How to measure micro-amp currents and have low impedance?
Bob Paddock wrote:
I'm not familiar with Patterson's work, and Google was not much help.
Will you point me to a specific paper or schematic please?
Its not particularly new, you have probably seen the circuit many times
(the name has been around for at least 50 years or so) see:
You're right, I've seen lots of those circuits, just never recall the
name Patterson
being mentioned related to them.
Thank you for the link.
Probably because it is sometimes misspelled as Paterson its mentioned in:
US Patent 7395308,
http://www.ee.bgu.ac.il/~pel/pdf-files/jour37.pdf
http://www.ee.bgu.ac.il/%7Epel/pdf-files/jour37.pdf
http://www.biochemj.org/bj/154/0277/1540277.pdf
I first came across it in a Phibrick application note.
Patterson's original paper appears in:
Review of scientific Instruments (1963) 34, 1311-1316.
Bruce
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@gmail.com wrote:
From: Bob Paddock bob.paddock@gmail.com
Subject: [volt-nuts] How to measure micro-amp currents and have low impedance?
To: volt-nuts@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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