Device for nA Measurements ? - Page 1
Forum section: electronics / testgear

author: Nicole01
author_rank: Newbie
timestamp: January 03, 2023, 03:03:54 pm
content: Hello Everyone,i would like to know how you make Measurements in the nA Range ?Which Devices can you recommend here ?You can't do that with a normal Multimeter.Even with most Benchtop Multimeters it is not possible.What Experiences have you here ?Many ThanksNicole

author: bdunham7
author_rank: Super Contributor
timestamp: January 03, 2023, 03:08:07 pm
content: There are many variations, but I use one of these:

author: Nicole01
author_rank: Newbie
timestamp: January 03, 2023, 03:12:55 pm
content: Thanks for your Help and the Info.I'm looking for a newer Device.It would be good if it was already Digital

author: bdunham7
author_rank: Super Contributor
timestamp: January 03, 2023, 03:23:17 pm
content: Well, as I said there are many variations. At the other extreme from my very old model, there are:https://www.tek.com/en/products/keithley/low-level-sensitive-and-specialty-instruments/series-6400-picoammetersIt depends on what your specific requirements and budget are. The active inputs on typical picoammeters result in a low voltage drop (burden voltage) across them, only a few mV in the case of my ancient 414A. If that doesn't matter and you have a higher-voltage high-impedance source you want to measure, a 10M input voltmeter actually works with 1V = 100nA. Also, there are some cheap converter units like the uCurrent from EEVBlog's Dave Jones and the Current Ranger, a sort of knock off. I don't know much about those two, but they've been discussed here at length and you can search for them.

author: Performa01
author_rank: Super Contributor
timestamp: January 03, 2023, 03:26:33 pm
content: Most DMMs with 10 Mohms input impedance can be used for this in principle.Just use the voltage range, e.g. 200 mV instead of the current ranges. This becomes a 20 nA range and you can measure down to 10 pA with a 20000 count meter.Some (especially true bench) meters have an unexpected high input current up to 1 nA. You can detect this by measuring the open circuit voltage with 10 Mohm input impedance; thus the meters own input current. If this is anywhere above 50 pA (= 0.5 mV), then the meter isn't usable for this purpose.Of course, the "correct" way to do this is using an electrometer; this allows you to measure down to the femtoamperes...

author: Nicole01
author_rank: Newbie
timestamp: January 03, 2023, 03:44:10 pm
content: I mainly want to Measure Voltages in the 1-30 Volt Range.I have also seen that you can install a Resistor in the Circuit and then Measure the Value.I don't want this, i want to Buy a finished Device.You're right, there are many different Devices.With many, however you also Pay for the Name of the Manufacturer.That's why I'm asking if there might be a Manufacturer who isn't that well known and the Price is okay

author: bdunham7
author_rank: Super Contributor
timestamp: January 03, 2023, 03:55:21 pm
content: Look up the two that I mentioned--the µCurrent and the Current Ranger.

author: DavidKo
author_rank: Frequent Contributor
timestamp: January 03, 2023, 04:38:15 pm
content: It is all only about the budget and how big voltage drop you can afford.I think that you have not understood the post from Performa01. Every voltmeter have internal resistance (that will be your shunt) and the measured voltage will be on its display/meter, rest is only ohms law. There are no ampere or voltage meters. Ampere meter is voltage meter with small internal resistance and vice versa. That is why small currents can be measured with voltage meter when you know its internal resistance (can be measure with another ohm meter) - common method with analog meters.

author: DavidKo
author_rank: Frequent Contributor
timestamp: January 03, 2023, 04:49:55 pm
content: Quote from: Nicole01 on January 03, 2023, 03:44:10 pmHow much can you afford? Should it be new or used?My Keithley 197A can measure nA, but that is not his job. You want to see nA or you want to have nA range and see pA (completely different story and budget)?

author: DavidKo
author_rank: Frequent Contributor
timestamp: January 03, 2023, 05:01:36 pm
content: I have another 2 meters, than can "measure" nAand are more on the budget (than UniT was my first multimeter cca 30 years ago).

author: mawyatt
author_rank: Super Contributor
timestamp: January 03, 2023, 05:05:20 pm
content: Quote from: Performa01 on January 03, 2023, 03:26:33 pmThere's another good way to measure the DMM Input Bias current if the DMM has the usual High Z Input (10G) feature on lower voltage ranges. Us a small 1nF or 10nF quality low leakage film cap (Polypropylene, Polystyrene), place the cap across the DMM inputs and short the cap. Remove the short and time the voltage ramp to a voltage within the range of the DMM setting.The input bias is calculated as Ibias = C dV/dT, where C is the used Cap, dV is the delta voltage reading and dT is the delta time to reach said voltage. We use this simple method to periodically check our various DMMs to "see" if the input has been damaged (dramatic increase in bias current). Just did so recently with three KS34465A, DMM6500 , AG34401A, HP34401A and SDM3065X, results were (11, 5, 4), 8, 25, 5 & 71pa respectively using a 10nF Polystyrene Cap.Best,

author: DavidKo
author_rank: Frequent Contributor
timestamp: January 03, 2023, 05:29:26 pm
content: Even with most Benchtop Multimeters it is not possible.Have you checked the tables inthe pinned topic? Other thing is that nowadays they want to pay you premium. Old meters had uA ranges and that UniT have the same 10nA resolution as HP 34401A (please do not comment this sh.t).Below you can see comparison between old tech HP 3457A and newer tech HP 34401A.HP/Agilent 34401A is a downgrade in this caseHP 3457A (you can see 10pA through GPIB)

author: tooki
author_rank: Super Contributor
timestamp: January 03, 2023, 07:18:43 pm
content: Quote from: Nicole01 on January 03, 2023, 03:44:10 pmThis type of instrument isn’t exactly super-common. You’re not just “paying for the name”, you’re paying for the expertise in designing these high-accuracy, low-sales-volume items.If you want one cheap, get a used one. That’s a fantastic way to save money on quality test gear. I’ll choose an old Keithley over a new no-name any day.

author: Nicole01
author_rank: Newbie
timestamp: January 03, 2023, 09:21:14 pm
content: DavidKo thank You for the topic LinkI think I didn't phrase myself correctly.You are right, many Multimeters can Measure in the µA and nA Range.But not very far down.I want to Measure at least to 1 nA.Many Multimeters only Measure 2 Digits after the decimal Point.If I now want to Measure 1 nA, this cannot be Displayed.The Price is not that Important, the main thing is that I have correct Results.

author: Someone
author_rank: Super Contributor
timestamp: January 03, 2023, 10:43:51 pm
content: Quote from: Nicole01 on January 03, 2023, 09:21:14 pmSo where is the confusion?Look through data sheets to find devices that will meet your accuracy specification.DMM6500 is probably the cheapest thing in that space. From there it is off into SMUs and Picoammeters.All those devices have limitations that may be "no go" for your unspecified end use (and other functions that may be value add) so it is almost impossible to provide any suggestions.

author: BeBuLamar
author_rank: Super Contributor
timestamp: January 04, 2023, 12:27:06 am
content: Quote from: Nicole01 on January 03, 2023, 09:21:14 pmI think the Keithley 6400 series would be good enough for you.