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

[Robert Atkinson]

If you are not interested in the operating (TX) current, use your 10K or 100K series resistor with a forward biased diode in parallel. At low currents there will not be enough  voltage across the resistor to forward bias the diode. On transmit the maximum loss will be the forward drop of the diode. This will be 600-700mV for a standard silicon (1N4001) or 2-300mV for a schottky. You can use a cheap DMM to measure the voltage across the resistor. This trick can also be used with an analog meter to protect it from current peaks.

Robert G8RPI.

--- On Sat, 26/9/09, Bob Paddock <bob.paddock at gmail.com> wrote:

> From: Bob Paddock <bob.paddock at gmail.com>
> Subject: [volt-nuts] How to measure micro-amp currents and have low impedance?
> To: volt-nuts at febo.com
> Date: Saturday, 26 September, 2009, 11:36 PM
> 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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