Digital Multimeter measuring range from uA to mA without selector switch - Page 1
Forum section: electronics / testgear

author: snapster
author_rank: Newbie
timestamp: October 01, 2019, 09:51:49 pm
content: Here is my scenario and problem with the normal multimeters.I have a board that sleeps for 55 minutes, wakes up and lights up LED, and then sleeps for 5 minutes. I want to measure sleep current which is in uA and normal current which is around 300-400mA.I would keep the selector switch in uA range to measure the sleep current consumption. But when it wakes up the current consumption is around 300mA, due to which the multimeter disconnects supply to board as it above the max range in uA selection unless I manually switch the selector switch in between. Hence I need something without me requiring to manually switch to measure from uA to mA.

author: rodpp
author_rank: Frequent Contributor
timestamp: October 01, 2019, 09:59:51 pm
content: Use 2 multimeters in series...

author: Gyro
author_rank: Super Contributor
timestamp: October 01, 2019, 10:03:35 pm
content: Quote from: rodpp on October 01, 2019, 09:59:51 pmWon't work - the series resistance of the uA meter shunt will drop the supply voltage too much when the board is running at full current.The simplest normal way is to use a low value external shunt resistor (with tolerable voltage drop at 'running' current) and measure the voltage drop across it. Unfortunately, in the OP's case, he is looking for a ~300000:1 ratio, which is going to require very high meter sensitivity.The other way is to put a short across the multimeter while it is in either mode and then measure them separately on the appropriate current range with the short removed [EDIT: even if means modifying to code to keep in active state for long enough to measure the current].

author: snapster
author_rank: Newbie
timestamp: October 01, 2019, 10:20:16 pm
content: Thanks for the reply Gyro, Yes I tested out by putting a short across multimeter selecting the appropriate scale on multimeter and then removing the short to measure current. This way I was able to measure sleep current as 20uA and normal current as 300mA, however I want to do this testing for long range of time like few days and log the measurements, so I cannot manually remove the short.Do you think some kind of DMM like this would workhttps://www.mouser.com/datasheet/2/403/tektronix_04232018_1KW-61315-0_DMM6500_Datasheet_0-1324940.pdfThanks

author: Gyro
author_rank: Super Contributor
timestamp: October 01, 2019, 10:30:36 pm
content: Ah, you have some budget available then!It depends whether your circuit can stand the burden voltages of the current shunts (Page 10), how fast the auto-ranging is and exactly how it operates. A reservoir capacitor, downstream of the meter may buffer your circuit from supply voltage changes during range switching but it will 'blur' your results too. You would need to measure its leakage resistance before and after the run to try to estimate its effect on the uA reading.P.S. It might be a question for Tek technical support before you spend the money.

author: 0culus
author_rank: Super Contributor
timestamp: October 01, 2019, 10:41:31 pm
content: If you have budget for big purchases, check out the Keysight N6705C. It's a plugin based mainframe instrument so you can get plugins that match up with your needs.

author: GerryR
author_rank: Frequent Contributor
timestamp: October 01, 2019, 10:44:51 pm
content: You can probably pick up a used HP 34401 multimeter for short money on ebay that will do that for you. (Don't let me discourage you though, from picking up a new Tek meter!)

author: snapster
author_rank: Newbie
timestamp: October 01, 2019, 10:46:33 pm
content: Yeah I can spend up to $1000 for an equipment, but just wanted to make sure I am going in right direction as far as selecting the equipment goes. I will check with TeK technical support, thanks for your insights.

author: MosherIV
author_rank: Super Contributor
timestamp: October 01, 2019, 10:49:55 pm
content: How about using a 6 or 7 digit multimeter in voltage mode to measure the voltage on a shunt.In 1V range, it should be able to measure uV.

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: October 01, 2019, 10:56:24 pm
content: The Gossen Metrawatt "Metrahit" meters have just one socket for current and the selector only switches between AC and DC.E.g. the X-TRA switches automatically through all ranges from the 100µA range to the 10A range.

author: 2N3055
author_rank: Super Contributor
timestamp: October 01, 2019, 11:00:33 pm
content: Metrix MTX3293 also have single current socket and autoranges from uA to 20A without user intervention

author: alm
author_rank: Super Contributor
timestamp: October 01, 2019, 11:08:55 pm
content: Depending on how much voltage drop you can tolerate, a (Schottky) diode across the DMM set to uA range might work to clamp to voltage when it wakes up. Could put a second DMM in series set to a higher current to measure the wake current, if you care. You could get fancy and use remote sensing on your power supply to compensate for the diode drop, but current through the sense leads will likely affect your measurement.The Keithley and HP/Agilent mobile communication supplies (Keithley 2302/2303/2304A/2306andHP/Agilent 66309/66311/66319/66321) are made for this exact measurement and use a similar mechanism with multiple shunts in series and a clamp across them. Can sometimes be found for ~$100 on eBay. Computer inferfacing is GPIB only.You could try a DMM that can autorange without changing inputs, but I doubt that if the device wakes up, the DMM will range up in time before the device under test's brownout detection triggers.Last option that comes to my mind is indeed a DMM with enough resolution to measure uA on its A range. But make sure to look at the accuracy and resolution figures. The DMM might have to be on its 2A range, in which case for a 6.5 digit DMM a single-digit uA current is down in the last two digits, which may be down in the noise.

author: threephase
author_rank: Regular Contributor
timestamp: October 02, 2019, 04:33:11 pm
content: Have you looked at either the Mooshimeter or the Joulescope? Would either of them meet your requirements?https://moosh.im/mooshimeter/https://www.joulescope.comI do have a MetraHit coil meter and whilst the current is on one common input, it is extremely slow to settle and by the time you have forked out for the software and IR connector, you are probably getting close to the price of a DMM6500 anyway.Kind regards.

author: mjkuwp
author_rank: Unknown
timestamp: October 02, 2019, 05:41:35 pm
content: You could put two DMM in series and banana-jack jumpers to wire them up. You can use a jumper to bypass the uA meter after you are done measuring that portion of it. This will leave the jumper and meter in parallel but of course the current will mostly flow through your jumper and on through the 2nd meter.Also have you looked at this?https://www.eevblog.com/projects/ucurrent/

author: snapster
author_rank: Newbie
timestamp: October 02, 2019, 06:13:37 pm
content: Quote from: 0xdeadbeef on October 01, 2019, 10:56:24 pmThanks, but my sleep power consumption is less than 100uA around 20uA.

author: radiolistener
author_rank: Super Contributor
timestamp: October 02, 2019, 06:30:20 pm
content: just buy Brymen BM867/869 it allows to measure up to 500 mA with 1 uA resolution with no need for manual switch.

author: cgroen
author_rank: Unknown
timestamp: October 02, 2019, 06:55:25 pm
content: Get a Joulescope! I have not regretted buying this piece of equipment! As much as my Salaea logic analyzer is running pretty much all the time here, my Joulescope does the same!

author: colorado.rob
author_rank: Frequent Contributor
timestamp: October 02, 2019, 07:18:21 pm
content: I use a Siglent SDM-3065X for this. It's a 6.5 digit DMM and in the 200mA range I get 100nA resolution. My latest project draws 18mA peak. In the 20mA range I get 10nA resolution, which is needed to accurately measure sleep mode current. And at that level, the battery self-discharge rate overwhelms the current consumption.

author: Fungus
author_rank: Super Contributor
timestamp: October 03, 2019, 05:25:36 am
content: Quote from: radiolistener on October 02, 2019, 06:30:20 pmWas going to say the same thing.I've used my BM857 for this exact measurement. It isn't 100% accurate down to the last uA when you're on the mA range but it will be within 2 or 3 digits. You'll definitely know if it's asleep or not.Quote from: radiolistener on October 02, 2019, 06:30:20 pmNot quite. They're only 50,000 counts in mA mode so you get 10uA resolution on the 500mA range.

author: Gandalf_Sr
author_rank: Super Contributor
timestamp: October 03, 2019, 09:05:00 am
content: If you've got $1,000 to spend, consider the Keysight 34461A; and use a power supply with remote voltage sensing with the meter burden voltage eliminated by the remote sensing. You won't have buyers remorse if you invest in the 34461A.Full disclosure:1. I own a 34461A2. I have zero financial gain relationships with any test equipment OEMs or distributors.

author: nctnico
author_rank: Super Contributor
timestamp: October 03, 2019, 09:28:56 am
content: Another option is to get a PSU which has uA current measurement resolution. Like the Keysight 66311B or the much newer E36310 series. The latter has build in logging onto a USB stick. The upside of using a PSU is that it doesn't have any extra burden resistance in series with the power input of the circuit. If you need fast sampling to catch small peaks then the Joulescope mentioned above or the ​ZS-2102-A IOT Power Profiler (https://www.anglercircuits.com/) seem like useful products to me.

author: radiolistener
author_rank: Super Contributor
timestamp: October 03, 2019, 09:38:43 pm
content: Quote from: Fungus on October 03, 2019, 05:25:36 amyes, in 500 mA range it shows current with 10 uA resolution. But when current go below 50 mA it automatically switch to 50 mA range and shows current exactly with 1 uA resolution.There is no need for manual switch, it selects 500 mA / 50 mA range automatically. I don't think you will need reading of 500 mA current with 1 uA precisionFor currents below 50 mA it allows to measure with 1 uA resolutionAlso it has separate 500 uA mode with 10 nA resolution, but it needs manual switch and it cannot handle more than 500 uA in 500 uA mode.

author: mliberty
author_rank: Contributor
timestamp: October 14, 2019, 12:19:38 pm
content: Disclosure: I am the creator of JoulescopeI designed Joulescope exactly for this high-dynamic current range measurement application. Joulescope maintains a burden voltage of 20 mV or less across the shunt resistor and can auto-range in 0.8 µs (typical) to keep your device under test working well. Most other auto-ranging test equipment takes much longer which can cause your device under test to brown out.I have attached an image showing Joulescope's static current specifications. You can download theJoulescope User's Guidewith the full specifications.If you do go with a bench multimeter, very carefully read the specifications. With high dynamic current range measurements the key concerns are burden voltage, resolution, accuracy and autoranging time.Thanks @cgroen for the kind words, and great to hear that you are enjoying your Joulescope!

author: rsjsouza
author_rank: Super Contributor
timestamp: October 14, 2019, 02:18:39 pm
content: As others have suggested, in my experience I have used the following:- Keysight E36300 series, which has excellent resolution across a very wide dynamic range.- Keysight 34460/34461 6-1/2 digit multimeters.- If you don't mind going the used route, a HP 3478A has 300000 counts and operates at 300mV/mA scale with 1µV/µA of resolution.- The combination of tools from TI:XDS110+XDS110 HDR. Although the XDS110 is a JTAG pod, it can be used as a standalone power supply (1.8 to 3.3V) and measures with a dynamic range of 100s of nA to 800mA.

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: December 21, 2019, 12:51:05 pm
content: Disclaimer : I am the creator of ZS-2102-A.The IOT Power Profiler tool (ZS-2102-A) is built exactly for this purpose. It can measure from few uA to 1A with 1Msps rate. It can actually give you the record for the whole 55 minutes as it uses advanced data compression. The whole data would not be more than 1GB. The series element is a 0.1Ohm resistor across the whole range, so it would not trip the load.

author: David Hess
author_rank: Super Contributor
timestamp: December 21, 2019, 11:38:50 pm
content: I use a different method to make this sort of high dynamic range measurement. I put the current shunt resistor on the other side of the power pass element in the regulator. This may include building a test regulator into the circuit. Now the voltage drop across the current shunt resistor can rise up to the point where the minimum voltage drop across the pass element is reached without affecting the output voltage.

author: exe
author_rank: Unknown
timestamp: December 22, 2019, 08:57:57 pm
content: Quote from: alm on October 01, 2019, 11:08:55 pmI wonder if sensing current is constant (because output voltage is approximately constant). If so, it's just an offset error that "can be compensated in software".

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: December 23, 2019, 03:20:24 am
content: The voltage drop cannot be compensated in software. What if the drop is too high and causes the load circuits to trip.

author: exe
author_rank: Unknown
timestamp: December 23, 2019, 02:26:11 pm
content: Quote from: EEEnthusiast on December 23, 2019, 03:20:24 amI meant consumption current to be compensated in software, not voltage. Voltage supposed to be constant because of remote sensing.

author: Vovk_Z
author_rank: Super Contributor
timestamp: December 28, 2019, 05:12:11 am
content: I don't know how Joulescope works, but I want to add one more way to do: there is some kind of circuits, with name something like "impedance converter" - they have low input impedance, but you can connect to it's output high impedance load (high-Z ammeter). What we need for that task is precision powerful ( or boosted >300 mA) operation amplifier.I can't find exact schematics right now. I have used it with 1500 Ohm milliammeter, and lowered it's impedance to 10 or 100 Ohms - shunt value (I used Lm258, but with better opamp shunt will be much less value).

author: exe
author_rank: Unknown
timestamp: December 28, 2019, 10:03:44 am
content: Quote from: Vovk_Z on December 28, 2019, 05:12:11 amIs it about using a transimpedance amplifier (TIA) as a virtual ground (current sink)? The output voltage would be proportional to current, the gain is set by a feedback resistor.Something like first circuit in this article:https://www.analogictips.com/faq-converting-current-to-voltage-the-role-of-transimpedance-amplifiers/. I think the current source doesn't need to be grounded.

author: sailah
author_rank: Contributor
timestamp: December 28, 2019, 12:09:43 pm
content: I have a Keysight N6705C with N6781A SMU designed for exactly this type of work. Was around $10k. The N6781A is the really sensitive module for battery drain analysis up to 3A.https://www.keysight.com/en/pd-2747858-pn-N6705C/dc-power-analyzer-modular-600-w-4-slots?nid=-35714.1195294.00&cc=US&lc=enghttps://www.keysight.com/en/pd-1842316-pn-N6781A/2-quadrant-source-measure-unit-for-battery-drain-analysis-20-v-1-a-or-6-v-3-a-20-w?nid=-35714.937223.00&cc=US&lc=engMy device has sleep current around 10 uA, every hour wakes up to do 1 min of sampling (ultrasonic). Every 8 hours it has a packet send of ~135mA for 500ms.I needed a very accurate way to power profile this for lots of reasons but mainly to determine battery life for this IoT device. I had borrowed this exact setup from Keysight as they are a sponsor at our lab. After using it, I realized I needed it so I bought one myself. It has already paid for itself.The interface is pretty nice, you can download the software for free (14585A Control & Analysis SW) to view datalogs. My engineer is in Europe, my Keysight is in Boston. I can run the device and send him the datalog in Google Drive so he can dig in and look at the activity in detail.It has very good dynamic range (10,000,000:1) to be able to autorange from nA to mA.If you want to send me your device I'll run a datalog and send it back to you along with the file in Drive.Otherwise I'd buy a Joulescope

author: Vovk_Z
author_rank: Super Contributor
timestamp: December 28, 2019, 10:59:40 pm
content: Quote from: exe on December 28, 2019, 10:03:44 am- yes, I think it is exact what I meant (and what I've used).

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: January 02, 2020, 06:15:01 am
content: For the N6781A, the measurement accuracy is limited.For the 100mA range, where most of the low power wireless devices operate, the measurement has a 10uA error above the %error. This would imply that a 15uA current drain could be read as 5uA or 25uA.Measurement Accuracy (at 23°C ± 5°C):Voltage, 20 V range: 0.025% + 1.2 mV*Current, 3 A range: 0.03% + 250 µA*Current, 100 mA range: 0.025% + 10 µA*Current, 10 µA range: 0.025% + 8 nA

author: mliberty
author_rank: Contributor
timestamp: February 03, 2020, 10:16:45 pm
content: Quote from: Vovk_Z on December 28, 2019, 05:12:11 amDisclaimer: I am the creator of JoulescopeJoulescope is a shunt ammeter, which is a fancy way of saying that it measures the voltage over a known resistance to compute current with Ohms law. What makes Joulescope unique is super fast autoranging (switching resistors) and very low voltage drop (< 20 mV). Joulescope also has 250 kHz measurement bandwidth. You can find more details in the "Theory of Operation" section of theJoulescope User's Guide.All About Circuits has a goodintro articlefor transimpedance amplifiers. If you need higher currents and capacitive loads, transimpedance amplifier stability is a concern. Shunt ammeters have no stability issues.