new uCurrent - Page 1
Forum section: electronics / general

author: sairfan1
author_rank: Frequent Contributor
timestamp: January 13, 2021, 02:23:20 am
content: I want to know when new version of uCurrent is expected?are you considering real time logging auto range change like from nA up to mAoptions that can help evaluating battery life etc.

author: alex-sh
author_rank: Regular Contributor
timestamp: January 24, 2021, 12:50:26 pm
content: Check out this one:https://lowpowerlab.com/guide/currentranger/

author: charlesrg
author_rank: Contributor
timestamp: February 16, 2021, 04:21:14 pm
content: I'm also interested in a new version, auto ranging and display would be great.

author: danielbriggs
author_rank: Regular Contributor
timestamp: February 16, 2021, 08:52:38 pm
content: I've bought 3x of these for use around on projects; they're brilliant.(I have no other connection to n-fuse other that buying their products).https://www.n-fuse.co/devices/tinyCurrent-precision-low-Current-Measurement-Shunt-and-Amplifier-Device.html

author: SMB784
author_rank: Frequent Contributor
timestamp: February 17, 2021, 09:10:01 pm
content: Thanks for the links to alternatives guys.One thing I find a tad bit frustrating is the quality of the datasheets on these alternatives.For example, they quote 1pA resolution on the datasheet with a 5.5 digit multimeter, but they dont give any useful units for either the RMS noise or the accuracy.They claim the accuracy is 0.05%, but out of what? I assume they mean 0.05% of the full dynamic range at the specified current sense setting, but they dont specify this anywhere. At 0.05% accuracy on the full dynamic range, that means you are only accurate to 1.65 nA for the CurrentRanger, or 625 pA on the tinyCurrent, which means that its hard to gain any useful knowledge with the 1pA resolution.The noise figure is even worse. The CurrentRanger doesn't appear to even list it (maybe someone can direct me to a better datasheet for that product), and the tinyCurrent lists it in nebulous units of dBV. Is that dBV on the full dynamic range like the accuracy specification? How much averaging was performed to get that noise figure? Listing it in dBV/√Hz would be much more useful, or at the very least provide a plot of noise vs frequency like Dave did in his original design. I will assume that since they list it as dBV, the noise figure is quoted at a 1 Hz bandwidth over the full dynamic range. That would mean for -90dBV the noise would be something like 40 pA on the nA setting, which makes their resolution figure of 1pA even more dubious. Not only do you not have the accuracy to make use of the 1pA "resolution", the noise is too high for it to be useful too.Anyway, the gist of this is that due to a poorly specified data sheet, its impossible to do a proper comparison of these units, much less verify their rather outrageous claim of 1pA current resolution.

author: vanthome
author_rank: Newbie
timestamp: February 18, 2021, 09:48:28 am
content: Thanks for mentioning our Product, the tinyCurrent, @danielbriggs.Let me chime in to answer some questions:QuoteWe provide the figures for noise and THD on the website:https://www.n-fuse.co/devices/tinyCurrent-precision-low-Current-Measurement-Shunt-and-Amplifier-Device.htmlQuoteThe accuracy tells you, how far off the output voltage (and this the current) can be from what you actually should see in the respective range. The major factor here is the tolerance of the shunt resistor.QuoteI agree that we should create a proper frequency-response characterization and I will do this if I find the time. Regarding your concerns about the precision. I can just tell you that I have done several real-life measurements in the nA range with a high class multi meter or a scope that could average the signal and although there is noise, the obtained figure was pretty much matching the calculated power consumption of the devices I tested.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 18, 2021, 10:08:18 am
content: The concept for the µCurrent is good for relatively high currents, like more than 100 µA, but not the best choice for small currents. For those small currents, there is the option to use a trans-impedance amplifier to get better performance (even lower burden and less noise).With only 1:1000 steps for the shunts it is natural that it does not work at ends of the ranges. There is a reason that better DMMs have the shunts in 1:10 steps. With a shunt resistor the dynamic range is limited and with a worst case factor 1000 down before the next small range can be used, there is not that much resolution left, especially if a small burden is priority (e.g. because one wants to save on a high power shunt).A 10 K resistor used in the original µCurrent has a natural thermal noise of som 0.77 pA/sqrt(Hz). So one can not really expect pA resolution unless with lot's of patience. So it needs a different concept, not just higher grade parts.

author: SMB784
author_rank: Frequent Contributor
timestamp: February 19, 2021, 05:54:02 pm
content: Quote from: vanthome on February 18, 2021, 09:48:28 amThank you for such a prompt reply, I really appreciate it and it speaks well of your commitment to your product.QuoteThank you for directing me to the datasheet for your product, that will be very helpful for others.QuoteI understand, so this accuracy is with respect to the reading, not the full range. So the minimum accuracy on a 1 nA measurement is +/- 0.5 pA, is that correct? That seems pretty excellent, perhaps too good.QuoteThank you for addressing this, but I am still left wondering how you got this -90dBV noise figure. Is it just the average in dBV of the noise floor over the entire measured bandwidth produced by an oscilloscope's fourier transform of the signal? If so, how large in amplitude was the input signal you used to measure the noise spectrum? Or is it the RMS noise over a bandwidth extending into the white-noise region of the noise spectrum? I appreciate your assurances, but I need some data. Please update us if you have time to take the measurement.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 19, 2021, 06:29:06 pm
content: The quoted accurcy look like the accuracy of the scale factor / shunt. There may be even additional error from the gain - so not sure the number is actually correct. From the schematics there are resistors with some 0.05% resistors for the shunt and the gain stages - so more like 4-5 resistors contributing to a little more than 0.1% for the better ranges.The mA range may show first signs of self heating and thus reduced accuracy at the upper end (e.g. more than 200 mA).The accuracy, especially for te smaller ranges should have both a part proportional to the reading and one fixed one. I would expect some 1-5 µV additional fixed error for the input and for the nA range maybe another 10 pA of possible OP bias / leakage. With a 4.5 digit meter with 200 mV range and thus 0.01 mV resoltion 5 µV would be hardly visible, more like 1/2 LSB. So one may have to add the meter noise resolution limit too, at least for lower resolution meters.The higher burnden should apply to the mA range, not the nA range.