Precision Current Measurement in Multiple Ranges - Page 1
Forum section: electronics / projects

author: cyounkins
author_rank: Newbie
timestamp: April 01, 2015, 04:59:12 pm
content: Hi there,I love my uCurrent, but for most applications I want higher dynamic range than what is available with the uCurrent gold. My system has sleep current consumption of ~1 uA and peak current consumption of 25 mA. If I use the mA setting on the uCurrent, I can't accurately measure the sleep current as it is too small relative to noise. If I use the uA range, the peak current consumption exceeds the maximum of the range.* How can I damage the uCurrent with excess current on the uA or nA settings? Is it just power through the resistors? (I don't fully understand op amps)* Could I safely use the uA range on the uCurrent if there are peaks of 25 mA for ~100 us in there? (at 3.6V, so 0.09W). The 10R 0.1% resistor that is used in the uA range is a 0.1W.I'm interested in average current consumption in different states of my system, so I was going to record the output of the uCurrent with a USB scope and do an integral in a scripting language on my laptop. What I really want is two signals - one for each range of current consumption, and for the lower one to max out ('clip') without damaging it. Then I can switch between them in my post-processing.Unfortunately I don't think I can put two uCurrents in series with two analog inputs on my scope because their grounds will not be the same. I ran into this issue previously when I connected a mains-grounded scope to a uCurrent on the high side of a mains-grounded circuit (just like Dave says not to in one of his videos!).* Is is possible to modify the uCurrent to output two signals, one for each range, with a common ground?Thanks for any support!Craig Younkins

author: Marco
author_rank: Super Contributor
timestamp: April 01, 2015, 05:56:59 pm
content: You can put two uCurrents in series, just use a single shared ground connection to the scope for both probes (I'd connect the current inputs black to black, then pick one of the blacks on the outputs for the scope ground connections and probe the reds). The output from the second would simply be inverted.AFAICS the only problem would be that it would take around 3 msec for the opamp of the uA range uCurrent to recover from being overdriven. Also burden voltage would be 0.25V at 25mA, and it would be too much for the nA range.

author: babysitter
author_rank: Frequent Contributor
timestamp: April 02, 2015, 05:56:00 am
content: Does your system have a free GPIO where it could announce its awakeness? some switching magic with FETs and you might e.g. short/disable/bridge the input of the low-range µCurrent between wakeup and sleep.

author: cyounkins
author_rank: Newbie
timestamp: April 02, 2015, 03:16:17 pm
content: Quote from: babysitter on April 02, 2015, 05:56:00 amUnfortunately that is not possible in my situation - the system has an MCU and RF module, and the RF module has a separate MCU with no sleep/wake status pin available. It is difficult to disentangle the current consumption waveforms of the two parts.Craig Younkins

author: MLXXXp
author_rank: Frequent Contributor
timestamp: April 02, 2015, 05:11:31 pm
content: By my calculations, which to my knowledge no one has disputed, the µCurrent Gold should be able to safely handle your peak 25mA on the µA range (but not on the nA range). Here's my reply to a similar discussion, with links to my µCurrent safe input calculations.https://www.eevblog.com/forum/projects/ucurrent-micro-amp-measurements/msg599366/#msg599366

author: Marco
author_rank: Super Contributor
timestamp: April 02, 2015, 05:16:21 pm
content: Quote from: MLXXXp on April 02, 2015, 05:11:31 pmAnd as I said, you can just connect the uCurrents back to back and use the "center" connection as your ground for probing (of course the DUT has to be ungrounded in this case, but it should be any way to avoid a ground loop).

author: MLXXXp
author_rank: Frequent Contributor
timestamp: April 02, 2015, 05:43:56 pm
content: Quote from: Marco on April 02, 2015, 05:16:21 pmBut as you've pointed out, the problem with putting two µCurrents in series is that your high current reading has to pass through the 10R resistor of the µCurrent on the µA range. This creates a fairly high burden voltage, which is one of the things the µCurrent is designed to minimise.At 25mA, there would be a 0.25V drop across a µCurrent on the µA range. Since supply voltage requirements haven't been specified, I don't know if this would be a problem in this particular case, though.As long as the system has a consistent and repeatable current draw pattern over time, I'd just take readings with a single µCurrent on different ranges and analyse the readings off line, as the O.P. has suggested.

author: MLXXXp
author_rank: Frequent Contributor
timestamp: April 02, 2015, 07:05:42 pm
content: If you wanted to use the µCurrent to measure on the nA range, while safely handling high out-of-range currents, you could possibly put a standard diode, forward biased, across the inputs. This diode wouldn't conduct significantly at the low, maximum 12mV full scale, voltage but would clip at below 1V, thus protecting the µCurrent. Note that the capacitance of the diode could become an issue with high frequency current fluctuations.If you have both positive and negative high current swings, you could use two diodes, back to back.This is assuming that you only have high current flowing while the µCurrent is powered on. If you want the µCurrent to be safer when powered off, you could consider adding Schottky diodes to it and/or the "off short" modification, as I have, described here:https://www.eevblog.com/forum/crowd-funded-projects/current-gold-on-kickstarter/msg448208/#msg448208You can also just short the inputs externally before powering off the µCurrent.

author: Neganur
author_rank: Unknown
timestamp: April 02, 2015, 10:54:37 pm
content: I think the 10R current sense resistor will only tolerate 12.5 mA due to its 0.125 W power rating (0805).

author: MLXXXp
author_rank: Frequent Contributor
timestamp: April 02, 2015, 11:31:25 pm
content: Neganur,I don't know how you made your calculations. Power P = I^2 * R12.5mA through 10R is 0.0125^2 * 10 = 0.0015625 WHere's how I calculated it:R9 is 10R and is rated 0.1W, but to be safe I'd derate this to at most 75% (0.075W). At 85mA (0.085A) R9 would draw about 0.072W and the voltage would be 0.85V, which is safe for the input, when powered up.0.085A^2 * 10R = 0.07225WSo the maximum safe current on the uA range would be 85mA.

author: Neganur
author_rank: Unknown
timestamp: April 02, 2015, 11:34:17 pm
content: forgot the square. Details!

author: Neganur
author_rank: Unknown
timestamp: April 02, 2015, 11:41:59 pm
content: on a side note, now that I have my maths straight, I was looking at a replacement amp with similar offset, drift, input bias and BW but greater rail capability.Maybe theMAX44241could be used to increase the dynamic range without looking at log amps. 36V and r2r.Could also use the dual amp version of it and tap in between the two amps to get both the 10x and 100x range at the same time. (for example on a scope)

author: kjs
author_rank: Regular Contributor
timestamp: April 03, 2015, 09:03:49 am
content: you could also use the LTC2050HV and replace the GP OpAmp with one which can handle 12V supply. Then operate it with 10V and you got your larger range too. Just make sure that you don't exceed the max power ratings of the sense resistors.The LTC has the same spec as the MAX but is actually measured and guaranteed and not just a vague spec derived from some Cadence simulations (we know how accurate that can be). Also available as a dual: LTC2051HV.