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

[Bob Paddock]

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?


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