100 nA that's not an issue (unless you ask for ppm uncertainties). What the 8808A *does* have, and it's unusual for its class, is a IV-converter based current measurement. That offers very low voltage burden which is advantageous in some application (possibly also in the one under discussion).
Measuring base/collector curreents is not a task that requires very low uncertainties. hFE varies about with 1%/°C, and the collector current is usually quite temperature dependent as well (e.g. intentionally "proportional to absolute temperature" (PTAT) for precision opamps). All we want to do is distinguish 150 nA from 180 nA, and not affect the circuit operating conditions too much. No metrology certificate needed. 😉
Samuel
My bad. Will institute new rule; no posting before coffee.
Had even grabbed up a couple of 0.01% bridges I made 20 some years ago and then bam, reality check right between the eyes!
Cheers
Had even grabbed up a couple of 0.01% bridges I made 20 some years ago and then bam, reality check right between the eyes!
Cheers
Attachments
The Keithley 2001 is $4000-$5000 new, and even used, they go for over $1500 in good condition. That is the price you pay for 7.5 digits, and it is not something most hobbyists need to have on their bench.
I almost forgot this thread:
Multimeter spreadsheet - Page 1
Wytnuckles over at EEVBlog has compiled a huge spreadsheet of multimeters (both handheld and benchtop) and their various specifications + MSRP. You can look through it and find the best meter to fit your needs.
Multimeter spreadsheet - Page 1
Wytnuckles over at EEVBlog has compiled a huge spreadsheet of multimeters (both handheld and benchtop) and their various specifications + MSRP. You can look through it and find the best meter to fit your needs.
The Keithley is just a fancy op-amp with an electrometer tube input you can read the manual. It has a "fast" and "slow" mode one of which is a transimpedance amp using virtual ground this essential for measuring junction leakage where NO voltage burden is essential.
You can but several op-amps with 20fA input current and one I think at 3fA and use a big feedback resistor. You could also synthesize a big resistor with a T feedback network. The Keithely gets around 10 atto-amps or so. If you just want 10nA or 100nA full scale a chain of 1 or 10 10Meg resistors would give you a volt full scale.
If the shunt resistor approach is necessary floating everything on batteries will work, some Keithley's used mass quantities of batteries including one of those 90v B+ batteries from old radios.
Lots of very good, Lab quality suggestions.
Fine, didn't expect less.
Most seem to have skipped the:
for God's sake 🙄
Well, although technically correct, you don't actually need
but you might apply an obscure and often forgotten piece of knowledge called by ancient alchemists the Ohm's Law.
I think you are designing simple transistor gain stages and want to bias them properly.
In that case, get a Protoboard, a 9V battery, and a 10M resistor (I doubt you will use biasing resistors higher than that, if anything because they become somewhat harder to get and simple flux residues on the PCB, in a humid day, will cause you problems).
Conect emitter to ground, base to +9V through the 10M resistor and measure current flowing from +9V to collector, which I'm certain is within your Multimeter's capabilities.
Hfe or current gain will be Ic/Ib where Ib is 8.4V/10M=840nA
Do you want to measure it at 10X lower currents?
Use a 100M base resistor, although at such impedances and currents, leaks and losses will dominate the field.
Even if physically possible, I would not design such a pedal which would be difficult to produce and easy to drift and fail.
Just curious, can you post the schematic (or the relevant part) you are trying to design?
Because Keithley is a hollow branding this days and part of DANAHER group, most of their recent products is re-branded meters from Taiwan.
A bit of extra research could lead you getting what you like in a friendlier price.
Probably best to consider the ones on ebay. You could even buy one for parts only and substitute a modern electrometer op-amp. I expect this would work down to the pA level.
Me, I have my trusty 610C for going on 35+yrs.
Essentially this is what I advised a few posts back. Using the handheld DMMs built-in hfe tester does exactly the same.
I am actually in the midst of working through a number of pedal designs and mods and have determined, for the amount of time it takes, it would simplify moving transistors in and out of designs with a little foreknowledge of what to expect of the transistor under operational conditons, it is not about determining the bias point as much as determining how big a punch any one transistor is going to supply. Hit or miss trial and error, in my estimate, is a waste of time that could be put to use elsewhere. I am accumulating and hope to catalogue the gains on a number of transistors and then, knowing the enemy, work from there. There is a lot of different information out there. Everything from real world to incredibly over the top hype. I believe I'm better served if I just find out some of the answers for myself.
Amplifying the voltage on a shunt is no different. In either case sub-uV chopper amps are readily available.
Which model are you referring to? The 2001 mentioned above doesn't go to electrometer territory.
Samuel
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