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| Measuring small current, complicated |
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| Gavin Melville:
Thanks for all the input. Some thoughts. What I'm trying to do is work on the idle current first, so getting the 300nA down. I have a 34465A meter (but not a 34470), as pointed out the dynamic range isn't enough. Have looked at the Keysight Battery Analyser. Wish it was there when I got the SMU, but even by SMU standards it's expensive - from memory around $14k US, with the mainframe and just one low end SMU module. I'd like to short the shunt, but trying to that, electromechanically or active device is going to be hard. When the CPU wakes up I don't have enough warning without _really_ disturbing the idle current. Even trying to get a few 10's of msec will cause more problems than it solves. I think my best path is to break up the currents, and us a SMU to supply the idle current areas one at a time. The SMU is quite comfortable with the small currents. Thanks, Gavin. |
| SiliconWizard:
--- Quote from: Gavin Melville on November 08, 2018, 05:11:47 pm ---I think my best path is to break up the currents, and us a SMU to supply the idle current areas one at a time. The SMU is quite comfortable with the small currents. --- End quote --- This is obviously your best bet. It even makes sense if you're specifically tracking down the idle current, as it will be just one measurement you'll have to deal with, and you can use "conventional" equipment to measure it. You could probably even use a µCurrent for that. Your overall problem is a very common one that has been discussed quite a few times here. A few threads (there are probably many others): https://www.eevblog.com/forum/testgear/battery-energy-consumption-how-to-measure-it/ https://www.eevblog.com/forum/testgear/high-dynamic-range-current-vs-time-measurement-at-low-cost/ https://www.eevblog.com/forum/oshw/metering-both-sleep-and-active-current/ Doing that kind of measurement dynamically with such a wide range is a very challenging endeavor. The most usual approach is taking it sideways: measuring the "idle" current separately, and/or measuring the average current during a typical scenario (which would include both idle and run periods). |
| duak:
I'd try the the two power supply approach. The SMU provides the idle current at the nominal voltage and the other provides the high current at a few millivolts below it. If the SMU doesn't cut it, the microamp supply could be an opamp transconductance amplifier that would also give the idle current. The high current supply would need a low leakage precision diode or clamp that could switch fairly quickly to keep the output voltage up. This could be realized with an opamp, buffer and a few diodes artfully arranged. If this isn't clear, I can try to cobble up a sketch. Cheers, |
| spec:
+ Gavin Melville Attached is another offering, this one straight from the drawing board. It has not been optimized, prototyped or developed. The circuit uses a conventional voltage regulator circuit (N1), and a current to voltage converter based on a virtual earth shunt feedback amplifier (N2). The circuit eliminates the requirement for a nano amp ammeter by generating a voltage proportional to IS, the unit supply current. As the voltage is at a zero Ohm impedance, neither does the voltmeter need to have a particularly high input impedance: 10k Ohms upwards will be suitable. There is little current drain from the 5V reference line. Under low IS conditions the voltage regulator output is low, so the low-leakage diode D1 is slightly reverse biased. This means that the voltage regulator has no effect. Under these conditions IS is sourced from the summing node at N2's inverting input, which is supplied with current by the feedback resistor R3. Thus the summing node acts as a 5V regulator, maintaining a 5V supply for the unit. At an IS of 7uA the summing amplifier out put will be up against the 12V supply line (saturated) and will be unable to supply any more IS. At this point the summing node voltage will drop. When it drops by a milivolt (V DELTA) the voltage regulator, N2, takes over and maintains the voltage across the unit at 5V- 1mV (a VDELTA of 1mV might be a bit tight: 20mV is probably more realistic). The voltage regulator has the potential of supplying in excess of 200ma, except that a BC337 is limited to around 143 mA, due to heat dissipation, so this needs to be sorted but it is not a major issue. And that is how it works- I hope :) When I started the design you only had a couple of replies, but by the time this design was ready for posting I was surprised to see so many replies: should make some interesting reading. :) PS: the OPA192 is pretty good, but there are possibly better opamps for this application. I have initially kept clear of switching opamps on the grounds of low noise, but they may offer some advantages. The non electrolytic capacitors are polypropylene dielectric. All capacitors are for decoupling. |
| bson:
The transients constitute over 99.5% of the energy, so I'd ignore the idle current entirely if it were me. But the measurement problem is interesting and I run into it all the time. |
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