# In which scenarios is it important to measure microampere?

With the situation of finding a new multimeter, I found myself lost to the number of available devices on the market. For sure, to find the most suitable device I have to set some requirements. While comparing them, I came to the following point and by this to my question:

Most pro devices have only ampere range with a resolution of 0.001 A (1mA), while semi/hobby devices have ranges for milliampere and even micro-ampere. I saw device reviews on YouTube, where the presenter complained about missing micro-ampere range. While another person on YouTube told the audience that milliampere range is sufficient. So, here my question to the experts:

What kind of scenarios require a measurement of micro-amperes?

For example: Looking at a data sheet an AND gate has "input leakage current" and supply current in micro-ampere range, but when is it necessary to measure this tiny current?

• Have you ever heard of devices that run for 80000 hours with a 2000mAh battery? Oct 15 '18 at 15:02
• Not an answer, but it's worth noting that the test equipment company Keithley makes ammeters with a resolution of 10 fA, and Keysight's B2980A series has a resolution of 0.01 fA, which is frankly quite ridiculous. Oct 15 '18 at 15:47
• @Felthry: I had my hands on equipment that would count electrons. And it had to be recalibrated afterwards... Oct 15 '18 at 16:04
• Dave Jones, from the EEVblog, also had this problem and developed the uCurrent. Oct 15 '18 at 16:24
• @Jeroen3 Actually the development of the uCurrent wasn't driven much by the need of low current ranges, but by the need of low voltage burden in low current ranges (see my answer). The uCurrent has an admirable 10uV/mA (10mohm) voltage burden in the mA range (~100 times less than the usual hand-held pro-DMM) and a respectable 10uV/uA (10ohm) in the uA range (~10 times less usual DMMs). Oct 16 '18 at 18:43

One of a line of products I worked with and designed for was a smart payphone; think a microcontroller that operates as if it were a payphone.

These had to operate on an ordinary telephone loop, with a guaranteed 20mA supply (but not guaranteed to be higher); in the on-hook condition the unit was permitted only a few microamps of leakage current as the central office would otherwise detect a line fault.

In response to the comment on leakage; due to the harsh environment (outside in very hot, very cold and high humidity) the boards within the payphone housing were conformally coated and used moisture sealed connectors.

These units clearly needed to be tested as the difference between on-hook and off-hook current draw is order of magnitude different so confirming just a few microamps on-hook was quite important.

Another application is in new, really low power microcontrollers (typical part linked) where I would want to confirm the actual current draw in the various modes of operation and some of those modes are in the microamp range (or less).

Lots of possible applications, this is just a couple.

• Telephone example is pretty surprising. At 50V, even 5 megohms would produce "a few" (10 in this case) microamps of current. I'd be surprised humidity around joints didn't produce that effect, or even 10km of cable insulation. Oct 15 '18 at 18:47
• The telephone loop is ~48VAC. Not sure what the back-of-the-envelope leakage is for that...
– user65586
Oct 15 '18 at 20:35
• @Peter Smith: It looks like community votes for your answer. Thank you for giving those examples and sharing the link to the low power microcontroller. It gives a good impression about where to measure µA ... Oct 15 '18 at 21:20
• @jdv - Phone supply is -48v DC not AC Oct 15 '18 at 21:23
• I will mark this as final answer, because it has the most votes. It does not mean that all the other answers are wrong. Thank you to all for the answers and comments! Oct 16 '18 at 9:41

A lot of battery-operated devices need to optimize for power consumption, and µA currents are frequently involved (sometimes even nA).

To give an example, consider wireless remotes. They may have just a 3V, 200mAh battery. If you want this remote to work 10 years without necessitating battery change, that's just 20 mAh/year. Or 0.054 mAh/day, or 0.0022 mAh / hour. We cancel the hours and it's a shy more than 2µA continuous idle drain. A lot of contemporary micros and RTCs are way better than this, but you need to measure your production run to verify the device works as intended.

You'd say "isn't battery lifetime dependent on the number of operations of the remote" - well, it could, but the idle consumption may be more significant. The wireless transmitter and the MCU inside the remote may consume 10mA for a brief period when operated. Say less than a second. So that's 10mA but for a very short period, so the energy consumed from the battery is quite minimal. In contrast, just the 2µA idle drain for a whole day requires more than 16 times more energy.

First, your assumption that professional multimeters don't have a microamp scale is wrong. A Fluke 287, for instance, will happily measure microamps. The Fluke 116 only has a microamp scale for current measurements.

A lot of professional multimeters are designed for specific use cases. The aforementioned Fluke 116 is targeted at HVAC systems, where (apparently) the only currents they need to measure are from flame sensors. A high-end model like the 287 can do everything. I used one to measure reference currents in the 0-20 uA range back when I was working on flash memory process development. For battery-powered systems, microamps are important. But for most use cases, you don't need the microamp scale, so you don't pay extra for one.

• You're correct. After more research, I realized that Fluke has multimeters specific to the use case. As you said, Fluke 116 with only µA range. It was confusing me, that some multimeters (for example UNI-T) just come with µA almost by default and in the professional area, this range is not available on every device. Oct 15 '18 at 21:04
• The UNI-T is an order of magnitude cheaper than the Fluke. The specs are probably a lot worse, and quality control will be as well. Hobbyists aren't too picky about that stuff, but if you're a company with millions of dollars on the line, you're willing to pay for quality guarantees. Oct 15 '18 at 22:25
• "...you're willing to pay for quality guarantees" And for guaranteed safety levels, as reliable CAT ratings. So that your employees won't die while doing measurements on some nasty industrial thing just because their DMM arcs-over due to a power line spike! Oct 16 '18 at 18:10

When you are developing low-power devices, every nanoAmpere is worth to be saved. For example, when using a CR2032 coin battery you have around 200mAh of capacity. Once I developed a device powered by one of those batteries and I had to check that the microcontroller went to sleep mode (0.6uA) most of the time. I Also needed to check that when active, the current consumption was in the range of 10uA. In addition, I had to check that the sum of every component in the PCB (in their low power mode) matched the sum of the quiescent current stated by their datasheets.

In summary, if you want to get the most of your power source, and be sure that you are handling your hardware/software you have to measure low-power performance of your components, and usually this rate is given in uA or nA.

• Thanks for this answer, it gives a good example and it is easy to understand. I like it with all the other answers here. Oct 15 '18 at 21:15

The voltage burden of a current range of a DMM is the voltage that is dropped across the DMM while performing the measurement. It is expressed as V/A or mV/mA or similar units. Note that this units are equivalent to ohms and is the standard way to express the internal resistance that the DMM presents to the circuits in that specific range.

In some application is not so important to know that your DMM is capable of measuring in the uA range, but that is capable of doing so with low enough voltage burden.

This is extremely important in low-power or micropower applications, where microamps of current are drawn from low-voltage power rails.

In fact imagine a DMM having a 600uA range with a 100 uV/uA burden (like my Fluke 87V): if you measure 100uA drawn from a 10V rail, you just introduce a 10mV drop in the rail, which is negligible. However, if you measure the same current on a line that carries a 100mV signal, then you have altered that signal by 10%, and this may also make your circuit stop working.

Seen from another POV, it is not only the current range that matters for doing a measurement in a low-current application, but also the impedance of the circuit in which you are going to insert your ammeter. If the ammeter has too high an internal resistance (high voltage burden) it will significantly alter the measurement or even the working of the circuit under test.

So while choosing a DMM and examining its current specs, you should also take into account the voltage burden as a parameter.

• You could have read my mind: While digging through the data sheets of DMMs, of course I found the voltage burden value. And if it comes to µA measurement, this burden has to be considered. Thanks for the input and hint, I'm sure this will help others as well. Oct 16 '18 at 18:31
• Wow, I learned something today and I just want to add this: eevblog.com/projects/ucurrent - this "adapter" has a voltage burden of 20 µV. Oct 16 '18 at 18:50
• @TobyN. Be careful to understand what is burden voltage: is not 20uV, but 20uV/mA, i.e. 20mohms, and that is on the mA range. On its uA range it is just 10uV/uA, i.e. 10ohms. Not to say they are not good values, they beat most professional hand-held DMMs, but it is not SO better as you seem to imply. And keep in mind that the uCurrent is not input protected as a DMM, so you can damage the thing if you are not careful. Oct 16 '18 at 18:57

Often when performing characterization and modeling of semiconductor devices, leakage currents (which are critical to creating a useful and accurate model) will fall in the micro-ampere range. Typically these measurements would be performed with a Precision Source-Measure Unit (SMU for short). Such measurements are also commonly used in technology development to evaluate the fundamental performance of a given semiconductor process.

• Good point with SMU. For hobby electronic (even dealing with low current devices) it might be not the right measurement device from a cost perspective. So in your personal opinion: Is a multimeter a good alternative or do you think mA range is sufficient? See also the answer of Adam Haun and Peter Smith - interesting stuff with focus on low current. Oct 15 '18 at 21:34
• It depends on the application in question. Other answers highlight some specific examples of where the mA range is just not sufficient (e.g. production testing of low-power battery-driven circuits). If the multimeter has the accuracy and/or precision necessary for the measurement, then sure, it's fine. Perhaps it's even feasible to build a circuit using e.g. an instrumentation amplifier to convert a $\mu$A range current to something detectable by a cheaper multimeter reliably. Again, it's very application specific. Oct 16 '18 at 1:10

When operating an electron microscope, it is often desirable to know the beam current to the resolution of a few picoamps. Beam currents are small because the goal of an electron microscope is to focus a narrow (and thus low current) beam of electrons on the sample, in order to have the beam interact with small features.

This is accomplished by connecting an ammeter between an electrically isolated sample stage and the microscope ground. Such an ammeter must, of course, be able to measure in the range of currents used by the instrument.

This is more of a niche case than you're probably interested in, but for completeness: high-voltage physics experiments often involve currents in the microamp or nanoamp range, for example many photomultiplier tubes have saturation currents in the 1-10 uA range, with response curves like so (from this Hamamatsu info handbook):

Generally these are read by high-impedance amplifiers to get a useful voltage (~1-10V) proportional to the current, but I could imagine cases where you want to figure out which of your PMTs are broken and just want to connect a multimeter and wave your hand over the tube to block light and see the current drop.

Similarly, anywhere that you try to maintain a high voltage (few kV) bias on something (eg an electrode in vacuum) you will have a leakage current which must be supplied to keep the voltage steady, this is usually in the microamp to nanoamp range as well. Again this is something you're unlikely to be in a position to measure safely with a handheld DMM.

The "pro" devices?

I think by "pro" they actually the "electrician" meters. When someone is working on home 120V wiring, or working on a car, usually they're dealing in amperes, or sometimes mA. The microamps are important in electronics, but not so much in professional "electrical" work.

But for engineers and scientists (heh the real pros,) microamp meter scales are incredibly important. The same is true for hobbyists, or anyone working with transistor circuits. See all the examples in the answers here. Transistor base currents, photodetectors, op-amps, and just anything involving resistors over 10,000 ohms, etc...