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

[Bruce Griffiths]

Use the Patterson transdiode connection with npn and pnp transistors as
feedback elements.
It works much better/more predictably at low currents than when signal
diodes are used.

Bruce

J. Forster wrote:
> 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
> meter.
>
> FWIW.,
> -John
>
> ============
>
>
>   
>> 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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>>     
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