[volt-nuts] How to measure micro-amp currents and have low impedance?

J. Forster jfor at quik.com
Sat Sep 26 22:47:00 UTC 2009

I'd look at a low offest and bias current opamp with diode FB to make a
current input log current meter. The input is a summing junction, so is
very low impedance. A pair of good back-to-back parallel diodes would
protect the inputs. You might think about a bar graph display instead of a



> I hope a current (I) question isn't out of place on the Volt-Nut list. :-)
> The fundamental question I have, is how can I measure current in the
> low micro-amps, 1uA or lower, range and have a low impedance, of say
> less than 10 Ohms?
> I'm looking to build up a few test fixtures to profile the current (I) of
> Boards Under Test, on a production line.  I'm looking for problems
> like flux trapped under a part, and baseline Iq testing.  I plan on
> profiling known good boards, to compare against the new board builds.
> We are talking about quantities of boards of 50,000 to 100,000 per
> year, so the test has to be easy and quick, that can be done by
> minimally trained monkeys (not my choice, I have to work with what I'm
> given).
> The products are all battery based, so keeping the current consumption
> down, especially in Sleep mode, is important. We've found over the
> years that measuring the system current is a good way to find
> production problems, such as flux trapped under a IC etc.
> In an idea world I want to have a current (I) data acquisition system
> with the following specifications:
> * Current is measured on the high side.
> * Works with source voltages as high as 32 volts.
> * Has continuous current scale of 1nA to 1A, with a low source
> impedance (less than 10 Ohms, maybe 100 Ohms).
> * Cost less than $50 per unit to build.
>  (Please don't recommend high end meters the boss will never spend
> money on, not even from EBay.  We need five to ten of these units).
> * The product under test can not be modified.  Some of them were
> design ten years ago, and aren't going to change.
> I'll settle for 1uA to 500 mA, in multiple scales, as long as the
> scale switching is automatic, at 12V.
> The reason I need a low impedance is that my products are part of a
> sensor network, which transmits data using RF. The RF section wakes up
> at random intervals around thirty seconds or so. The time is
> deliberately random to avoid RF packet collisions. You never know when
> the current meter is about to have its needle wrapped around the end
> stop with a nice satisfying "Thunk".
> If I try to measure low currents with say a 10k or 100k resistor I get
> my current measurement, but when the transmit comes on the system
> crashes because it does not have enough current to sustain it. I want
> to be able to run through a full sleep->transmit->sleep cycle without
> crashing due to current starvation, and track the current throughout
> the full cycle.
> I've found various ideas on Internet such as:
> Measuring nanoamperes; Measuring low currents can be tricky. Clever
> analog-design techniques and the right parts and equipment can help.
> By Paul Rako, Technical Editor -- EDN, 4/26/2007
> http://www.edn.com/article/CA6434367.html
> 10nA to 10mA using a LogAmp:
> http://circuits.linear.com/Precision/.../Eight_Decade_Current_Sensing_Log_Amplifier
> TI has their LOG10x series of LogAmps as well.
> Once we get above 10mA things are fairly simple, lots of ways to do
> that. The fundamental problem is the micro-amp measurements, while
> maintaining a low impedance.  Having two different parallel measurement
> systems would be fine, on of mA and one of uA.
> What I want to know is that any of you have been down this road
> before, and what suggestions you might have?
> --
> http://www.wearablesmartsensors.com/
> http://www.softwaresafety.net/
> http://www.designer-iii.com/
> http://www.unusualresearch.com/
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