[volt-nuts] Cheap Oscilloscope measurement of small current?

[Oz-in-DFW]

I'm doing quite a bit of this and like most things, the answer is "it
depends. " 

What I'm doing right now is using a resistor in series with the _input_
of a low leakage micropower Low DropOut (LDO) regulator because this (15
uA) circuit will run off of a regulated supply.  The LDO has a steady
state consumption of a few microamps and that is largely a function of
input voltage and operating temperature at the low operating currents
I'm dealing with.  My workshop temperature is constant enough.  This
works well below a uA.  I just use my plain old Tek scope with two
channels in differential mode.  If I neede teh extra channel I'd brew a
diff amp or use something like this:

http://datasheets.maxim-ic.com/en/ds/MAX9938.pdf

I believe you have a much more complex problem.  You will be operating
from battery or solar supplies that likely have a defined voltage and
internal resistance over time.  This can have a profound impact on
operating time, particularly if subtle considerations are as significant
as you imply they are.  You'll either need to take a lot of measurements
and model the supply carefully, or you'll need a good simulator of the
supply. 

On 11/4/2010 6:07 PM, Peter Loron wrote:
> Hello, all. I'm working (very slowly, but working) on some designs that
> will be battery (and possibly solar) powered sensors with a wireless
> data backhaul (likely 433 or 915MHz).
>
> As part of my never ending quest for more tools, I'm starting to educate
> myself on what I may need to get measurements on current usage over time
> for these devices. I want to be able to quantify power usage while it is
> asleep, sensing, sending data, etc..."does changing this code shorten
> the radio on time?"
>
> I'm sure Agilent would be happy to sell me a fancy oscilloscope current
> probe for more than my condo cost, but I'm looking for something more
> cost effective.
>
> Suggestions from the hive mind?
>
> Thanks.
>
> -Pete
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